Animal Behavior Alcook An Evolutionary Approach.7.Edition

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SEVENTH EDITION
AN EVOLUTIONARY APPROACH
A
ARIZONA STATE UNIVERSITY
SINAUER ASSOCIATES, INC.
SUNDERLAND, MASSACHUSETTS
TM C_
A~ itIk ('OIlwy1'~ informatioo. about .... fiP1~uponry 10 ri,'~
INln.~aphby Ilruc'l' lyan.
1M FrontiJPi«~
1lw lhrNt d;,.pa.y of tIw l-WnadJyas Nboon noion many qun"'- for
n 'olutionary~~aph by C K. 1.oftnz-
Animal ~hovior.An Evolutionary Approach,~nfh Edifion
C Copyright 2001 bySiN..... AS6Oriare.. lee.
All rillh tll ........".<"d.
This book may"''' be ... prod lK't'd in who'" <>f ill pan I.... any p urp..... wha l"'·....
without wriUffi f"-'rmiss ion from the publisher. For inKmnaotion or 10 ord..r.
odd""", :
Sinou~r A."..";,, t,..._Inc., P.O. 8.." 407, Sund..rlond , M.s.., ,,hu ..,Us. 01 375-0407
U.S.A. Fa. , 4 1 ].. .~!l- 1lI 11 .
IIll~mct; pub li. h... iN.....r.com; http:/ /wv.w.•iN u~r.('<>m
Akod, John, IQ.l2-
Anim.a\ bfoty"ior / John Akork.-7lh <"d.
r·mI.
Indudor.. bobl"w'aphiuoI ...-ft,,,,, la'S.
1S8~ 0-8",,3-011-6
I. An,m.a\ behoo\'ior-E..olutioo.. I. Titl<-
QL751 .A5Il 2001
591.5-.k21
lO'l H7 65 43 21
To Tht Naturt COl1st rvancy
Contents in Brief
1 An Evolutionary Approach To Anima/Behavior J
2 Proximate and Ultimate Causes of Behavior:How and Why Birds Sing 22
3 The Developmentof Behavior: A Focus on Heredity 50
4 The Developmentof Behavior:A Focus on the Environment 76
5 The Control of Behavior; Neural Mechanisms 1/4
6 The Organization of Behavior: Neurons and Hormones 154
7 Adaptationand Antipredator Behavior /82
8 The Evolution of Feeding Behavior 214
9 Choosing Where to Live 248
10 The Evolution ofCommunication 280
11 The Evolution of Reproductive Behavior J 1f>
12 The Evolution of Mating Systems 300
13 The Evolution of Parental Care 394
14 The Evolution ofSocialBehavior 420
15 The Evolution of Human Behavior 456
Contents
1 I An Evolutionary Approach To Animal Behavior 1
Qul'Stions about Behavinr 2
II.,U' Qurs/ions nh",t Proximatf QWSfS 3
Wh!f Qut'ShOlls nwut U/h"",tr (nuSt'S 3
Ans",· ..ring Pru~ima t<' and Ulhm.ote Qut.."tiOll5 abo",
Bt'h.wior :;
8tn..:Jll'rS n..J H""""1l Btftn.ivr 5
Gulls and Egphrl/ R,."mt",1 9
Da rw in iolfl Theory and Ulti ma te Hypoth......" 11
v..n.t,jnWtl Logic and 1MStudy of &Nwim 13
TM Probkm ""tlr Group sn~f;"" 14
T....ling AltemahH' Hypot~ 16
(Lm,i"'y,,,,1i $rim« 18
2 I Proximate and UltimateCauses of Behavior: How and Why BirdsSing 22
D1ffnmt Songs: Proxim.oloPQ..-; 2.-
s..o.d Uprnrna"'" Softg Dr,..Iop".", 26
nr AlIi.Pl Soflg C",,'rol S~..... 28
nr DtwIopmtnt of''''' Sortg Conlrol 5~..... JI
Sorr,~ Drffrrmcrs Bt1,....... ,,,, So-uo Jl
Awiding" MisunJm;tlllldill!i 34
Oifftonont Song$.: UltiINIe c.ll'O.'S J.I
1JIrAd<Ipt4t...."" ApyI_1I J5
I'.'lry DoOnly ,"",In: SINg ~ 39
N/ltrmlJ S<-I«tiorl.Pl4 o..I«ts 42
Proximal .. and L%m.tlec.~Are
Complemen tory 45
3 The DevelopmenrofBehavior: A Focus on Heredity 50
TIw Go.onetic5 o f Beh.....io! 52
Gmt1i(" Dilfrrrnas Imd H......." Bt/Ion"", 55
Goorr.1ic Diffrrmcn ",,<I IQf),~ 57
H_ Mmy Cmdic DIjfrrrncn A" NtrdftI to Pralwcr
• &lwt.....aJ CJiffrrma~ S9
1l\I' h olulion 01 Bt>h.villf 65
Difrermt SelKtion ",""UlYt'. Diffnnlt Centos.
Diffnnll8eh.lvion 67
4 I The Development of Behavior:A Focuson the Environment 76
T1w InIWKti\'e' T1wofy of Dr\-eklpment: IiorrtIorll'!o
.lind fltoh.o,ior 78
W/ItIl to l!«tIrIIt'.I FCItIl~ irt • HoNy 8ft' Colony so
W/ItIl to~. Tm'itor¥/~ 82
E"~.nd Bt-hil"iof.1 Dr\'elnpmmt ll..f
u rly Expt'Tietlc~ ,m4 ~Ilioll of RtlatilYS 85
I.tIIrm"8 as Bthal'wnll Ott",I"!'me" t 90
holution and fltoh.o,ior.ll Dr\-dopment 92
Thr~ino V.t..,.o/ Dt(lflopll ..~t"" f/nibility 94
Sn Dljfrre't1a5 ;rt S,..tWllNtrtillK AkJity 98
Thr EroIuhorr of Iwocioftit", u..mirtK 99
Thr Erolu/ioll of Ott",I",, """'t~t H~ts15i5 10j
Thr Ada/'tit'" Value vf Dn>t1"P....... t,,1 Homro>lll$i$ 106
5 I The Control of Behavior: Neural Mechanisms "4
How Sn1n:Jm Cootrol BeN\v 116
StnMW)'~.,....rIJ 5..rrm./ J19
~Ilg ..w ~illg '" Stn50ry /I'pIIl U6
UIIt" P.t"", GmmIIon US
TIlt s..tg of IW Mid.<Jfipllwll fM 131
TIlt Pron""," &s;" of 5ti","II~ fillm"g J31
$ttttIoJ ... FiI/niJIg '" A"",1ory RlaJoton 132
5Ii","11I5 fil"""g i" titt' Tctil.- M<oit JJ4
A<Upti\~ Prlninwte"~ of 80"'••1\_ 137
AMptiPrMtdlm...... of,...."uitwl 141
All OIji1ctory Map 145
6 I The Organization ofBehavior; Neurons and Hormones 1S4
How N..ural Command C..n tM'! OrganiZt'
Ikha\' ;ur 156
CW MtduIllisms.lId 8rlrtlIOt>NI5drt>iwks 159
H_ DI10 Circ:adillll MKIloIllislllS WorA:? 161
w..g·TmnCydncfBml..... 165
T~ Ph!f.'iC/2/ EIII"rollmt'IJl llIfl"fflCf'i; LoIIg-TmII
Cydl:S 167
Changing Prioritit-s in Changing Sucial
Erwironl1>l"Tlb 170
TIlt Effrr:ts of u,n.l.litwI iii Iitt' C...... Anok 171
H~tklp~i:.tSocMlBoIMI'" 17J
71Adaptation and Antipredator Behavior 182
T1w MNlning of Adapl.llioo 184
T1w Comp.uati\·" Method for T..,;ting Adapl.lliooist
Hypotlll"Sol."S 187
~ Impvrlana of Dit\'rgffll ft\liw'iQII 189
TIlt Imp"',an.:t uf CoIl""":<l'tIl EPOiw,ioll 190
The Di....rsity of Antipl't'darn. Ada ptations 192
~ Va/wt ".f Body "Dtroralioll.'· 194
TItt V.I....ofW/lTIi'lig~ 197
H<JW To51",.. Pun..". 200
TItt V.llIt ".f Vrgi"'lIO' 202
Tht Dillllioll E/ftclalld lilt StIfi..oJr Hmi 1Q.1
fightlllg Back 207
A Tactic of Lasl RI:S<,.,? 209
8 The Evolution ofFeeding Behavior 214
Incolhng Food 216
Tht 0rig11UofPr-ty-l..oal'"g MtdMfoisllts 117Cortt,,,, Htlpfrom e-,..."'" 119
Tht History of~&r DrlIIIaS 111
Tht A40tptin' Va.... afHonty &r £l,m,;r.o 225
ThtI~ U111" HypotltniJ 117
L«/JI'"g Prry I1y Dtmt 219
Sel«ting " 'hal lO Ear 231
H_ '" Cltoost"" Opf.......a.r... 23J
H_ '" Cltoost"" Opti-t MIIUd lJ4
Cnti<"is>m ofOpti-t,ty 17wvry 136
TIttEroIllhoft '"Al/mW11A" OW/!; 136
Conwming "'hal You SeI«t lO Eat 238
H_ to CJI;1tIt. Wlwlk 240
Why DI10 HII""'IU eoo..l1..... Akohol. 5Fn.
Il1ld Dirt? 141
9 I Choosing Where to Live 248
H..bitat Selection 250
HiIblwt P'r,Ii"'iUS "'" Ttrntorilfl Sj;wlr5 ill
Dr<prningfrom Onr P/lIa '" Anotlorr m
M,gration 259
T1u' Cosl. ofMigralion 260
TIlt 1k'Nfit5 uf Migralitm 262
MigNfi<ln iIO .. QmdiliolW TiKti.:" 2U
TemtorWity 266
TrrTiiorilllity ..ptJ c.lorits 269
TrrTilorUzl Puuks 271
Why Do Tf'Trill1'Y Ho/dm AI..."", Alw"y' Win? 272
10 The Evolution ofCommunication 280
The Ori gin. and Adaptive Value l>f a Sign<l l 282
C~ ...~lIIlit.. SrlKfitm 4P1d Mllllirk awngt5 283
TIlt AJoortiR' VoIlwuj hit o...ng<'< 284
Cllnmf AJoortItY V.w 286
11>0' History of .. Sign.al-Rf'mving M..dwusm 2'l7
n", History of1n"d Wing. 289
s"n.<lry hpl<litltiun of Signal Receiv,,"
by SigNlet's 292
x-y Ptrft',Joa'f~~ flw £llCII~"'" of..
SigfwI 293
n", Panda P"n,ipk 297
The Ad"pta~tAppl'OlCh to CommWlK..tion
~m
Yo1ry Oct &by Bmls&x SoN<tislJjI for Food~ JOO
Ukg'/I_I~ Rt'l:tn",.s 30S
Adapli,,,, Sig>1al Rt'Ui,~n)( J07
Ricrivtrs Mily RI'1j~il'tH"IUS/ Sigrwls JOIj
Why~ lktJ'Iio>r O<nor~ JJI
11 The Evolution ofReproductive Behavior 316
"Jh!o holution CIt Dillt iE ....fl if! 5el< R'*'" 3111
Tnh"g IIw£llCIIIII'""""Y 11II't>fyujSn Di/frrmtYo ill
s,,'ual 5ell"Clioo and Competi tion for Malt'S 3rl
SOCUZ/ Do...i""nct' and Millt FillUSs .HI
Altl"l"lllllivr Milling TiKtics 332
Ae-'i~ Siraltfl/ll'ifil AllnnaliR' Molti,,!
TiICficJ J.U
J1rI'tl' DrsIi>od Stnrtrgin , J1rI'tl'MoItlflg r.-t'" liS
SavM smrtio>r I/ptJ 5"...... Coo'Ipt1" .... JJ7
Malt CWlrdin)( JJ8
s", u..l s"lt'ction ..nd Mat.. Choice 341
f~ .MIlt Chc>ict' ,,"11101<1 M11lmo1l &nqi15 J.I6
Ttstirrg IIwHnIlflry .MII~. Good Gnrn. ..>04 R~""""Y
~~J5J
Sex......l Conflict l;oen.·een !\.tales ..nd Fem.l~ 353
12 j The Evolution of Mating Systems 360
Is Monogamy Ad..pti,· ..? 362
M.mllllamy III .MI......../. J64
M"nllllamy i" Bird. J66
ulrl/-Pai, C"P"iIIfio>r.: n",.MIk Prrsp«fi.". 369
£xItI-Pllir CDJ'IUIIfiorr£ TJw FtrItIIh P....ptdit'" 370
Poly.andry without Pol)·gyny m
Pol)·gyny J77
f"""/t [)o/nst Polygyny: Tht f"""lt Prnp«tiror 379
Rt50wIlY Dtfr- PolygyrIy J80
RrsouIlY [)%tst Polygyny, n", f(mak
Prnp«riw JSl
Saamlrlt Cc>mprlilion 1'oIYSY"JI JS2
Uk PoI!fSY'IY J85
Yo1ry Oct,\.Wts Aggrtpft;.. Ltb' J86
Why Oct MIIny F~ M111~ ll'illl flw Sa..... /o.Wks 1/1
Ub? JS9 I
X T. l>I~ 01Ce>n lent.
~ ,13) The Evolution ofParental Care 394
,
Why Is Parenl,ll Care More Often Maternal than
Paternal? 396
EXCl7'tion~ to tlw Ruit 397
Wh.~ Do Mille Wattr Bugs Do All tlw Work? 400
Discriminating Paren tal Cart' 402
Offip"ng Rt>:ognition; C"",,,,,rati,,,, St udi<'s 403
Why Adopt Gmetic Strangtrs~ 404
TIlt History of Intersptfi/ic Brood Parasitism 406
Why Accept a Parasite's Egg~ 408
Can Adoption &neftt fostl'l' Parmt.? 410
The Evolution of Pan'lltal Favorit ism 411
•
14 The Evolurion ofSocial Behavior 420
The Bent'fits and Costs of Social Life 422
1be E\'olulion of Helpful Behavior 427
R«iprocal Altruism ar Pl'I'sonal GIlin? 429
Altrl/ism and Indim: t Sd«tim, 433
TIwA larm Cail of Belding's Ground Squirrel 434
The Concept of Inclusive Filness 435
Inclu,iPf Fitn,,, and Helpers at tilt Nest 437
Insect Helpers al tilt Nest 442
1be Evolution of Eusocial Behavior 445
Testing tilt Haplodiploid Hypotht<;,s 447
Vrry Clll:'( Relattdnr5S I., Not E"-<tntial for Eusociality
to [ 001,,,, 449
TIlt Ecology of [usocwlity 451
15 The Evolution ofHuman Behavior 456
1be Adaptalinnis! App roach 10 HUllliI n Beh. ,"ior 4';8
TI,e SociobWlogy Co"t""",rsy 459
[1'lJlulum and the Diversity of Human Cult,,,,,, 462
Sociobiology versus Arbitrary Culture Theory 464
Adaptive Mating Oed sions 466
Adapli"" M ate Choice by Women 466
Glossary 489
Bibliography 495
lIIustrarion Credits 529
Index 53'
Adapti!'" M ate Choice byMe" 471
Conflicl btfw«tl the Sexes 474
Coerci,,,,Sex 477
Adaptive Parental Care 4IlO
Hel,'ing Chl/d,..n Marry 481
Preface
An"th~r f"ur y~"rs or so ha ve passed ' in<:e the f'r<'viou" <.>d ition 01 Animal
Beha,,;")]" dppe.>n'd. Purin!\ thi, time, much thai L, d"Preso;ing hils occurn.'<l on
II><' polilk<l1 and """;ronm.",,,,1 fronts_L..1 us IlOI d well on the;e ""<'I1l>;, but
i""tt'ad ronsider the pooitive side of the It'dl;t"f,which includes the many exce]-
\tonI P"J""T" on animal ochdvior that have been w ritt<>J1 over Ill<' last I<.>W years.
1ndt>e<I, the rMeat which impurt.,nt d iscowries about behaVK"-aTt' bt-inKmad"
has st('..d ily inc",.sed thn"'K"'''' I Irn, Iit.,'lime of my textbook. Just pid ' up a
n'O.",t ropy of any ;0"",,,1 in the dis<.~pli"" and compare the " ....arch reports
tlw", with tlu _ in the ""'" journal 15 or 2Q years p,,,v;ou,ly_Thu•• in 1982.
t.....journal A"i"",r Bmwwur publisht-d 1264 f"' ges oI ...ports; in 20ClJ the t,,"'1
was 2180 raK'''>, a fig"... thaI unde""brnalt"!' the actual inc",. ... bi>cau,<, the
iuunt/ll substantially enlarged its p.lg e siu in l'l'lll . \1o"-,<,,, ('r, the quality of
tho> work, d" well as i t, qU.Jnlity, to,.ch.l.nf:"d . The a,-.,tage """"'I paper on
~nimal bet"",ior i. mo ", "'Phi,Ii.;",tt-d ~nd inl<' ''-'Sting than tho> ~ver~ge paP"'"
wriU,'n in It", 1'1805. SCi......lists ....~ lly d o b uild ,m what olh~", haH' acrom-
plisl-...d, whi.;h ""tl; lho.' b.lr hi~l><'r and higl><'r ~Mh y~a,.
A. ,,-'S<'a,,-'he.. haw ""'" the chal\e.-l~,' of ~xp., nding our al"-"'dy 'ub.t.mtial
""',,,,ledg.' ,,j animal b<'ruvior, theyhaw gt"Ot"'a l<'tI an ~mNrra"""""'t of riches
for a texlb""k " 'ri l<'1". As I han> ",viS<'<! my bo"k ag~in, my m~in difficully has
N"-'" whalto I"",v~ "u!. nol who,t 10 put in. Although there ha ,'~ alway. M 'fl
m~ny mo ....~ood pa!"," than spaC<'a"ailablc fur my .ummari.... Ihis time
anlUnd lho.· pn>bk'm holS se<1ll00especially aeu n-. ~vt"J1 the numbo' .. of ..xcel-
1...,1 ......arrh "'P',rts c"'-'pl''tI wilh my desire to limi l th., I""gtb of my boo k,
D<->pite <'Iforts in th., p"'1 10 k,,,-,p th~ number of pa~,.. of texl und~, " mImi.
my book had grown bigger over the y""rs. nu. lime Jhav~ m.u..g,>d 10",verse
the lnend, which should make the book '. conn-nls mo", d ig,'Stibl", although
it d,,,.. m..an tha t "'''"C good ,tori,.. w......omitted. Read~1'!l "f my book can,
ho"'~\'('r, ,'xplo", any b<'1""'ioral tOPKoj Iht-ir choice by turning 10 tho> origi-
nalli tetatu",. In this ""Hant onE" tool-the Web of Science (htlp: //www,w..oo/-
5Ciel\C<'.com)-il; lrt'm('lldo usly valuabl.. because il proviJ... such ~n ~~sy and
quirk ",ay 10 Irack down lhe wurk of particular "-'SC'an:h<o'" and 10fo lk'w the
trail of p"p.,rs that rit,' a "-""'arch "'f'Ort of inu,resl. I ram">t """",mrn''fld this
...arch me<:hani'm 100 sln>ngly.
In addition 10 b,in~ng Ih~ boo k up 10 da t.. and """uclng ils I""gth ,,>rn,~
whal, J how" bc\>n abl.. to Inrorrx-"'dl~ ....,1'" illustration. ltiroul':h<-.ul tho> text,
Ihanks 10 Ih~ w illingn ..," of mv publi.her, Sinau~r A''''lC;at...., to m~k.. th..
mow f,om hlM k and ",hitt, to full rolor.This change not only add, 10 Ih<' a,..-
thelK val "" of tho.' book, bUIshuuld .1"" ht-lp ,....d...,. gr.,-;.p tl><' poinl of graphs
xu P,e/ace
more quic kly and easHy. The photograph. may also help .tudents why
so many bclt",ioral biologi.l.low .lI.ldyinlll'l'al anilT4lls in ,..,al''1wiroror nts.
The many dI<Inl<;'" thaI I have made '" !hi, .."""th edition of my bo,l<,k ha,'e
all bteoen ft\dde with one primary goa l in mind; 10hel p my reade......... how
........ rche... have been able 10 reach .., tisfaclory =Iu.ion. aboul how and
why animal. do tht' things theydo. I hope thaI it>;"""mple. 01 tht' wonderfully
inte""ting ........reh d,me on animal beh.vior will ft\dke the point th.t sci",,·
tiflC logic offers a powerful means to gain an underst.wding of nalure,
Acknowledgments
The author of .ny texlbook depe nd s on cooperalion from a surpri.ingly lal')\"
number of pe",p l" . A, I have rewriU"" lhe bo,l<,k again, I haw, been very for -
tun. le in having many ge nerous colle. g ue. willing 10 give me th.. ir lime,
advice, pho togr aphs , and other form. of hdp, Mike Maxwell was especially
helpful in prov iding informalion on m;c" ldlile analy" ",. All the cha pten<
of th e boo k have incorporated chang uggested 10 me by Ihe following
revi"we.... : Alex IMso lo, Eliot Brenow ilz, Ken Cat.nia, Robin Dunbar, Bruce
Lyon, Jim Marden , Bob Montgom..rie. Randy Nd"m, Gabrielle Nevill. Don
Owings, Steve '\Iowkki, Ko:m JU.e, .." G:n<' Robin""', Tom Seek>y, Nancy Segal,
David ""'''''!neat, and leanne Zeh. Special thanks to Bob Montgomerie for read·
ing tht' entire manu"':Tipt and for offering "x!remely helpful .uggestions about
how 10do thin"" better.
A host of other colleagu... have provid ed m" wit h f""rmission to use iIIu<--
tration. that originally apf""a""" in their papers . and some have given me
the iIIuslrations !h<'m...i"..... I wanl to singk- out Bruce l yon for speciallhanl<s;
his su p>:rb p hotog raph. ap!"'ar in many pta"" throu ghout the text as well
a. on the cover of the boo k, I ha ve ackno w led ged all supplie... of pholograp hs
in the !<>xl allhe appropriate figul'l'S.Acknowledgments 10the pu bli.hers who
ha"e also g''Jl,,,ously gra nted f""nn iss i,m to use their copyri l<;hted material
.ppe.ron p.a~ loca ted between the Bibliography .nd the Index.
~fy ed itor al Sin.""r A.sociates, Pete Farley, has done the hard and often
unglamourous work of keeping the project mov ing ahead. I am wry grateful
to him as well a. to all the other Sina uerian•. especially CheL.... Holabird, J"an
Gemme, Ch ris Small, David Mcintyre, and Mara Sil"er, Norma Roche , w ho
cop yedl ted this ed ition as well as many othe.... ,..,ally know~ how to fix mis-
takes. Readers of my boo k are luc ky thallh<'}' get to read lhe copyedited ver·
sion rather than the original dr.ft,
Although dI<Ipter " ·v i,,,,'e..., pho tographe ...., editorial.talf, and presidenl.
of lhe United Slates often change from ed ilion to edition. som e Ihings s lay lhe
"" me, which p"wid....
a certain reassuring stability t" my life. My wi fe Sue
continues 10 cope with my many idiosyncra.i..., .Iillli.tening thoughtfully
whenever I vigorously del\OUJ\('(-' the likes of, say, H enry Kissing..r or Antonin
Scalia, sliU willing 10live in a cramped ~amp',rvan for month. at a lime w hom
we are in W...tern Australia for .nother round of bee research. My younger
son !\;ick is .till in town rr..mpe , Arizona), and he ta kes time olf from help-
ing lho6<' acruse<! of driving under the influence 10he lp u. eat dim"" and play
ping-pong, A. in the pa.t, he lets me win onee in a while so that I can retain
som.. small measure of S<-'11... . t<'<'m, thaI most important of mod..", com-
modities, My older son loe is dose .'nough (Albuquerque, !\lew Mexico) thaI
he ra n join uS 00 occasion, helping me ma in lain the iUusion of being .ur-
rounded by family, In addition, I .m happy to report that none of my friends
at Ariwna SI.lte (among lhem OO"e Brown, Sl,'v .. Carroll, Jim Colli,,", Stuart
Fi.her. Dave Pearson, and Ron RUlowski) has yet go"e IIIhis reward, which
means tha I we can aU g<>I tog<'lher at lunch and som et im... on Frid ay aft"r-
....... iDrt-.. juot~ _ t..,...toowl. doing iDrdoocada AdmitWly.1Iwamount
cl ........ INI _ (OMUmi" Iwo~. dIOWb,& from tt... pildwr P""" pn"iOn
cl tt... good old d.I~ 10. sJ-or two rurTmtly. WI tt-e \I<lf"O undford.........
....... (Ihr nWJWltlUO dN<h-,",,~ cl grttinp; oldo.-f MIdIhr trout>Ift o..-d by
dri<rti,·.. <:<>IIoIo~) t..,............. lrloI"d IIw ........... INnk goo<tr-. TCI my f...,-
i1y .and fnend,. INro. y"".
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Pf><J'''9raph by Ma,l; Mofftrr
An Evolutionary
Approach to Animal
Behavior
or hundreds of thousands of years, humans
observed animals because their lives depended on a
knowledge of animal behavior. Even today, the sub-
ject still has great practica l significance. Information on the
reprod uctive behavior of insect pests, for example, may
ultimately lead 10 their control, while knowledge of the
migratory routes of an endangered whale or shorebird may
enable conservanonlsts to design adequate reserves to save
the animal from extinction. Moreover, an unders tanding of
the evolutionary basis of our own behavior might help us
identify why we so often damage our environmen ts, per-
haps enabling us to reduce ou r destructive tendencies
(1251). But even if the on ly beneficia ries of studies of ani-
ma l behavior were the persons who conducted the
research, 1suspect that work in this field would continue.
Learning how and why animals beha ve is an intrinsically
fascina ting business. Perhaps you can imagine what it
would be like to be the first person to discover that male
damselflies actually use thei r penis as a scrub brush to
remove the sperm of rival males from their ma tes [11 771; or
maybe you can put yourself in the shoes of the person who
first showed that female Seychelles warblers could control
2 C~,pler \
lhe ....~ of It....i, t>fkpring ... as It> ha .." daug'u.... al Ii....... wlwn il Wa6 rnosI.
adv."UIt......s [6411.
In the pa~ ahNod, you will"'am aboul thew and l'IWly otht-r~
dio<>:J\~ The point of this ""'I. ""-"'.... if; "'" onl)' It> introd...,. you 10tt-
~ findlllp. but ...... 10help you~ h<- ocimIi!;b hi,.., d&oIfr.
mlrW'd that 1:1\0'~y~ ......_ as .. romp<'tlli>... ,,","pun of!l<XtS or thai
the l.'OOIrol of off>pnng ..... ratioby the~warbW ..., adapulion",.;,.
• puticu1a.~. I t>rbr>.., thai the f""'C"" of doin& ocWn<-." ",,-.rybit ...
~ .. the mdif>w> that an> its mit product-If Ian help you w ..iu_id
the Iogio: of ""'"""', .......u as af"f"'l"ciallO ttw~ di \enil)' of animal
boN•• , my_~ will ha>... d<Jrooo it> job
Questions about Behavior
lli>"\'dfor one """'mo:"I" in Monl\"\"(>flje. a tiny communiI)' in 1:1\0' I'f\<>IIfIIain: of
C""U RK 1.;,,;1. ....,. f"""""-'d by pac;fi>tQua~ f the L"nilo!'t:l sea...
around I n of ttw Korean War. Whilr I ...as t a frwnd Ioarwd .
black liK!'1. which 1h\U\K up by a wrule """'" on I back pordoof our .
The ultra>'ioIo:t ,a~ of the lamp altractN hund~of rnoth!oe«h nighl.and
many olayed .... the .t-i until I could insp«I them. 50.,...,.. fI\Of1\ing> I found
a h"l/;'" btijlht Y'-~k"" rnolh b<-longing the gm....A~'"",,""" <linging 10the.hoe!.
In the <hilly dawn. lhe sluggish moth did n<>, s'nJltl'iW if I pK-ke<! il up car.-
tully But if I j<",tk-d il sudd<'fl1y. or poke<! it sharpl y 0" ih thtlfa~. the root!'l
abnJptly lift,>ti illl fonowinK' and held thl:m up It> "~I"_ illl p", viously ""'"
,,'ale<! hindwinjls. n,., hind,,'inK' we", ma,,,,,lou,ly d''<'Ofat,,,J. wil~ orrular
"..Id..... IM t 1,.,k,,,J lik.. two ey<'S. which """",<'<.I 1<> .1..", l;>.K'k al n,.. (Figlll\'n
Any"".. ....-injl Au/omeris abnJplly "~I""" i~ hindwin" ....y<'S" will hA"e
som.. qll<..tion••boullhe l><-hav~ ,.-. But no matk-' ho..... ""'II the Iislof quo-stior6.
..""I. quo.-ry , a rl b<- ",,,,,igned to one 01 two tundamt"nUlly di lll........t cal<'goM:
"h"w '1..-i""o,· abOUI the p ro.lmak- ......:hani._ ir\sid.. the rnolh lhat an>!ll'
the bo·ha.it.... n, · why 'lU<'5tions: aboul the u Ulmal" n, ....·oI ulion.a~· ..,...,...
for I~ boNvior 177~_ 8S'l1. "H" .., '1.....t~· abou l boNvior ",~k MlIr an indi-
"ld""l maN!\" tn ' . rry OUI an acti>'ity; 1M , a l.-gory o f '1"""lions~i~
e>.plan.alicJno, aboul how an animal ', in........ meo:han"",,~dt'o.""""J"ed and how
1 Aul<HNrlornot~ from Co"a Rica. ILotI) n.. moth In~. ~ing 1>OIIbor'r. _
_ ""0'9 ' _ ""'" the~ lRiglJl)....It.. b<oin9~ In the .hot-... m.""""
~" ~. 1o<ewI"",.IorwMd.t ..tIic~ .Ime the "eye>"on the ~~ bHomo~
PhoI"'l,.ph.l:ry the ."'.....
"'n Evolut ionary "'pproa,1> to Animal B~l>a_io, 3
!ho>}' thm cau"" tl><' a nim.ll to beha_~ in a <:eTtain way, In contra,t, ' w hy que"-
tions" abou t b<,·ha_ior ask why the anim.ll h.l.s~volvro th~ m<.>ch.l.nism, th.l.t
und..,.li" illi action"
How Questions about Prox ;mote Couses
Consid,'r th " following que,tio"s aboul th.. wing·nipping ,,'action of an
Aut"",..,,, moth to a sharp poke:
• How do th~moth's mo,.;ll$ m..ke i" wings move, and what control,
Ih""" muscles?
• How do,," the moth know when il h..s been louched?
• Did the fonds tho.- moth at" as a caterpillar influence how II behaves a,
an adult'
• Did tl.... moth inherit this beha"ior from ii, mother or father?
What [he"e qU<-'Sti"ns ha"e in common, d<'SpiteI""ir dive""ity, is an intt'f\'St
in the operation of mechanisms u,thi" the moth that cau"" itlo pull ill; forewings
forward, re\'ealing the amazing hindwin~. n.., diversity of proxim.Ite ques-
tions is gn'at '''''-'ugh, I-.<,w","'",", thai we can subdivide them inlo two rompk~
m""tary gn>Uf'S, on<> dealing wilh the interactive dk<:ts of he n"Jity and """i-
n",,,,,,,,t on the d,·v..lopment of the mech"nisms und,'rlying wing -flipping, and
the oll><'r dNling with how the /ully dewk'f""i physiological mechanism. actu·
aUy op<'ra\<- when the beha"ior occurs.
n... d,,,,elol'menW side of the "lu.,tion has todo with 1-.<"" the moth's he..."j-
il)'-I'" g''TI<''S, its DNA -influ<'fl<ro tht- pr"lilt-ratiur, and specialization ofcell,
Ihat occurn."J as a It-rtilized egg ga"e rise 10a ca""rpillar, which grew inlo an
.>dull with a partiruLir lUnd of nervous sysK-m.n.., opt'rahonal ,-ide , >I the equa-
lion has 10do wilh how ""ural mechani,m, within the adult muth detect cer·
tain kinds of slimulation and how m",,",11"" are then relayro 10 activale mu..
cular .....ction•. Research on the de,,('lop mffi la l and physiologica l a'f"Xt5 ,>1'
behavior ",,,,,,in 10 be carried oul for Auto",..,.;" but ",-,meday we m.ly learn
aboul both
of tru.-s., proximate caufoeS of its be!>a"ior.
Why Questions about Ultimate Causes
Even if we already knew everything II",,,, was to know about the proximal<'
cau","-" of wing-flipping by Aulom,ns moths, we could stm ask many more
q"",tion~,
• lNhat do loday's moths g" in, If anylhing, by wing-flipping?
• Has the behavior changro over evoluliunarJ' time'
• If so, what were the prcd"",,".",,.,; "I today'. wing-flipping ""'ron-"'?
• If lhe behavior has changro, what ca used the cha nges?
TlJe,;e q"",tions all invol"e the ","oluliona')', or ultim.lte, reasons why an ani·
mal does something . Why docs the moth suddenly lift its wings and e xpos<'
it!; eyespots whE.'TI it i, mok>sled' n... British so::i<.>nlisl David Blesi suggesl<'d thai
lhe action sp......d be<:ause in the past wing·fIipping frightened off some bird
predators when they mislook the molh's eyespots for the eyes of INi, enemy,
p..."jalory owls [116].
If Bl<'St was right and wing-flipping behavior saved the lives of molhs in
the past, then the e"olullonary procl'55 ha, rontribut<-"J 10 the pe.,;i.tomce of
the proxi"",l.. mechanisms that enable today's moth. to bt'ha,'e the way the y
do. Particular ge""" present in the bodies of conl<'mpora ry Aut""""" moths
ha"e been "'Plicated and passed on from g..ne ralion 10 g..,..,r ation, pcrna!",
becauS<' they hc lp<>d Ih.. molh d,"elop an ability that frigh tened away prcda·
lory birds, enabling illo h e long "",-'ugh 10 Iransfer Its hercdilary informa·
hon 10 somt' d~~end.nl". This pt<lCtS< could help expla in why Aulo rn,,;,
molhs living in Montevf'rdf'IOday recei.-e gen", from lheir p"t'l'nls thaI pro-
mnle lhe development 01 wing·flipping behavior. n,.. de",·lopm~",'.l plan,
and lh~r\'f"", Itw beh.lvio,al .biliti..,., 0/ each membe, of lhe sF""';"" i•• ,..,.ult
01 difl~"""",," among individual, in thei r "'producli.-e 'UCCI"" over evolu·
I;,maty Iitn<'(Figu... 2).
lhe cunrnl functioo 01. b<-ha,-ior olf<:no In,ighl inlt> it>. f"--mle u!M'fulne!is
in IheP""I. whi ch could helpexpl.'lin why the lrail.p.... d and ...placed oth~",
ov..,- time, CharaCh'rishcs lhat cur .....tly adv.nce tlK'"'producti"e chances of
individu.l. cou ld plau. ibly h.lve h.ld the ..me .-fIt'<'! in lhe P""I, the..,by aflect-
ing the "ou.se 01~""lution. Bul what traits pl'fff'ded 11K' wing-flipping beha,-·
ior of modem Aufomm,' II we could go b.ck far enough in tim~. we would
fmd an anc.-slor 0/ the moth that did not exhibil tlK'b<-h.l,-ior. l'~,.haps lhe ori·
gins 01wing-flippi"l'l invol",-d wing mowmen", a"...",iated with taking nighl,
movemenl" tha I have bef'n alt"ted during the moth'. history, ju.t a' th~ colo.
p"ttem of the hindwings has certainly d"'nged O\,~. tim~ [115]. A lull und.,.-
. ta nding of Ihe ultimal.. Cau><>5 of wing-fl ipping ""lui"", invl'Sligation into
the inilial fo .m and sub'""'!uenl evolutio" of the behavior, a' well as the
p1"<X'\'S",," ""'f"-lI1Sible for the dlang....
You should now be able to discriminat" proximate (mt'<:hani,lid qut"Stit.",-,
from un im.lt.. (evolutiona.ry) 0""" (Table 1), If you wanted to find oul how the
ne'vous syslern of Aufomf"l'is moths control. the wing·flipping ft'Spon"' , you
would be inl.......ted in tlK'proximait' b.>sis 01 behavior. as i. any""... ,,,,,,,,"rned
with how g..n<>tk, developm,'nl.!, neural, '" h"Tmonal mechanism, work
within an animal'. body. On the othf'. hand, if you we", int.-"-'Sk'd in whetht'r
wing-flipping evol""<i beca"... of p".t p..-d.,tion p"""""', you would be dNI-
{
G<n<- prod"""
G<nr-mv,""'""",t
.'0"'-
Gffiffi< 1Nl"~P
THE lNDJ\'lDl!AL
GENE 1'0 01. OF ~EXTGIl'IRATO"
I
C..... " .".mi,.;m. (n_)
I
PHYSIQUX;JCAL
SYSTEMS
1
lJl;V~LO"'thNTAl
SYSTEMS
4 Cllaple, 1
2 Proxim.,••nd ultimal. ","u..,s of ....Mvl"'. "'"he
""",imat. _,v"Jou. Inw,..1mecMni.m. ""able an
"'ufOmfrl. molh 10e _ ule ~. win<j-fl ippin<j ....~, "'I
lhe ultimate level, the moth-, fNCtion <0 bird pred a'<m
delermine. ~. reO'oo",,;,,. """'.... . . mHoured Il'; how
many <opO" of in~ fNCh III< next 9"""rali"",
ReO'oou<. i".di_.. among Indl¥id""I. .....~hd_.
pro. imate mechan""" determine whkh 9""'" a re ava il·
able to Infl~ til< _I<>prnenl of Indl. kl ",, '. In til<
next gene<ati"".
An Evolutio na ry Approach 10 Anima l Behavior 5
TABLE 1 ~15 of aNI)"i. in the.tudy ofanlmal behavior
I. C"""""ic-d,,,,. lopmen,,,1 m<'CMn;'m,
Hf,,,,, ",- """..J ;,~ '"' bo.'h.l,io.
]) k>r""-,." 0>/ """">CJ'-motor
. y "'''''' "",~•.,,,,,,,,,,,,,,
m ..-li"".
2-o;,.,,""Y-m,,'ut mo.d"'.ni,,,,-'
1\:,.." 00... 'ystems f<>t ,he d"'edit",
,-./ en'·'''''''m''''t.1 .~m\lli
It"mw.... ""'e""it". .J,u,""~
""f'O""i.·..,...., '0 env,,,,,,menlal .timu l,
Sl.elel.ll_mu""uldr 'Y'''''''' f,,,.
e, fry i llJ! out ""i""''''''
U~i_leC.u>e.
1. Il;,~>ri<ol p",hWOV' k..din~ h, 0 CUtrl.'1\l heh."'''".ol''ail
bo",,,, ocrurring "n·t "",~u~,,,,
ft"m Ih<- ",.;~; n "t ' he It, ;, h, '"" P""''''
2. "" · '~fi," pn ....._~ ..tupin~ lne hi.,',,,",' 0>/ 0 b.-h.w ;'"., 1Ir."
I'a" and CUtrl.'t1' u..-!ul.......". ,I><'
_,'K". i n r""",ohng IiMi"",
"1'"><lucti ..,, ",<,<..",
ing with olI\ ultim.l'" i.;.su(>. as i5any'......•who wanl>o the .""w..... 10'll><'S1it1l\S ab<>..J1 f
the nl',oduclivc val",,' of a tr.il .rod il>o ta~l..ric...l founddtions (_ Figun>2j.
Mo,,~"'~<, il ..",,,,,,,,,, ..... ~'" 10 claim that work on It-", evolutiolldry Nsis oi
winl\-Ilipping beh.\vioi v~minalt.'<! 11K- n.,.,d kl~""W'Cq"",lioru; .b""t the pill""-
ioIogical b,'5;s of the bcll.w ior_you would II hope ) ,*'jl't.1 "n,nuOI.Isly..r rox;male
and ullim. te hn .."t-s.re "'lnlplem""la'Y,_nol mutually_" , clusive-a con-
ce pllhaf many I""" plv lind dilficun to l\ra,p. For ex.m r1e, I ,m"e ... ad Ihat
caput.'hin !IlCIIlkcys rub tho.·oil~ from citru, fruits ,,,,10 their fur because the chem-
icals mayhdp heal skin wuund,.11>eauthurthenadJed,"Of course_ the mon-
ky. may .imply ""i''Y.the ......"'"i,..," as if this ""planalion n",.nt that w,' could
ill""'" the ll'It.'didnal bt.,..,fil hypoIIK'Sis. Th.I ....ould bt- a mista ke. At 0 p"",i-
malv l,'\'v[. monk~ys may indwd d,'rive pl~.~u"" Imm a pplying cvrtain suI>-
sl_lnn'S 10 Ihe;, bodi,.,;, bul this explanation d'.... nul "'plac<' th" ultimal~
....ound-....alinl\ hypo>thvsis. It in !ht. past monkeys Ib"t hk<-d tu ",b citTu>; uib;
O.. lhe" skin had vwn slil\h.IJy l\n·,'l~, " i" ud ucti ve ~",,,,,,,s than individuals
lhal w~... ;ndifn-n'nllo oily ",-n,",-lion., we would bt-lI," undvrstand why all
c.puch;"" tod.y u'"' citrus ,~Ls in a paMkular fashion. The full .'Ildlysis of .ny
behavior involves answ"riJljl;bolh p,oxim.1t<·QIlJ u llimalto 'l u"",bons.
Answering Proximate and
Ultimate Questions about Behavior
Jt i.~ ,,,,,, thin!; 10 0., curious aboul a nlffh.lnism of bt-havio, or its ,""olulion-
ary foullda tion and anolhN thin~ to ""ti,ly un~'. curio~ily. G~tti"g va lid
an,w,'''' to biolu~c.l qu....lion• .-...qui"" a p.irlicu iat approach, calkd the .d-
.."ti / i.. m..thod , who"" logic musl t«> undvrstood if you a"" to )l;r.,p .... hy
biologists acrt>pl somv conclusions but not oIIK·". We ,.hall .. . plo'" this is>uv
....ith ,-xamrles tal"m fnml I...." .ludi", don.. in t.... middle of the twrntivth em-
t"tv by the h'l\'al behavioral bio"'lIist Nikn Tmbe,!\",,- one on a proxima te qu.....
lio" and the " thv«'" an ullimalv """.
Beewolves and Homing Behavior
Tin"",;;en helped mdkv the sludy of animal behavior a part of mod.-m biorollY·
Althou"," Ch."k'S l>'lrwin ;n",,,,ligali-d e. rth ....orm burrowing bt,ha " ior, bum-
bId....., m.ti",," b"....~rbird d i,play s, and Ihv fadal vxpr<'SSions of d"il' and
6 Ch.pler 1
3 TN! founde... of eth ology. from Jtofl to ,,,ht: NikoTin~" Konrad l",,,,,...nd
Ki<1 von F,i""'. P'ho'ogr.pn. b)' (i<'Itl 9,T",h.., .. (middlel Sybdle K.I.".nd 1,,,011
0, von F"",h.
human'. nO scientific journal, ...ere devoled to b.,rovioral n.,.,an:h until tho>
mid -1930s. At th" tim... the field
of elhology originaled und.,,- lhe guidanIT
01 Tmbergen. a "" live of tho> Nelherland,. and his lril.'lld Konrad LA'""""••n
Auslrian, They and tho>ir <:t'Uea"".... inn",tigaled both proximale and ultim.",
qu ....tions about Ihe behavior of gulls. jacW....s.hut"'rf!i•.,. snow hunlings.
greylag g_, m"lh coI'·'Pillars. and many other animals in m.,;r natural t'rwi-
ronmenls [720. 11371. Th....., pion.....ring elhologisls ultimalely received Ihe
Nobel Priz.. in M""idne in 1973, which Tinb.,rg"n .nd Lorenz shal'l'd wilh !Cdrl
"on Fri",h (Figu....3). an AuSlrian l't'S<'an:her famous fo' hi, work on honey
l>ee communic.tion (""" p. 220).
On" 01Tmb<'rg.-n·' rliest el!lt,logical ,tudies !><-gan in 1929, mo .... Ih.>n 40
yean;bef"1\' he wa> a arded the Nobel PriJ:", when he d~,",o,-..red a I"rge num-
ber of digge' wa,ps ",.,ting in th<' "''''d dtml.'S ""ar Hulsr.OfSl, ""Iiand. n.....,
wasps so fascinated linberg"" thai ht' and rus tellow ".".,an:hers Spenl w.....ks
livin!! in a primiliv.. camps;t.>and hicycling up 1070 mil,,,,a day in om..r 10I<>a'"
m",,, ah""t tlv:rn [1137]. The sf""'i... of digger "'asp thai cought Tinbergm', eye
was Phiumlhu' lriangulunl, the h,..,,""01l. so ""me<! becau,", il captu~ and para-
Iyz.es honey l>ees hy stinging them (Figu", 4. k·t1). Female b.'<'Wol""s transport
captun-d b.'<'S 10an lIDderground """,, ...he", lh.rya ... slored in hro<>d ITIIs off
the ""'in tunnt.-l. 1be l>e<...... ,"'mtually eatt>n by the wasp', offspriog wlwn the
HitI<' grub hateh.>soul In"" an egg laid on a b.", l>y the nt'Sling r..m..le.
Som~ s.:mddunt'S in Hl1lshorsl w~", doUN wilh hundn-ds olhurrows...ach
m.rk"" with a low m,,1IDd of Y"llow "'nd thai the hom.1e b...,wolf had Iran..
port<'<I1o lilt! surfac.. whim ~xcavalinl'l ht'r 0<"'1. Tmbergen noted thai wh....,.
"''''''''01/ It>ll he r horrow 10go """ hunlinl'\. ,he ('(lwred up the Of""'ing hy rak-
in!! sand owr it hiding itlrom vi..w, and y..t wheo she carne b.",k • half hour
or an hoor latN carrying a paralyzed hon..y bee. sh .. dart~d di,,,,,tty to her
hiddt'll nest ..nlra""", ignoring aU tho; <>thers (Fii\Un'·f. ri!V>,). Bygiving fumales
unique painl marks, Tmbergen v..rified lhat ~",h w"'p buill and provisiOfl<.'<!
only or... r"'stal a time.
Th.. skill wilh which Ihe mark~d b.·.·w"I,,"" found Ih.. ir hidd..n 'onn~l.
inlrigued and puzzlffi TInbergo>n. How co"ld thl'y IoW' horne so easHy? The
wa,p" p",vid<-d a hint 10. p"'sihle answ.,,-: ...heo a f""",l~ ldt ,,,,r ",..I. par-
licu larly on her firs' flight 01 Ih~ d.y, 'he oft,'n look ofl,lowly and l"opt"d'
"n'r Iht' "'''I. flying back and forth in a""wf I;'V"",incn,,'inll k'Tlj;th and heigh!.
Af!t,r a f..,..' ,.,',,>ods, she .bruplly It1rno>d and zipped "ff in a straighlli"., (10 lho.'
beo>--hunting gn>lIDd', which w""" ah"'" a kilo",ct,'r away). Tmbergen '''''f'''ded
thallhc wa,f" "aclually look in tho; hoatu"" "f tho, hum,w', ,urroundin"" while
....n Evolutiona ry .... pprooch 10 .... nim.1 a..h.vior 7
cin:1ing. bo".." the co" ..red no>st ""Ir........, .nd that by memorizing loc.l land-
marks, such as the sticks.nd grass dumps 'iCatlel'l'd in lilt' ..nd, theyWt'r" abl..
10find lhe "",t .."traJla' upon their ",tum.
Tml>.:rg"" ",.lized that if this hl'f'Olho'siS---lhis possible explanation-was
"'""-'<:t, he oughl tn be able to make il hard lor .fumale 10",locate her Tl<"S1 by
changing th.. local landmarks around the burrow. So h.. waited until ..,m..
l<ma\es left 10hunt lor b..<-s, tho:n c.",fuUy 'w"Pt the .....a .round their burrow
",,1r.!"IC't-"S, moving .way tuft. of grass, p<>bbles, . nd sticks that the bt",wolves
might use tn orient themselves. His tesl showed Ih.l1 he was right about Ihe
wasps' dependence on ,'isual CUt'S. When pr"y-I.d"" females c.me rooming
b.lckck_ I" tht-irne<;1S within the 1>ndmark·free wne, they appeared confused
.nd hovered in mIdair .boul. meter .way lrom their nest mound ocfo", cir-
ding out 10"'pe.1 the appJ'OOCh again and again. Only by dropping their pnry
and ",'arching mo....or I......t r.odom on the ground w...... some f<>males ....""-
toally .bl<:10 find their nesl ""tr.nce.
Tmh<>rgen's simple <'Xperirnef11confi"""" his suspicion thaIbt'\'\Vol,'es !<>m>ed
a ,isual image oflhe a",a imnlt'di.lll"ly around Iht-ir ......Il;, which theyu......-l to
pinpoinl th<' «",..red ""Ir.""", when cnming bo>cl with pnry. But he wanted to
best hili hYJ'<,the;is in another way. 10be Su....thaI I>e had it ril;hl. As he wrot<'in
Curious N"tur<liist, [1137]. "The it'Sl I did "'"XI was again quit<'simpl<:. If a w.sp
u",.,j landmarks it should be p''',ible to do m"....lh.n m......ly d isturb ht>rby
"'n>wing her beacons aU over the place; I ought to be able 10misl...>d h<'r, to mak..
h<-r go 10 the wl'Ol'lS p1a<..... by moving the whoI<' constdlalion of .....r landmark.s
OV<'l" a CO'rtdin disLu"-",,," In othtT words. T~t.., pn,><.lieted th<' """,Ill; ..... """"ld
g,>j fmm. manipul.liVt' exper imrnt if his landmarks hypothesis was correct.
y.,'hton ..... did the experiment, tI<' found 10his delight thai it worked like a d"l111,
Bycamully movIDK an ""ti", ,\'I of kx,allandmark.s around a nesl20 renlifllt"-
1<.... 10tht' "''''11\.;'0'1, ..... induced th<' I't'tuming ltemale to land 20centimet<'N 10
!he southeasl of l>er ",al ",'St mtr.....,... VI'h<-'n h<'~ lhe wasp away. and then
shilted all lh<' "ronway beacons" back 10ItI<'ir ori!,;i...1localion~, lhe bt...woli
circhl a"",nd and ca",,' down ril:hl atl'lt'r ",-,"I entr",""" ,
Tinbt");.", then did yet .notl>er experimrnt, this time 10..... if he could mal..
the wasps train lhemselv<'S to landmarks 1h.>1 1\.;, p",'·idtod. H~ put a ring tlf pi""
con.'S around son1<' n.'Sto whil.. 111<' nt"'! OWnt'," were in,id.. tht'ir bmww,.
v.'ht-n lhe w"'P' Ca"'" "ul, lhey Ioop<.-d b.lckand forth "wr lh.. "''S! a",a b.>fon>
heading off to hunt, but Ilwn carri.>d on as usual. Two d<,ysbter. Tinbt'rg,'"
n>lumC'd to d i'pl..... lhe circl.'S 01pin~ ({'nt'" whi lt' Iht- f""'al", Wt..... tlff hunt-
ing. H~ e~pt",h>d that illht- w"'P' had l"acned tht- ,,'perim..ntallandmarks,
theywould land within the moved circle of pine COnt.'S, rolh", lho",at Iht~r nt"'ls.
Tht' ~'perin",nl worked ju~t a, Tinbt");.", Iht",ght it would (Figure 5) 11136[.
4 Th. __If w..p. (lfttl ......a<ll
with a honey~ thot ~ i. "in9in<J-IRicJl1tI
.... wa.., at ~. ~t .-ntralKO', from whId1 it
ha. removt'd """,;"'} ...n<1.
P'hot"9r.pt>. n." E rd Stroltm,
8 Ch"p'e, 1
5 S.,..'i al lHmlng by a~w",p. Ti~l>o!tg<'t' ~"",h IAI
,imple ""P"fimenIIO ,.., hi, hypot"-,si' ",'" fem.lle t>e<o-. (AI
IN''''''' 1"-,10<..1"'",,,ll.ondm,,,k, "round tIM!;, b«rrows "nd (BI
uS<'d tni.lntor""'l;on '0 find ' "'" "',,' ~~tr"lKe when ''''trming wilh
1>'.... from d i.'''''t hun ling ground•.Aft~, Tin bo<,'1"~ [1136].
'"'
Thus, T1flbt>rgen was ..bit.- to answer a pro~im..k-que;IK>n "bo.>ut h"w hunhnt;
bo.Tw,~"",1':'-1 b.1Ck 10 Iho>ir ",",hi. H" """wed thai lema'" bo.'<'WoiH'S ""'f'<nd t"
ce1ain vi"",1 CU<'S in !he """irorutt.'nl and lh.lt lh...y stotr,·l.tndmark infom"lliolt
for u.", wh.,,, huminI" V.\!h<>ul dis/;o.,.:'lir~ a .ing!<> w"-'P.T.~ wo>s .1>1... lolell
u. ">mo.-Ihing.bout what &"""1Il inskk!h<'S<' cr....tun'S. s<>rn<'thtng ab"ul!h<~r
,..,..,.IIT1{.......nism~mctIun!\ "1x,,,1 lhe p",,,im.ll· caIN'S ,tI bo.>haVKtr.
Ttnbt.>rgen pntdumJ th(s n'Sult by u.ing Itt.> sdt'flhtit: ntt"th<rd. Fin;!, ho.. a.hxl
a q<t<",Iic<t: How do """w,~,'",home to lho.~r ill'S! ''11lr"",:,,? n"", hi: p,,'P'....x1 a
1<,'111. hw ",,,,W''I", a hypOlhesis, "I><"'t th<' (Au-, ... of horn int;: lite bo...'Woi,,'" F"-"''''''''
a visual .ysn·m Ih.lt t;.,IIl<""and stOll'S injormatit>n .b"ul k>cal Llndrnarks in lhe
nesting an", f,tr u.", wht-rt n'tumin,; 10lhe nesl ''11lrar..,... To l<";1 thi. hyp,,!h<'S;s,
I", de"dop<>d logical ""F"-"'tahoru; or predictions lrom it: for "".mpl". ' ''p''ri-
!'tI<'f1tal di.pla<em<.'111 of lhe locailltldmark, """,nd a o<'S1 should cau..... a hom-
in,; I.>mal" I" ,hift her landing
,;1,' accordingly. Thi, predi'1;on, wh•.."I.'SI,..1.
pro\'Cd 10be corn'Ct, which 1:""" T'inb<'t]I:,'11 confid<.'''':l-' !h.>thi. original hyp<>th-
""is was in"."'" riglll. H" tested th<' hyp"lho.'S;. again with lhe pin<' co,.... "'peri-
menl, which ga,,~ him furtho.'I" confidena>!h.>t hi. ~ondusi(1Ilwas {'{'"''Ct .
Thus, w" Cdn di. Sff l lhe pmcedu,," linb<'')';"" joUowed inlO d ",ries " I'.t~l":
1. I I" slar!<xl wilh a c~ u •• l q.....lion abl-'ul a nalural ph<'fIO"",non.
2. H.. d.." ..lop<oJ" hypolh..,.is 10 ~"plain what h" ,",w
3. &""-.'1 on lhe hYF"-,Ih<"i., ho.. g..,,,,,alcoJ p ....diction. '" "xp',.-ttoJ ,,'SuIts.
4. H~ cdrri,x1 oul" 1",1 "I' hi. pnxlicti,ms by Jo;athc.,.ing dCludl ,,",ull. 1m com-
p.tri.on wilh 111<' {'xp<x1ed o""-'S.
An hOlul ionafy App,oach to Animal B~h..;o, 9
[ 5. InSQ doin~ he ",a<:Ju,da sci .... ti fic <ond u.ion about lIN> validity of hi. orig-inal explanation.
Ttnberg.... ·s acceptance of hi. hypoih<'Sis would h.Jv" bt"'" ",,'en stronger if
he had systematically tesled and ""i<'Cted set'fl'al alb'maliye hypoll><>s<>s. "ach
wilh its own mutwlly "xelu.i .." pl'l'dictions. How"v"" eVNIby wo,king with
juslOIl<' possibl" explanalion. he mad" "'al pn'g..... ' in und"rstanding bee--
wolf homing behavior. thank,_ 10!he 'imple, effC'ct:ive k>gic of science.
Tinbe""",,'. """arch is typical of mosl sci""lific inn,'Stig"lions. Indeed. lhe
scientific m<>!hod is familiar <'f\OUgh 10be used intuitively, r"Ih<" lhon formally.
by aU sortso/ prop!<-, not just scit.'I1lists. Lislt'llIt' Click and a.ck, !he"" rnech.In-
io on ~alional Public Radin, a, lhey try 10 figu....out whal i. cau.ing a 19B7
Volvo 'ldlion wagon 10slaU Ul1<'Xf"'Cl<.>dly as ils dri"'-", Bill from Bedford. Ma,...
achusetts. motors down til<> highway. Or "naly'" how air a.-h expt'"" alb.-mpt
10d"lerm;"" whe!h<'r fliKhl 123 Cam<' In an unhappy end becau"",, of a terrori,t
bomb, rIl<'Chanical failu", of a p.>rticular p.u1, or fi", in Ill<> lUI\8"1':" hold
Whal",-"r you can it. tIw scienlific method work., il impn",... our under-
sL'nding 0/ 1Ill'caust'S of thinKS, liy und,-rslanding what i, meant by hyp"lhe--
sis. pr-<'dicti,m, It''l. and ",nelu'ion, we can beller undersldnd how science
w",ks and why ,...rt.in conclusions a", juslifi,'d.
Gulls and Eggshell Removal
I.e!", examine annther e""mple of scil'flCC in aclion.•1", pro"idoxl by Tmbt'r-
gen but this lime from a sludy of Itll' ulli"wlr cau",' of a beha,'int, linh,'q;""
obscrved Ih,'1 bli",k-h<-adoxl gulls a'move bn,k..., ''K!I'hell, I",m their nest,; a
,hort tim" aft"r th"i' young,I".. have hatched . Allhollgh Ihis action might
"",m In"i"I. since it t...... only a few "-",,nds for a ~ullio fly olf and dn'p lhe
eggsl"'l1 a short dislance away from Iho.' ",,,1.1,,, wa, ,till curiou, aboul il. Aliter
.11, whil" Iho.' gull is away fn,m the ,,...,t. ill"a....,; it,; babi... ungUdrded, and lhe
w"rld of black-hroded gulls is full 01 pa'dalors, such as ollll',. laTK<'r gulls lhat
like n"lhing NoII"r lhan 10dirK' on ""wly h"tdIed chick., Then'fo,,', TinNoq;,'I1
suspeclc'd that a'moving ~!\gshdl, mu,t pmvirJ...om.. <on,id.. rabl.. "'pm-
ducti,'" bt'nefit III oil)].an argumenl "'info"'ed by tIN> findinK lhal ""my olher
bird. behave lhe same w.y (Figun:' 6).
In CSSt'11<\'. Tinb~rg.'Il ",",urned that ''''''''' up"n a hm<', bl.-K-k-headed gull' (or
the n"w-exhnct speril'S Iha! eventually evoked inl" black·lll'aded gulls) did
,ratrem",'e~Ils from their Il<'Sts. llIt'l11he first "AA"hell ",mo':,,, aware<!.
perhaps •• a /\'Suit of a g,'11('tic mutah"n that had .om...mall bul particular
6 NeS1in g Ioon>, liile l»a<:k-
_ 9ulI~ remow compiw-
"'" emp'y "99~I< f,,,,,, ,~,
fie,,". PI>o1"9'~P/> by 8tu<~ lyOn.
10 Chapter I
infl""""", un the dev"lopment of the bird's Jlt'n..,>U.~ systt-m. 'The new gene, with
its dL,tindh'" d"velopmental eftect, could not MV" l"'rslst.>d unless the indi-
viduallhal first c.rJit>d it "'l'n>ducro mo", successfully th.n other birds, with
their dift.,..."t g<'<"'" and ...'iOci.>!ed willingn=; 10ig"o",,~l'"in their .....ts,
If the mutant bird's descrndants C\mtinU<'rl to haw mo.... su rviving chicks on
.verage than the o"""typical birds, the mutanl g""'-' would have made ilst>1f
mol'(' and mo", comm,,",, But if the "'l'roductiv" SlK'Cl.,.. of "KlIshell ....movers
was Inwet on ..'erag<> than th.,t of birds that iK"0red <>ggshells, thc mut.tion
"""".,.Iying the new =p<'"'"' w"uld hav" dL<dppean>dfrom the "f"-'Cies b<.:""se
its OWlK'rs would have failed to pass it on,
In oil"", words, linbergen assumed Ih.ll "KlIs(""U ",,,,,,val <.'\'olved beeau...
"f past difte"""... amon!'> II><' .ncestors of bl..ck·headed gulls in lheir b<'h..··
iot and "'l'rodoctiv,' succt'SS. 1he ultimate question he ask<>d WaS, what millht
have caUSl>d the sp,..,ad "f eggshell ....moval in an aJlC<'Stral F"'I"'lalion in which
the ""IM"ior was (Jl'ICt> r......? He could nol!>') baek in time to study what h,'p-
l"'ned in lon!,>'gOO<' ge",.,..tions of black-headed gulls, bul he could determine
whelher Ihe bt'h"'i,,r "!ferro .om" "'producliv...dv.nl.ge lI) the cur",nt
i'I''fl<'r.tion.lf so, he could more plausibly elaim lhat this advantage was resp0n-
sible fo r the cu......nt maint..""",," "f the trait, and also f"C'5Sibly for Ih.- sp",.d
of lhe trail so",.. tim .. in the past. Ifnot, he would know that 0..... p<-...sibke'pla-
Mlion for Ih<· evolution of t>ggShe1l ...."'ov.1 w,lS probably wnmg
linbergen sugg...tro 1h.1t C:AA'~I L~,:,,~I .I>Y ,t<>day'. black·h..aded gulls
mighl be "'l'nxloctivdy .dv.nta8""U> beeause i.t eliminal,>d. visual (Ue that
could giv .. II", nt.'St away t<, certain pn>d.llors . BI.ck-he.ded gulls tlt.'St oul in
the op"", but they-awar to try to hid.. the "''Stin whaten'" vegetation is avail·
able. Mon"'ver. Ih,· color of Iheir eggs and chicks is ,1Il inconspicuous ",otlled
brown and gray. In contra,t. the white Inner part of.n op.'nt'd K"ll eggshell is
highly C\m,pi<'u<"", and it might"''''e as a "",'SI-id",,tifying beacon to carrion
en)WSand other pred"tOrs if nol Il'mo,'ed by a p"n>nt gull.
To k'St the hypota.,.;, trot the eggshell MllO\'allrait w,,-~pa_oJ down thn"');h
g<.'f1<.,..ti<..... ,It bIad-hrodted guUs OCca'-'S<' it /oiledpn"j"t<"", T~ fitsI ,j,>""I·
,>pt'd d pn'diction from the hypothesis: if <"gI,'>hell """mal was an antipn-.:lator
d.,,,",,,, tht'tl Ill<-' pre;ence of brnken~lls shoUldhelp pn'dalors k"",'" k....l
He dlcckt.>d this prt'dicti"n with a simple expcrirn<-rtl. H.. ,1,,1"!iOme intact t>ggS
from "",I>; in a colony of black·h..ad.>d gulls and sca ttered them Ihrough Ih..
sar>d d"""" that ".".,...~ukuly patrolled by t'gg-..ating carrion Cn:>Vl'S. By some
of the unhalea"; ''gp he placro bmkffi eggshells a short distaro: .way; by oth·
ers, he droppt'd the~ILs farther away, 1he intact eggs that we re clotlelt to
white "lQ;'hell bilS w"'" moll' llkely to be found and ",ten by foraging croY>'S lilan
those lilal were Iar!l><'r oway from .n~ll giv.....way rue (Table 2), Sino: this
finding matched the predicted result, TmbergtTl coro;cludedthat eggshell mm·a!
b~' "''Sting black-headed gulls could .... v......·oht'd OCcau"" birds that pp<'f>l"<l
to behave this way l05t f",,'t't offspring I<> preddtt>rs. 1heir surviving offspring
beeam.. the aJlC<'Slors "f t<>day'. gulls, which pos.""" the h...redit.>ryproximate
m<.,<:h.ln~sm,;associated with n:producti,·.. sU<'«'S!> in the pa-'t 111401,
TABLE 2 Effe<t of the p«»:imity 01~gsMIII on "'99 pred..tion by crows
Oi. ",,,,", from
.....g.tiell"' .....g « m)
"
'00
zoo
Soot"", llnb<-'rgffi ( IH O]
E99st~k~
by <row.
"
•
"
Egg<nol ",k~
by~ow.
"',.
""
"
"n
An Evolution.ry Approach 10 Animal B ~havior 11
I'.:ot~ llt.>l.lthough thi. sludy c~ an ~\'olutionary or ulhm~'" q"""'.
lion, Ih~ procedu", for an~wering il did not differ fundamentally from the
method Ttnbt-rwn u-«'d when he .Iudiro the p"nimalt' basis 01homing behav-
Ior in the I:>eewllil
wa,p, He beg.n wilh (1) a qoeshon; \'iny hdd black-head,,,j
gulls l.'\IoIwd a . pecial respon.., to the egg>hells in their nests? He then gener-
al<'<! (2) a 'p«ul.lhw aruMet, a working hypothesi", maybe eggshell ....mew.1
It.>d .pread in lhe p••t beeau"" il help<'<! p"">Jlt gu n, conceal their offsprinl\
from crow. and other \'i.u~lIy hunting predators. Thi. h)'pOth,,,,i. lro logi-
cally 10 (3) a p,,-diction; he expectro 10 ..... crow. and other pT't.'d31ors o.ing the
con.picuous roe' oft~red by broken eggsh..lls tll n.rrow th~ir ...arch for
unof"""'d o.>ggs. Tmbergrn th<.>Jl (4) checked the p.-..drlion, using an expo.Timent
to find out what crows aclually did. By matching the ['('Sulting d~la a l\ainsl
the pl'l'dictro llU!<X""",,, he leIted his hypoti....~is and ",ached (5) a scientific ron-
dusion. nam.,]y, that his original explanation was prd>dbly right.
DarwinianTheory and Ultimate Hypotheses
""'hen Tmbo.'rg.,n <k",.'k,p•.,j his hypothl,"Lsabout the ."'ol\"ro function ofegg"",n
o"lllO"ai. he was s-rronglyinlI~ bv I"kllural ""k'cti<1I\ theory, which It.>. de!<.'r-
mined the w.y biologists haw explo'""j ultim.1t' qo'-"Stions since 1859, In that
year, Charlei o"rwin (Figu", 7) publL<J>ed 0.. In"Ongin 0/5~ !2I:!61. w ith it>;
sw.rying ,"plal"kllion of how l.'\Iolutinndry change might ,-.;nIl' within sJ-"-'ci....
o..",;n'. gn.-.>t idea re;1S on ttwe commonly ,lbs..,''-.,.] fealul'l'S of llvtng things:
1. Variation: Ml'mb<'rs of a sp«ie!I diff~r in thei r ch,"<lCI<'rlshcs (Figure II ).
2. Hert'dity: I'a"",ls p.,s on some 01 their dislincti"" ch~tactt.,ti,lks tn their
oH'prinl\'
3. DifI~rt'nti~1 ",production : Ik<:dU".. o f th<.irdi,tinctive inh<.'ritedcM rael.'r-
i,tics. some individuals within a p"pul.linn ha,'e mo ....su",iYinll offspring
than others.
h ........ Darwin"lIenius to po.'TCrive that '-"\-'oluti",,,,ry change is ;>u't~t"blfwhen
tht'!IC tho... cond i!it"" """'" in a~ (Fillure 'I). [f ",me blad.-headro gulls,
lor insla""'-', pnldOCO' """'-'<:>&.pring th.,n ,>tht-rs, and if their .>dulloffspring haw
inherilro the trdil (such as ..j;j\s....U",moval ) that ad vdncM thei r sucres~fu l
reproduction. tllt>Jl th<_ offspring will also spread thai reproductio,,",:nhancing
trait . n.... other side 01 lhe coin is equally clea r. If ,,>me gu lls I"ave I"wer off-
Sprinl': than others becaliS<' of their i".... ritt..,j chard<:te rislks, tho", of their off-
7 (harlos Da rw in, ,horIfy aft.. ,,",urll-
iog from hi. a,ound-_ ..,.,._ on
~ 8N9"" bef",~ he wrot~ On 111<' O<i9jo
"'~. Copyright SC"""'e Photo l il>r..Y·
8 A va,iabt~ ,~io>,. Thr ladybird _io> ,*,_oqr;diI ..hibrt, hoted iUlry
va""'",,, io in colorpan~rn, Photog'"i>/'>' by MikeMajHo.
12 Chapt er 1
Percentageof
Generation
•
populat ion
it,
-, ., ,:_J • -I
..
".- , -.~ ~ 50% 50%I , I , , , , I
I >:: >:: >::
I A >:: , II I , -, I \ I
I I , , I \ I
2 ••
-,
• •-I
., 40% 60%
--I I I I I
- 0 I - -
I >:: I >::
I
>:: 1+' I >::I A , I, -, I \ , I ,I , -, I \ , , I
t 'j
-" "" '/
•it •·iI
,..... (
• •3
. 0
•• 30% 70%I I I .. , I .- 0
>:: >:: I
I
>::
I
>::
I Ii,I A I ,, -, I \
'cI r
, , I I \
, I ,
i ' ~i • ·f •
y':'.;r i-, ~i4 ~ - • 10% 90%I I ~ I - " I>:: I I >:: I ~, >::, ,I \ I , I
I \ I , I I -, I
•jf•il iI
, .:.( j \-'
-.j5 'j; .0 . 0% 100%
.-
.0
9 Natural select ion. If the differencesin the color patterns of ladybird beetles are
hereditary,and if one type of beetle leavesmore surviving offspring on average than the
other, then the population will evolve, becoming more and more dominated by the
reproductively successful type.
spring that do surv ive may inherit their disad vantage and thus may also leave
relatively few surviving progeny. As a resu lt, traits that compromise lifetime
reproductive success will beco me progressively rarer over evolutionary time.
Darwin called this process natural selection beca use he saw the elimina tion
of traits unfa vorable to reproduction and the spread of beneficial ones as the
natural consequence of hereditary variation. Thus, Darwinian logic lead s us
to expect that evolutionary change will always be in the direction that promotes
successful reproduction by individu als.
Darwin developed the theory of natural selection before critical discoveries
abou t the nature of heredity had bee n mad e. Genes are now kn own to be
nucleic acids that faith fully encode the information need ed for the synthesis
of proteins, which are critically impo rtant for all living thin gs. Genes can be
cop ied and transmitted to offspring. When this happens, the genes themselves
are essentially reproducing, with the organism merely acting as a mortal vehi-
cle for the process. The modern evolutionary approac h to animal behavior
applies Darwinian logic at the genetic level in much the same way that Dar-
win applied it to individuals:
1. Genetic variation: Genes can occur in more than one form . When alterna -
tive form s, or alleles, of a gene exist, the different alleles ma y lead to the
production of slightly d ifferent forms of the same protein.
2. Heredity: Alleles can be transmitted from parent to offspring.
3. Differential reproduction: Some alleles are better than others at producing
effects that cause their bea rers to tran smit copies of their alleles to subse-
quent generations.
If these three conditions apply, then alleles that help make indi viduals more
reproductively successful will become more common in the population over
evo lutionary time. Other alleles that confer less reproductive success on indi-
An Evolut iona ry Approac h to Animal Behavior 13
viduals w ill eventually disappear. (We ass ume that populations cann ot gro w
exponentia lly forever, so that only a limited number of copies of a gene can exist
at anyone time .) The logical conclusion is that selection on individuals will
favor alleles that help bu ild bodies that are unusually good at promoting the
propagation of those alleles-or, as E. O. Wilson puts it, a chicken is really the
way that chicken genes make more copies of themselves [1247].
Darwinian Logic and the Study of Behavior
No ma tter how the logic of natural selection is presented , it is a blockbuster
of an idea. It means that humans and all other living things have been shaped
by pas t selection favor ing characteristics that enhanced their ability to repro-
duce and pass on their genes. An understanding of this point helps us identify
questions worth asking while at the same time shap ing the kinds of hypothe-
ses that we will test.
Let's illustrate the utili ty of Darwinian theory with the case of infanticide in
hanum an langurs . These graceful primates live in band s, which often consist
of one large, reproductively active male and a group of smaller adult females
and their offspring (Figure 10). From time to time, the resident male is pushed
ou t of the grou p by a newcomer, usually after a series of violent clashes. After
such a takeover, infants tend to die. Although the cause of death is often unclear,
the new male is the prime suspect in many instances, and males have been seen
doing the deed several times (Figure 11) [132, 553].
The phenomenon of infanticide is precisely the sort of thing likely to attract
the attention of a Darwinian biologist. Why should a male langur that has just
spent days in a dangerous mooing battle with the previous resident male tum
on the offspring of the very females he has finally succeeded in joining? Hav-
ing avoided incapacitating injury during his battles with the rival male, why
should he now take the chance of being bitten by one of his female companions
as she attempts to protect an infant (Figure 11)? In other words, how can infan-
ticide possibly adva nce a male's reproduc tive success?This question jumps out
at an evolutionary biologist precisely because infanticide by male langurs seems
so unlikely to have evolved
by natural selection.
Indeed, the behavior might not be an evolved trai t that contributes to the
male's reproductive success, but could instead be a social pathology brought
on by overcrowd ing. Under high-density conditions, males may encounter each
lOA band of hanuman langur
females and their offspring. Males
fight to monopolize sexual access to the
females in groupslike this.Photograph
byS.Nagendra.
11 Male Infanticide In hanuman langun. Ileft) Aninfant·killing male
langu' fI.., Imm a f=\ale belonging,o,~ bond~ i, a"emp'ing '0 pn.
lR;ghtl Anur<ing boby langu, 'M' ha, bH'I1 paralyred by 0 bi~ to t~ >pi""
(note ,he open ....,.,ndl by a male langu',Thi, Infan, wa..tu<ked ,..p"' ed-
l)'~ a period oI ......k~ Io<lngan eye ond ~nall)' n. li~ at age 1a months,
Photographs by (~I Yolk... Som"",", Item 5<>m""," [t08S1. and lright)
c.,oja 8oI'1iM.
utM so "fum th.ll fightin); bc<:om<'S romn,onplace, with the hypt',..U"""-.ive
m""-'s then ..""'ulting nut ju.,t ri.... ", 0/ tho: ....,Tl<' S<' ~. but al", f,malt'Sand thcir
offsprin);. In fa,:l,. this J"tO<"O'VoIutimary hypc.>tho.-;is wds the fiN e~planati<.. ,>ff<",'<l
by "'''''''' Iangur w,TtCh<'fS. who knew thdll.mgurs w",", often fed by Indian vil-
lagers, and "', pt'rhaps had """,h<'<l unnatur.lly high population dmsiti,'S.
But .mother stud.....t of Ian"","" Sa,ah Ifniy. beli<,,,-'ed that inf.nticid.1 bl."""v·
ior by m.lIt'S might haw e..olvM by Mtural selt'Clion, dc"Spile ils .pp....nl
lack uf r.,pnxluctive value for the nMI... She Ihollghllha l pt'rhaps infanlicide
sr"",d as .. ,,",ult of ,",'lIdl cumprlitiun betwwn malt'S (553). A kill,.,.mal<'might
inc"'.",, his "'Productive sUCC<'Ss if he elimina!t'd nUr"ling offspring sired by.
ri,'al maie.l,'aving the mothers of thO&<' inf.nts no olher option than to m..te
with him and ha,'. his babies.
For ,he mom,,,,!. Ict's not wo'ry aboul whether Hnty's hypulh<",is is ril'>ht OT
W""');. n... point i5 thdl her poI,,,,tial an,w"," to the pUZ.l1e of infa"ticide d,'fi\'t."
In"" Oa".,inian th<~,ry_ Hrdy's ,u~('Sh"n is lhat m.IN tend 10 gain d<'5Cen·
danls lhrough the selective p'''''tice uf in/anti<id,,_lhis ,u~<",';""d;ff~rsnln-
d~m,"'t.lJy from the 50Cial p"thology hypolhesi" which 'lates thai killer malt.,;
l'O"in "uthinl'> from inl,lntidde bc<:au,"" il is an abnormal beha\'iur induced by
unnatural mnditi{~1S.
The Problem with Group Selection
Darwini..n hypt,th""" aoout infanticide al"" diff.. r from ""me other ""plana-
lions for the betl<w ior, 5uch a5 th<' possibilil) ' thai kille, malt.. e""h'ed in order
tu Iwlp pn'''Cnl ov~rpopulation.which would d<'5troy the food ",""un.",; that
Hanuman lan""c !>amI,"""";tu ,uIViw. AlthuuKh this hyp"t""'i, al"" daim.
th.lllhe behi,.,ior sp"-'ad in Ihe pa.t beG!use of C<'r1ain beno>ftcial conseql1l'llC"",
it i. not baS<'<! on Dan.;inian II"-'ory, H~", tl1<.' b..'I\Cfi~iacy of irdanlicid... is nol
the mal,' lh.lt kills infants. bul th,' K"'uP to which ht, ['d"ngs. 11,.."""Iution-
ary mechanism 1M the sp""d of inf.nticide l. fl,>I direct ly link..u to diffe"-"'<-....
amoctg individuals in lheir "'productive .u<U'S. (as in DarwiniMl ""lural ",1",,-
lion). bul catht'r to diflt·n'flC1.'" among "coups in tht'i' .u,,·i,'.l via a p""''''''
lhat ha' M'n "lied grou p .....1"(li" n.
Group sel""lion throry l't'C<'ivl'd its form.1 p",sent.lion in An"",,1 Disper-
sion ilt Relatiolt 10 Social B"hat'i"", lJ2&>J. wrill...n in 1%2 by V. C Wynn~­
Edwards. Acrordi~ to Wyn",~F.dward,. only lh<_ K'''UPS "r ,pt'<'it", lhal P"'-
.........d pupulation-n;gu~,tinK nu.-ch.nLsntS muld ha"e sUl....iwd 10Ihe p"""nl;
others thall.cW these ll1C<:h.'nisms had su",ly Ix-rom<' ~xtinc t through ov~,­
~xploi L, tion of the critical ",""um" on whid1lh~y d,'f"""l''Cl. With groups rom-
petinK unconsci"u,ly 10 ,uIVi"e. only thos" popu lations wh"... members
reduC1.'d their "'P,oductive output would be likely to persi.t. Thus. Wynne-
Edw. rds argued that ...volutiOfMI)· chang\' "'gularl)' occurs because of diff...,.·
""""" am<"'K gn"'ps in tf>t,ir p'",_iun of sclf-sarnficing indi" iduaLs. which in
tum .ffed the .urvival ch.'Incesof tho> group,
This argum<'lll w.s chall~nged in 1%6 by G. C. Williams in Ad<lplal"", a>lli
....'oI"'alStIKtw., [123'1]. YI'illiam.showed th.,t tho> .urvival of all''TThlli'·.... U...I""
was much mo", lik...ly to be d<'lermiJt<>d by ",p",ducti"... di ffe""'",", among
gl'f\<'Iically different individuals than by survi"al diffe",nc... among groups.
The ba.is fo, lhis claim c.n lie iliustrat<'Cl wilh ref<'"""", to 1.mgurs.Irn.'gine th.lt
in tf>t, pa't th,·", ,,'ally were mal ... langu,"" p"-pa....u to ri,k lI",ir own li,',,,, by
killing infants in ord ...r to ....uu,,", tho>ir population for tho> long-te-rm henefit of
th<.'ir group, In such a ca..... group selection. as ddined by Wynn<.>-Edw,ud•.
w,,,,ld be said to fa"or It", allel'i.) f"r mal... inf.,nlicid,· N'(ausellie gm"p as a
whole would bent."lit Imm tho> n'TTlm'al ,>I exress inlants
However, in a p''Pul.t;on of I.ngurs with some gmup-beneliting in!'",t;-
cid,,1 m"I,..., Darwinian natural ",,1,'Clion would also be at work. SUPf"""" thai
lho.",· w,·,,' lwo g''flt'tically di'linct In''-''' "f mai,'S: the infanticidal type. which
spent its """rgy .nd .ometim.... ,hort<'fl<'tl its lile for the K,,,0<1 <>f the group,
and a",'lh,·, lype. which lived long," and Il'produced mo", becaUS<' it let the
inf.nticid.,l m.I"" 'any the burd"" "I p"pulalion ,,'Cluclion, Wliich of th,,,,,
two tyf"'" w"uld c"n,titule mo", of the next gen~ralinn? VVh,,",, hereditary
m.t....ial would he tran,mill..u beller to following g"""ration.? \-Vhal would
h.'ppen ow, ~"ol"hon"rytim~ to int.",ticidall''fId~nd... in our hypoth~tical
p"pulali,m "f langu,,"'
'The g"" ,,>ral p<.)int that Williams made L' lhat sd",;ti"n atting ,m diff,''''flC''';
among variant individuals within a population will usu.lIy have a much
,lrlmg'" ""oluhona,,' t'itwllhan sel,'(li"n a<tins on diff,'fl.·nc,'S am"ns <'TIlin·
gmups, F" c- lho.- g,,,o<I..,f-tht-gmup ""!ection can ,«ur. pn",id,'tl that K""'f"i
Il.'lain their inn.'gl"ily It" l"ng periods and diff.... in lheir !\enetit mn'titution in
w.y. lh.ll aff,'Cllho.~r ,urv;,'a! chaTlC1.'S, But il )';roup.('[eclio" fav"," • trait. such
a, rep,oductive ... If-sacrifice. while natu,al "ok'di<,.,""Is al\<,in.t it.natural ,,·It'('-
tio" _m. likdy ti.' win. a. we have ju,t ""'" in our hYp<.'thetitall.ngur exam-
pic, Although """,.,reh rontinues on forms of group sel,'(lion mo", compl~x
lhan lht' It"-lhe-g<,,>d...,f.tho.-gwup type P"'P'''''''Cl by W}'nn~+dwa,d, [HlM":,-
1245],.lm,,,t.1I b...h.v;oral bi"I"gi,ts ha"e ..........ti.'tl It-", arp;um,."ts "f William,
R..'S<.'an:h...rs now ca,dully distinguish b.."''''-'''ll group benefit .nd indiv;du.,i
(or )';''fle) b..'fIdit Iin",th,....". The oYl',vdwlming majorily "f sci''fIlio;t. ,tudy-
ing the <,,",,luti,,nof animal behavior empl"y Darwinian l""'''y, 'atilt" than
group ... lectio" theory in any of its I<,rms,
How,'",'T. w",c"uld. if w" w;,ho.'Cl. I,,,,t non-Darwinian hypotho.'S<'S using the
I,,!\ic of tf>t, srientilic rn'·I....0<1 that w.. illu.lr.hod. wilh Tinbe"'"t'fl', ,tudi.....The
16 Ch.pt.., 1
soci~1 pothology hypoth<-sis (whi<:h is neithe, o..rwin~lnno, ,,"-"'p S<'1<'ction-
i,t) And the population "'K"I~tion hypotho,,,,is (which is b""..'<! on th" tho,,,,')'· of
group selection) fo, inf.nli<:ide by m.le langu,", bull> hdpp"" tn yit'ld tllt' "''''''
M-y pl'<'die!ion. II high popul.tlon d~>fI.sity ",.Uy de.." causo> .bnorm.l beh.l,' -
iOT (tho.' ",,,,ial pathology hypotho,,,,is), or If it truly th"'~t,.,,s11x' su,...I...~1 of kln-
gur grnuF"'.nd so ""li ....tes ""U-..,,"';IIOng Inf.nh<ide (tho.. p'>pUlat~", "y;ul<>_
lion hypothesi.), then w" would " 'p'-'CI to s.'" inf.nticide by m.IL","lilly in a"""
in which H.mum~n klngur popul.,hons ,1'" ~bnormany or unusu.lly hi#>.Con-
tr.ry to this pR-'dldi."" inl.nlicid.· "'K"Ia,ly 'lC<"U'" In 1n"P' hing ~I m(".:kr-
.te or evro low denslli... in ""tu'.I ....... who",... they.... nol ftod by F"-">ple [l3L
83'11. this finding we.k<'IlSour ronfidt>tlC<' in both the ..>..i.ll F"thnlogy and F''l'-
ul~li,m n,gulation hypothcst.. lor lan>;u, Inf.nticide.
Testing Alternative Hypotheses
Dot'Sour l<",t.live rt-'jection 01tht-'se two noIl-D.rwini.n hYF"theses mt.'.n rh.ll
Hrdy's D. , w in la n "'pl.,,,,,tinn is co,,,,,.:1! C!<'<1tly not. First, studi 01 l.n-
gurs living und..,. pH'sumahly natural conditions an' ra"" '" the id.'nce
.f:dinsllht' social p;llhology and F'>puIaht'll rt')!;Ulati,'ll I>ypnrn."",. LS nol ultt.,.ly
compelling. s..",ond, Hrdy's inc...."'-'CI reproductive opportunlti<'S hYP',tl'Iesi,
is not th,· ('Illy possible ,"pl.n.IIon b.,sed.", D.lrwinian Ih,,,,ry. For e.ample.
pe,h.ps m.l... com mil Inlanti"Id.. alit', t~k",,,,.. rs in o,d." to e~nnibahz.,
Infants, thercl>y ...plroi.hing lhei, .kpleted eneq:y """'t\',,". If '0, kill", m.l...
could d."i h.mdits from their actions th.t could.....,p them .live to rep'''-
due" mo than mal,.. Ih.t did not kill and eOflSUm" youn)\st.,r.; wh<.., takin)\
ove,. bdnd. In orde, to ,,,,,1 .~mlide"l thai we hd,'e idt'Tttified lhe tru.. ultim~lt·
c.use of inf.nticide, w.. will have to tesl .ltemali..." hYF'll~ In ways IhAl
he lp u, ",j<'''1 th" inco,l'l'ct on....nd I'l-'t.In the ,il';ht "'pl.nation. if w" ""V"
included il in our 11,1_
I-Vt",n it com"" to hYF'>II\O-'-;i.. tt.."ting, nute tMI mo", than 0"" ..,planallon
can I".ld to the So,m.' pR-'didion••s w" just saw wI><." " ...mining the two non-
l1:uwinian hyp<lIht..... on infanticid<.-_ lI"th.,f th<.....hyp<-.tIl<'S<'S yi.-ldO'd the same
ptedkli,~" infanticide shtmld be limited III high-denslty populations. II we had
found th.lt m.les killed inlAnts only wh<'n F'>puIallo.... hAd ir":n"...,.j markedly,
we would not h.we b<...,n abl" to ,,,,e"pt one 01 lilt.' hypolheses .nd l'l'jecl Ill<'
olher. lik.. ,.,i....,. ou' two l)",wlnlao .It.·.....,tin.... the- incn'a...>d n"Pmduehw
opporlunili<'S hYF'>thffiis and the cannibdli.-.mhyputl><'Si>;. both pnoduce lht' p....
diction thdl infanticid" by mal... willlt'lld 10{",cur snn" aft.. , la k",,,,.,,,; . In the
first co",', w.' ("F"'d m.I.'S to 'lop killinl'; inbnts by the lim,,!ho.' first of tht>i,
own olkpring are born to tllt' ,..."id..o! It'malt",_ In th., ",,,,,,,od c~",.. we e~pt'Ct
males 10Slop killiog inlAnts .Ilt>r they ha"" .....m·.......t fmm thei, ..,...·'W'ti(.lIy
,"'p.,n..";"" t"k<'""",. 11><' f"'-1lhiu infantidd<' is ind",_'<! limlh'd ,,, the p'-'rMJd ""'ll
altt· , • 1.>"'..",." d,,,,. nol hdp us dt'Cid,' b.'tw......, 11x"" two alt,'m.lti""S.
If lhe cannibdlism hypnth...is is true, hnw"""r. w.. should .om..lim........
m.ll" n"wrom,'rs ",'linl';.n inf.nt. No such n'Cords ..,isl. bu l n>member th.1
obs.. t\·,ti,,,,,, of mal.", in the ilCl 0/ inf.nticid,- .... ra .... Bul if w.' could d"mon-
slr.te thal 1eLm~ do not C(1.""um,, dead infant>;. w,' would <-.t>v~,."lyn;'u
tl><' ad.'pti c<mnibdlism 'rgum""l.
n", incn'• ...-.d n"Produetive opporlunili<'S hYF'>thfsis pnJdu..... v.noo, p.....-
dieht'll', Including (1) all.>ekl"" mak'S should not kill Ib.'i, own ofkp,ing, and
(2) h:>mal... d'''Prived 0/ lhel, ynung Inl.nts ,J,ould mate with lhe "f''Y mal..,.
thilt klll.'<I tl><'i, offspring. ,,-'Sultinj\ in lhe production of new inl.nts sited by
tilt.' kill." m.k... lht.,." p,,->dletlons ha,'" """,ived ronfirm"tl"fl from v.rious
"'U"'eS [553, 10ll5[. ftl' ,,~ampl.., a ,...,..,.",her obse,,,;ng. lan>;ut group of
S('\'",.l adult mal.'S Ii"inl'; togdhe, with ...ariou. I"mal•., 1\'Ct>rd,'<! 16 c."", ,>f
J
An Evolutionary Approac h to Animal Beha vior 17
infanticide for which DNA samples had been collected from the presumptive
killer male and his victim. In every ins tance the killer was not the father of the
deceased infant. Moreover, females who had lost their young promptly regained
their sexual receptivity, and the new infants that resulted were fathered by the
infanticidal males, as was again demonstra ted conclusively through DNA test-
ing [132]. These findings provide strong suppo rt for the increased rep roductive
opportunities hypothesis for infanticide.
But the more tests , the better. If natural selection has produced ma le Hanu-
man langu rs that kill infants to gain more rap id sexua l access to females, then
we would expect to observe infanticide in many other species whose social sys-
tems resemble tha t of the langur. This pred iction has now been confirmed
through studies of various other animals in which newcomer males replace pre-
vious residen t ma les, kill infants fathered by those ma les, and then mate with
the females that have lost their youngsters [344]. In lions, for example, infanti-
cide often occurs when a new gro up of males ousts the males from a pride con-
taining a number of females w ith young cubs [943]. The incoming males hunt
down cubs less than 9 months old and try to kill them (Figure 12), although,
like female langurs, lionesses try (some times successfully) to protect their cubs.
Lionesses that keep their cubs alive give birth at 2-year intervals, but fema les
whose babies are killed resume sexua l cycling at once and mate w ith the killers
of thei r offspring . Since a ma le can expec t to remain in a pride and have access
to its fema les for just 2 years on average, the reproductive benefits of infanti-
cide from the male's perspective are evident. Indeed, male lions probably kill
a quarter of all the cubs that die in their first year in some populations [943].
The observation tha t lions and other anima ls commit infanticide under cer-
tain predictable cond itions supports the increased reproductive opportunities
hypothesis. If these conditions favor the evolution of male infan ticide, then
we can predic t that infanticide sho uld be practiced by females of those unusual
species in which sexual access to males limits female reproductive success . This
prediction has been confirmed for a giant waterbug whose ma les take care of
egg masses (Figure 13), which are sometimes attacked by egg-stabbing females.
12 Infanticide by a male lion. The male carriesa cub he has killed after displacing
theadult males that once lived with the pride.Photograph by George Schaller.
18 Chap ter 1
13 Protect ion against infanticide.
This male waterbug guardsa clutch of
eggsagainst infanticidal females that may
destroy hiscurrent clutch in order to
replace these eggswith their own.
Photograph by Bob Smith.
After the destruction of a clutch of eggs, the male associates with the infantici-
dal female , mates with her, and cares for a new brood of her eggs [5631.
Likewise, in a water bird called the jacana, males provide exclusive care of
the eggs and young. Territory-defending females sometimes attack the chicks
of neighboring females, forcing the brooding male to abandon these offspring.
He may then mate with and accept a new clutch of eggs from the infanticidal
female. When researchers experimentally removed some territorial females,
neighboring females quickly invaded the vacated territories and, in three of four
cases, either killed the baby jacanas there or forced them to flee. Within 48 hours
the males that had lost their offspring were involved in sexual liaisons with the
infanticidal females. By committing infanti cide, these fem ales had gained care-
takers for their eggs soo ne r than if they had waited for the ma les to finish
rear ing their current brood s [361].
Certainty and Science
You must have deduced from my summary of research on infanticide that I
think the increased reproductive opportunities hypothesis applies to langurs
and lions, wa terbugs and jacanas. I do-but I could be wrong, and indeed,
some other researchers believe that langurs and some other animals do not
commit infanticide in ways that increase their reproductive success [75, 118,
276]. These disagreements remind us that all scientific conclusions
must be con-
sidered tentative to some degree. In the past, majority opinion has changed
dramatically when a previously unconsidered hypothesis came along or new
data surfaced that destroyed an established hypothesis. When I was student
at Amherst Co llege, my pa leontology professor convinced me that the earth's
continents have always been w he re they are now located . However, as new
evidence came in, everyone, including me, abandoned the old view; now the
genera lly accepted hyp oth esis is that the continents "float" aro und the planet
on moveable plates.
The rejection of established wisdom happens all the time in science. Scien-
tists tend to be a skeptical lot, perhaps because special rewards go to those who
can show tha t previously published conclusions are incorrect. Researchers con-
stantly criticize their colleagues' ideas, in good humor or otherwise, sometimes
causing their fellow scientists to change their minds. The uncertainty about
Truth tha t scientists accept, at least when talking about other people's ideas,
often makes nonscientists nervous, in part because scientific results are usually
presented to the p ublic as if they were written in stone. But anyone who has
taken a look at the his tory of any scientific endeavor will learn that new ideas
con tinua lly surface and old ones are regularly replaced or modified. I repeat,
complete certainty is never achieved in science. The strength of science stems
from the willingness of at least some scien tists to consider new ideas and to test
hyp otheses repeatedl y.
I hope that you will keep this point in mind as we review the find ings of sci-
entis ts and their in terpre tations of evi dence in the cha pters ahead. We will
first examine the proximate and ultimate aspects of bird singing behavior (Chap-
ter 2) before lookin g more closely at the different components of a proximate
ana lysis of behavior (in Chapters 3 through 6). Then we tum to ultimate ques-
tions about evolutionary history and adaptive value (in Chapters 7 through 14).
The book concludes with a chapter on the evolution of human behavior. Thanks
to the small array of behavioral researchers that have attacked these ques-
tions, there is much to say on these topics, so let's get started.
An Evolutionary Appfoach 10 Animal Behavior 19
Summary
1. Ilos;c qu""ti<>n" ."'lUl animal beh.l.vior faU inlo two cal"l'ori"". "How qll<."S-
tinns" ..-qu ire answ..". abou l l~ proX'"",lf causes 01behavior. how do
lIffit'h<'-de"elol'"",nldl and ""n""y-molm m"d"",isms ca""" an indi"idual
to ",'haw' ' Why qU'''I;onS" ""lui ... ,U"Mers aboullho> ulti"",l r cau_ of
behaviOTc Why hav" c..-rta in g"""" and certain pro,imal" m",l"misms p<'r-
sisled 10 lho> p""""l, and why has evolution followed one palh insl.ad of
anot~r?
2. /loth pro'imdle and ulhmate qU""hons can be in"""hgated scienhficaJJy lul -
low;ng tI",,,,, sl't'"
I , We beg;n wilh a cau",,1qu""ti"" aboul whal cau",," """e1rung to
happm.
2 We d,,,,i,,, a working hypothesi,. or p<,,;sible an,ww to I~ qu,..ti"".
3, We p!'>'dicl what we ",pect 10oo..,rw in natun' if lho> hypot""'is is
I",.'.
4, After de,-eloping 'lUr p!'>'dicti,,"(s), we rolled lho> appropriate da la and
1eS1\he hypothesis by malchinl': tho> actual .....uhs al':ai"'llho> "'F""'tt'd
,~
5, We ",,,,,h a ",-i""hfi, conelu,"o" based on \he ",suit> of our lesl , rejecl-
ing hypolh..""" ill~ir p,-",;lichon(s) fail to be upbeld, and wnlaliwly
""",piing hyp<llh..'St'S who"" 1t,,1, a.., positi,'e.
). Chari... Darwin pn:>p<-.,;ed thai evoluti""ary change occurs by nalural ...' I.'C-
tion . Ac"Ctmling to Darwin's theory, if. species contai", ge""ticaU y diff""",t
indi,'idual. w!>."", particular characteristics cau,", them to haw diffe"'lll
numbers 01suni iving off'pring.. I~ typ<.. tlLlt "1'roduce mosl . u"","",fully
will become m"", numertl"" in .ub,.,quent g""""ation...
4. V. C. Wynn<-Edwa"h p"'P'".,d a Iheory 01 grnop selectio", arguing Ihal
""olulionary cha""" will occur if ,,"""'icaUy diff"renl groups vary in how
w"lIl!>.'y . urv i"" beeau,,-' ,lI g,·nctic differt,""", am",,1': th<m.
S, The theory lbala """,arc!>.'r u.... afft'CtS 1M kinds of hypoth<..... he or she is
li .~ly to pn'P'''''' and lest. V",'rs '" Darwinion Ih.."ry produce bypt>th........ on
how traits mil(hl prt>lI\Ote \he "m'i.. ,l 01 the g""'-""of indi,-idoal. with tht,..,
tr.its; U'l<'''' <lgroop ,,-'kdion theory prod""" hypt>th........ on !>._ tr.ilS mighl
adva""" \he .un.-i"" l 01 the gn'up '" spt-'Cit's 10 which \he indi,idual bt-l'>ngs
6. Toda}' olmost an bi'havirn-al biolog;sls U'l<' Darwin;,ln rath<'r lhan group
,.,It.'CIit>o theory as th~ loundation ft" their hypotl..".,. because ""lectio" ot
tho> k'vel of ind ivid uals should be" mo... powerful forre for evolulionary
change than S<'lection al the It,,-.,} of groups. A Ir.il la..o,-",;l by group ",!<'c-
tion mil(hll...d individuals 10 """rili<~ th,,;r "'Pn:.:fueti,-~ s""""," I".. lhe
good of the gn",p . If otho>. members of the group haw altt'l"l1dtiw traits lhal
be""r pmpagate Iheir different genes, theMe genes win "'pl.c..- the """" lhat
are beinl': s.\Crifit..-d for the be""fit of the gn"'p,
7_ The beauty 01 ""ier-.ce li", in its logical approach 10 testing altemati""
hypoth......... wh..'th..'r proximale or ultillLlle. who>ther based on theory X or
lhet"y Y. p.,"'>n' who uS<' lhe appn"",h can eliminale expianalion.< thai f.il
their ,,",ts whi l" ac«,pling other hyptltheses that have passed their ","hi,
Discussion Questions
1. VI'llydo hwnans ",I '" much candy and drink ..., many solt drinks? \'ihich of
\he follow;"" expl"""tion.s are pro'imate hypoth~_.and which dIe ultimate
h~?
•
20 [~apl~r 1
\
,
, a, Candy and oofl drinh axllain .ugar. which 100,1>" ,.w_1o p"'>ple.
, h Sw_ ta.le i. rememhe~as g<X>d;!he "'emory of pleasure lead.
f""'ple 10 ealDT drink m",,, 01 the same,
~ c. Sugar. which is p""""l in candy and sofl drink.." i. an mergy source
lhal helps k""l' P"''Ple ali"e
oJ d, Our primal<: a""",I,,,,, d"l""'ded on sugar-rich fruil.; In"" lhe!;<,
anceslo"" we ha"e inheriled Ihe same kind " I tasle p"rcept ions lhal
theyhad .
~ e. The genctk ;nlmmatio" in ",,' bodi....har<'" the de,'el'>pmenl of
ncry" cell . thai provide perc'1'liorn of .w"""""'" and pwa.ure,
\i f. In the past. Ih""'" individual. who liked sugar lell more d......,ndanl>
than lh""" who were indilf"renl to , w<,,' I_ta.hng Itx>ds,
r g, The sensory inpul lrom la.le ",",cpl"'" in lhe I''''llue 10 ...I<>clro hrain
<",II, leads 10 a posilively rcin/"tting """"'lion of sw,..-In<""
2. Lemmings an> small ,""""",li ke ,,>d...,ts m..llive in !he Arcl:ic lundra, n."y a",
""""'n /or e,tremt· fiucludlions in pupu1l.l:ioo .;,,,, Al high population d"""'·
lit",. Lnge number,; leave lheir homes 10Irawllong di.'I....,.... In !he course of
!heir ~>O .....'Y. many dil,. "'''''' hy dmwning •• !hey ' l!<"mpl ~, ,wim aer".s
1" 1«,,, and riWfS. One widely circulal<'d ""plan.atinol lm \heir behavior ;s that lhe
lra.'e1er.; are atlt-mpling to romm;t suicide to rel"""pn-ssure on their populo-
lion. II """"" die , lhe survivo,.,. will have ...>mt'lhing wit It, ...\. Whal theory is
the founda tion lor IhiJl h)'p<>thesis? ....1>01 would G. C. William; have 10say
. ht>ot it? How w""ld 1>0.' us,' Gary La,.,.",', cartoon (helow ) 10make his poin t?
c~~a~t,v
" l~ ,
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~9 '"=,"-==-~
3. In Daw.on '. burrowinjl; bet>, males rome in two . izes, 1a'8" and """It w;th no
inlt."m....liat"" [121, The large males fight 10 mate with ';rgin /crnales in plares
where \he lemalesan>~ from undergn>ond nest> (..... p, 21 , photograph
by \he author). The small males avoid fighting.. and in5k'ad patml 11<"'......,
whe... IJ,.,.' somctimeo find and ""f'IJlate with km.1.1es that have~edwilh·
ou t mahrlg, .....hy do small rna" Dow.",,', bu",'wing bees ....ha"e dlfi,,,,,,,Uy
An Evolut io na ry Approa ch to Animal Behavior 21
from large