Intraspecific Prey Choice by Amazonian Hunters

31 Pages·2010·4.27 MB·English

Wenner-Gren Foundation for Anthropological Research
Intraspecific Prey Choice by Amazonian Hunters [and Comments and Reply]
Author(s): Michael Alvard, Janis B. Alcorn, Richard E. Bodmer, Raymond Hames, Kim Hill,
Jean Hudson, R. Lee Lyman, Rajindra K. Puri, Eric A. Smith, Allyn Maclean Stearman
Source: Current Anthropology, Vol. 36, No. 5 (Dec., 1995), pp. 789-818
Published by: The University of Chicago Press on behalf of Wenner-Gren Foundation for
Anthropological Research
Stable URL: http://www.jstor.org/stable/2744021
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CURRENT ANTHROPOLOGY Volume 36, Number 5, December I995
?) I995 by The Wenner-GrenF oundation forA nthropoloeical Research. All rightsr eserved ooiI-320o/95 /3 6o0-ooo0$3.oo
It is commonly held that indigenousp eoples strive to
live in balance with their environments.W hile the
IntraspecificP rey
meaning of "balance" or "harmony"i s not oftenm ade
explicit, in essence these terms reflectt he biological
Choice by concept of equilibrium (Pimm i99I). Predator-predyy -
namics have been well studied among nonhuman spe-
cies form anyy ears,a nd the equilibriumt hats uch preda-
Amazonian Hunters
tors maintain with their prey is often explained with
referencet o populationd ynamics( fora review,s ee Tay-
lor I984). A stable or fluctuatinge quilibriumi s obtained
when a predatorp opulation is limited in its ability to
by Michael Alvard
drivei ts preyp opulationt o extinction( Taylor I984:I8).
Often,f ore xample, predatorsr educe preyn umbers,r e-
sulting in lower huntingr eturns,a nd this causes a de-
cline in the predatorp opulation that allows the prey
populationt o rebound.T hese typeso fp rocessesc an pro-
This papert estst he idea thats ubsistenceh untersu se selective duce oscillations in both predatora nd preyn umbers,o r
intraspecifipcr eyc hoicet o increaset he sustainabilitoyf their
a stable equilibrium can be maintainedw ithout either
long-termha rvest-in otherw ords,t o conserveI.t is suggested
thatm uch ofw hath as been consideredc onservationby native predator'so r prey's being driven to extinction( Taylor
peoplesi s probablye piphenomenalT. he misidentificatioonf ap- I984; fora n applicationo ft hese ideas to human hunters,
parentc onservationas genuinei s a resulto ft he focuso n sus- see Winterhaldere t al. I988).
tainedh arvestsr athert hano n behaviorst hatn ot onlya im at in-
Many human subsistence hunting populations also
creasingt he sustainabilityof t he harvesti n the longt ermb ut
also are costlyi n the shortt erm.O ptimalf oraginmg odelsa re exist in equilibriumw ith theirp reyp opulationsb ecause
hereu sed to generatea lternativpe redictiontso thoseo f conserva- they do not hunt their prey to extinction.M isguided
tion.D ata on the intraspecifipcr eyc hoiceo ft he Piro,s ubsis- logic has, however,l ed to the belieft hat such sustained
tenceh unterso ft he PeruvianN eotropicsi,n dicatet hatt heyd o harvestingi s a goal of subsistenceh untersr athert han a
not selectivelyc hoose sex and age typest hatm inimizet he im-
consequence of density-dependenotr independentp opu-
pact on preyp opulationsR. athert, heird ecisionsa re morec losely
predictedb y optimalf oragintgh eoryE. volutionarmy odelsd o lation processes as it is in others pecies (BunyardI 989,
not,h oweverr, uleo ut conservationas a strategyfo rr esourcea c- FeitI 987, HarrisI 979, Nelson i982, Reichel-Dolmatoff
quisition,a nd productivre esearchm ayw ell followf roma n exten- I976). Some have arguedt hat food taboos act to protect
sion of the logic and the rigoroums ethodologyof e volutionary
species particularlys usceptiblet o local extinction( Mc-
ecologyt o the identificatioonf truec onservationan d the con-
textsi n whichi t is an adaptives trategy. Donald I977, Ross I978). Othersh avea rguedt hatp atch-
choice decisions are made to avoid depleted areas (Feit
MICHAEL ALVARD is AssistantP rofessoorf A nthropologayt the I973, MooreI 95 7). Stillo thersh ave claimedt hath unt-
StateU niversityof N ew Yorka t Buffalo(B ox6 I005, Buffalo, ers selectively avoid certain age and sex categories
N.Y. I426i-0005, U.S.A.). Borni n I959, he was educateda t Colo- whose harvesth as a greatern egativei mpact on the prey
radoS tateU niversity(B .A. i984) and at theU niversityof N ew population(K ayI 985, JermIy98 3; see AlvardI 993a for
Mexico (M.S., I987; Ph.D., I993). He has donef ieldworkam ong more citations).
the Piroa nd Machiguengao f Perua nd is currentlsyt udyingth e
impacto f traditionahl untinga nd trappingon preys peciesa mong Simply observingt hat huntersr eturnf romt he hunt
the Wana of Sulawesi.H e has published" A Test oft he Ecologi- with seemingly adequate amounts of game does not,
callyN oble SavageH ypothesisI:n terspecifiPcr eyC hoice byN eo- however,j ustifyt he conclusion that native peoples are
tropicalH unters"( Human Ecology 2I:355-78), "PreyC hoice in conservationistsU. ntil recentlyt hereh as been no opera-
a DepletedH abitat"( Human Nature 5:I27-54), and "Shotguns
and SustainableH untingi n the Neotropics"( Oryx,i n press).T he tional definitiono f conservationt hat would allow for
presentp aperw as submittedIO X 94 and acceptedI 9 XII 94; the adequate testingo f its existence among native peoples.
finalv ersionr eachedt he Editor'so ffice3 III 95. In addition,f ew data are offeredt o substantiatec laims
of conservationistp racticesb y native peoples, and there
are no data on prey populations and huntingh arvests
that would demonstratew hethere xtant huntersl iving
in equilibriumd o so because of "natural conservation"
(AlvardI 993a, b). At the same time,e mpiricalw orkh as
i. This researchw as fundedb yt heC harlesL indberghFo undation, led researchersi n the fieldo f wildlifea nd conservation
the L. S. B. Leakey Foundation,t he TinkerI nter-AmericaRne - biologyt o question the previouslya ssumed isomorphic
searchF oundationt, he Universityo f New Mexico, and the Na-
relationshipb etweent he behaviorso fn ativep eoples and
tionalS cienceF oundation(G rantB NS-87I 7886 to HillardK aplan).
I am gratefutl o Hillard Kaplan,J aneL ancaster,J effL ong,K ent the goals of conservingb iodiversity(K ay I994, Redford
Redforda,n dW illiamV ickersf orh elpfucl ommentsI. thankT eslin and StearmanI9 93, RedfordI9 9I, Clad I985; see,h ow-
Phillips,K ate Kopischkea, nd Monica Gomikiewiczf ora ssistance ever,A lcornI 993). Reasoningf rome conomict heory
with data collectiona nd especiallym y wife,S haronG urskyf, or
also casts doubt on the notion thati ndividualsw ill con-
encouragements,u pport,a nd criticalc omment.I am gratefutlo
the Piro of Diamantef ora llowingm e into theirl ives and taking serve open-access resources such as those usually ex-
me alongo n theirh untsa nd to AlejandroS mitha ndE stefanC arpio ploitedb y huntingp eoples( ClarkI 990, HardinI 968).
and familyf ora ssistancei n the field. Finally, a number of human evolutionarye cologists
789
790 CURRENT ANTHROPOLOGY Volume 36, Number 5, December I995
have argued that individuals should not be expectedt o food allows the foragert o engage in other fitness
procurer esourcesi n such a way as to mitigaten egative enhancinga ctivities( Kaplan and Hill i992). Foragersa re
environmentale ffectsi f such behavior results in costs expected to maximize theirs hort-termh arvestingr ate,
ultimately paid in diminished reproduction (Alvard that is, the rate at which resources are harvestedp er
I993b,H amesI 987, Heinena ndL ow i992, RogersI9 9I, unit time (Stephensa nd Krebs i984).2 Fort he basic mod-
Smith I983). In fact,a small groupo fh untersi nhabiting els, no explicit or implicit assumptions about harvest
a generous environmentc an hunt sustainablyi n spite sustainabilitya re made, and thus decisions to pursue or
of the absence of any proactive conservationo n their ignorea preyi tem are made independento f any effects
part;t his typeo f apparentc onservationh as been termed that its removal may have on the fateo f its population.
epiphenomena(Hl unni 982; see also Hames I99I). Froma n optimal-foraging-theopreyr spectivet, he factors
The idea of "sustainable use" implies that the re- that determinep rey choice are the nutritionalb enefits
source in question is renewable; that is, it must be a and the procuremento r handlingc osts to the individual
living biological population. One does not talk of sus- hunter (Stephens and Krebs I986). While rate-
tainableu se ofp etroleump roducts,b ecause only a finite maximizing hunting and sustained harvestingo f prey
amount of petroleum exists on the planet. No matter are not necessarilym utually exclusive, any long-term
what is done,w hen a barrelo fo il is burnedf ori ts energy conservationi s assumed to be onlyi ncidentalt o the goal
content,t herei s no mechanismf orr eplacingt hatb arrel. of rate maximization (Alvard I993a, b; Smith I983).
Forests,f isheriesa, nd otherb iological resources,i n con- Huntersw hose goal was to conservep rey,i n contrast,
trast,c an theoreticallyb e managedi n a sustainablef ash- mighte mployn ot only selectivep atch choice and inter-
ion. Forest managers carefullyc alculate the biological specificp rey choice but also nonrandomh arvestingb y
productivityo f stands of trees to determinet he board age and sex (Getz and Haight I989). The idea that non-
feett hat accrue to the foreste ach year throughg rowth. human predatorsu se selective intraspecificp reyc hoice
If managersw ant to sustain the forest,t hey cut down to manage prey populations was debated in biology in
no more than this amount of board feet (Sullivan and the I960s and I970S. Arisingf roma schoolo ft hought
Cutter I972). Sustainability,t hen,i s a characteristico f that might be biology's equivalent to the idea of the
biological resourceh arvestingw herein the harvestr ate ecologically noble savage, Slobodkin's concept of the
is equal to or less than the natural growthr ate of the prudentp redator( i96i, I968; see also MacArthurI 960)
resource in question (Clark I990). Sustained harvests was based on the observationt hat predatorso ftenk ill
need not, however,b e managed; the huntingo f a small the old and young members of a prey population,
populationo f foragersc an be sustainablea s long as they avoidingt he prime,r eproductive-ageadn imals. Slobod-
are culling theirp reya t a rate less than the net produc- kin argued that this was a result of natural selection
tivityo f the preyp opulations.F or example,H unn (i982) favoringp redatorsw hose prey choice conserved their
arguest hatt he apparentb alance with theire nvironment prey populations. Implicit in this argumentw as that
oft raditionalS ahaptin-speakerosf t he Columbia Plateau prudentp redatorsw ould avoid killing prey types that
was the result of a highlym obile foragingp atterna nd had a negativei mpact on the preyp opulation,e ven if it
low population; they simply did not have the capacity meant ignoringh igher-qualityt ypes. Though the idea
foro verexploitationI. have argued elsewheret hat mis- was eventuallyr ejectedb y biologists,w ildlifem anagers
identifyinegp iphenomenalc onservationa s genuinec on- oftenu se selectiveh arvestingt echniquesv erys imilart o
servationh as led to the labelingo fm anyi ndigenousp eo- those described by Slobodkin to manage the size and
ples as "ecologicallyn oble" (Alvard I993b, I994). productivityo f herds (Hayne and Gwynn I988, McCul-
Identifyingg enuine conservation among subsistence lough I984, SchemnitzI 980). Native huntersm ight
huntersr equires testingf or behaviors consistentw ith conceivably use the same methods if they hunt ac-
conservationr athert han fori ts presumede ffectss, uch cordingt o the conservationistg oals oftena ttributedt o
as sustained harvesting( Alvard I993b). them.
Since it is clear thatr estraintis the essence of conser- I will begin by introducingt he studyp opulation and
vation, conservationc an be defineda s subsistenced eci- its prey.N ext, I will discuss selective intraspecifich ar-
sions that are costly to the actor in the shortt ermb ut vestinga s a way ofr educingt he negativei mpacto fh unt-
aimed at increasingt he sustainabilityo f the harvesti n
the long term( AlvardI 993b). The difficultyof applying
this definitioni s in operationallyi dentifyingr estraint. 2. It is oftens tatedt hatt het imes pano verw hicho ptimalf oraging
Optimal foragingt heoryp rovidesa frameworkf ord elin- theorya ssumesr eturnr atesa re maximizedi s the longt erm( Ste-
eating what to expect of huntersw ho maximize short- phensa nd Krebsi 986). This is a ratheram biguousa ssumptionb,u t
the time scale over which the returnr ate is maximizedi s very
term gains, and this allows the alternativeh ypothesis,
importanwt hen dealingw ithi ssues of conservationG. enerations
thato fr estrainta nd hence truec onservationt, o be oper- are oftenr equiredf ore xploitivep rey-choicde ecisionst o deplete
ationallyi dentifiedi n contrast. preyp opulations( Rogersi 99i). Fromt his perspectivet,h e span
Optimal foragingt heoryp roducesp redictionst hat are overw hicht hese analysesa ssume maximizationis the relatively
often mutually exclusive. It is linked to evolutionary shortt erm( overt he courseo f one or severalf oraginbgo uts [Ste-
phensa nd Krebsi 986:i6]). The assumptiono f selfishs, hort-term
theoryb y the basic assumptions that, up to a point,
ratem aximizationo f optimalf oragintgh eorya nd the assumption
moref oodi ncreasesi ndividualf ertilityan d survivorship ofl ong-termc,o mmon-googdo als impliciti n a conservationst rat-
and thatm inimizingt he amount oft ime spenta cquiring egyi s the majorc ontrastb etweent he two hypotheses.
ALVARD IntraspecificP rey Choice in Amazonia 79
N
tion
FIG. T s yt i10km
.40
Peru
Diamante
Studlayr ea
Shintuya
FiG. i. The study site and vicinity.
ing on preyp opulationsa nd go on to presentt he alterna- Dbtaint he majorityo f theirg ame (85% by weight)w ith
tive predictionso f optimal foragingt heory.D ata on the 3hotguns.
age and sex distributionso ft he Piro harvestw ill thenb e Diamante is approximately70 km fromt he biological
comparedw ith those of censused populations fort heir researchs ite of Cocha Cashu, wherem any of the impor-
concordancew ith the predictionso ft he two hypotheses. -antP iro preys pecies have been extensivelys tudied by
Finally,I will discuss the implicationso f the resultsf or rerborgh (I983). These preya nimals include manyt ypes
the studyo f conservationb y native peoples. ,ommon to the diets of native groupsa cross Amazonia
Redforda nd Robinson I987, Vickers I984). The Brazil-
.an tapir( Tapirus terrestrisi)s the largestS outh Ameri-
,an mammal, the largesta mong the Piro kills weighing
The Piro and Their Prey [55 kg. The tapir is usually nocturnala nd, except for
,he mother-offsprinasgs ociation, solitary.P iro hunters
The Piro communityo f Diamante is situated on the ;ake advantageo f nocturnalt ravelp atternso f tapir,u s-
Alto Madre de Dios River on the bordero f Manu' Na- ng ambush kill techniques.A lthoughe ncountersd o oc-
tional Park in the Departmento f Madre de Dios, Peru :ur duringd aylights earches,t he majorityo f tapirk ills
(fig.I ). While the Piro engagei n a small amount ofw age )bservedi n Diamante were by huntersw aitinga t night
labor and marketa ctivities,t heire conomyi s essentially ovithf lashlightsa t tapir trails or in shotgun trip-line
subsistence-basedH. unting,f ishingg, atheringa, nd hor- ,raps.
ticulturep roduce 95% of the calories in theird iet,w ith Collared peccary (Tayassu tajacu), which weigh
the cultivationo f manioc and plantains accountingf or iround2 o kg, provide the most meat to the Piro diet
7I%. Hunting and fishingp rovide the majorityo f the Alvard I993a). They most often travel in multimale,
proteina nd fat in their diet but only about 7% of the rnultifemalhee rds of i-io individuals (Kiltie and Ter-
calories (AlvardI 993a). The Piro are proficienwt ithb ow )orgh I983). Most of the kills observedw ere made dur-
and arrowa nd frequentlyh unt and fishw ith them but ng generalizeds earches,a lthougha mbushes were occa-
792 CURRENT ANTHROPOLOGY Volume 36, Number 5, December 1995
TABLE I
Animals and Birds Encounteredd uringH unts by Piro Hunters
AnimalT ype ScientifiNc ame CommonN ame
Ungulates Tapirust errestris Braziliant apir
Mazama americana Red brocketd eer
M. gouazoubira Grayb rocketd eer
Tayassu tajacu Collaredp eccary
T. pecari White-lippepde ccary
Primates Lagothrixla gothricha Woollym onkey
Atelesp aniscus Spiderm onkey
Alouattas eniculus Howlerm onkey
Cebus apella Brownc apuchinm onkey
C. albifrons Whitec apuchinm onkey
Saimiris ciureus Squirreml onkey
Callicebusm oloch Titi monkey
Saguinusn igricollis Black-mantletda marin
Aotus spp. Nightm onkey
Rodents Hydrochaerihs ydrochaeris Capybara
Agoutip aca Paca
Dasyproctav ariegata Agouti
Sciuruss p. Squirrel
Edentata Priodontems aximus Gianta rmadillo
Dasypusn ovemcinctus Armadillo
Tamandua tetradactyla Collareda nteater
Choloepuss p. Two-toeds loth
Carnivores Pantherao nca Jaguar
Felis concolor Puma
F. pardalis Ocelot
Eira barbara Tayra
Lutral ongicaudis Nutria
Pteronurab rasilensis Gianto tter
Nasua nasua Coati
Atelacynusm icrotis Short-eareddo g
Birds( Cracidae) Penelopej acquacu Spix'sg uan
Aburriap ipile Guan
Mitu mitu Currassow
Birds( others) Crypturellussp p. Tinamous
Tinamuss pp. Tinamous
Ara spp. Macaw
Ramphastoss pp. Toucan
Odontophorussp p. Wood quail
Psophial eucoptera Trumpeter
Harpia harpyja Harpye agle
sionally set up at mineral licks. The white-lipped male and several adult femalesw ith offsprin(gC rockett
peccaries (T. pecari) so importantt o nativep eople across and EisenbergI 987). Spiderm onkeysh ave a much more
Amazonia (Kiltie I980) are absent in the Diamante area, fluids ocial organizations imilart o thato f chimpanzees,
althought hey occurredi n the past. with groupss plittinga nd coalescing (Robinsona nd Jan-
Red brocketd eer (Mazama americana) weigh 25-35 son I987). Adults of both species weigh 5-IO kg, but
kg and are essentiallys olitarye xcept when mothersa re among howlers males are significantlyla rgert han fe-
with young. Capybara (Hydrochaeris hydrochaeris), males. Table i lists these and others pecies encountered
which weigh up to 50 kg, favora quatic habitats,a re of- duringh untsw ith the Piro (see Alvard I993 a and Alvard
ten nocturnali n hunted areas (Emmons and Feer I99I), and Kaplan I99I form ore details).
and were never observed during foresth unts in Dia-
mante. Groups studied by Herrera and MacDonald
(I987) in Venezuela rangedf rom5 to I4 adults. Conservationa nd IntraspecificP rey Selection
Black spiderm onkey( Atelesp aniscus) and redh owler
monkey( Alouatta seniculus) are the two importantp ri- Ensuringa sustainableh arvestr equiresk illingf ewera n-
mates in the diet. Both species travel in groups con- imals than are produced by the prey population. Ac-
tainingb oth sexes. Howler monkeysa re usually polygy- cordingt o the logisticg rowthm odel, the numbero f ani-
nous and are oftenf oundi n groupsc ontainingo ne adult mals produced above replacement depends on the
ALVARD IntraspecificP rey Choice in Amazonia | 793
reproduqtiver ate of the population and the population age-categoriest o limit theiri mpact on preyp opulation
size with respectt o the environmentacl arryingc apacity growth,t hey will selectivelyk ill immaturesa nd older
(Clark 1990, CaughleyI 977, RickerI 954, Savidgea nd adults, avoidingr eproductive-agaen imals.
Ziesenis I980). As the population approaches carrying Selectiveh arvestingb y sex. Nonrandomh arvestingo f
capacity,p opulationg rowthd eclines because ofd ensity- prey according to sex is another tactic that might be
dependentf actors.B ecause of this logistic relationship used by hunterst o conserve theirp rey populations. In
between the rate of population growtha nd density,t he particular, avoiding female-biased harvests has been
maximums ustainabley ield is approximatelyh alfo f car- shown to increase the maximum sustainable yields of
ryingc apacity.A s long as the harvesti s below the maxi- populations( Fowler 198I, Harrisa nd Kochel 198I, Nel-
mum sustainable yield, the preyp opulationw ill not be son and Peek i982). For most animal species, males gen-
depletedt o extinction( see Alvard I993). The impact of erallyh ave much greaterv ariance in reproductives uc-
selectivep reyc hoice on the sustainabilityo ft he harvest cess thanf emales.T his is especiallyt ruef ors pecies,l ike
depends on the contributiont he culled individuals are most mammals, that do not commonlym ate monoga-
expectedt o make to the growtho f theirp opulation.F e- mously and whose males do not invest heavily in off-
cundity and survival schedules differs ignificantlyb e- spring( Bateman I948). While most females who reach
tween the sexes and between individuals of different reproductivea ge reproduce,m any males do not. It is
ages. Because of this, males, females, young and old possible,p articularlyin species with polygynouso r pro-
contributed ifferentlyto the growth of a population miscuous mating systems,f ora few males to monopo-
(CaughleyI 977). Removingi ndividualst hata re expected lize females and sire the majorityo f the offsprinpgr o-
to contributel ittle to the growtho f the population can duced, essentiallyr enderinga significantp roportiono f
significantlyin crease the maximum sustainable yield the males in the population superfluousw ith regardt o
(CaughleyI9 77). populationg rowth.I n contrastt o males, femalesu sually
Selective harvestingb y age. If hunters kill animals providem ost of the parentali nvestmentt hat translates
that mighth ave died anyway or that are not expected into offsprings urvival (Alexander and Borgia I979).
to reproduce,m ore can be harvestedw ithout causing Thus, the number of females in a population has a
extinctiono f the prey population than if prey are har- greatere ffecto n overall reproductiver ate and offspring
vested at random (see Caughley I977:i88). This pat- mortalityt han does the number of males. As a result,
tem was apparent to Slobodkin (I96I, I968; see also selective harvestingo f males can significantlyin crease
ErringtonI9 46, MacArthurI 960), who argued that the the sustainabley ield of a preyp opulationa nd reducet he
prudentp redatorw ould harvestt he memberso f a prey threato f population extinction( Caughley I977, Down-
population with the lowest reproductive value. ingI 980, Haynea ndG wynnI9 77). Male-biasedh arvest-
Reproductivev alue, definedb y Fisher (I958), is a func- ing is commonlyo bserveda mong nonhuman predators
tion of age and is the number of offsprintgh at remain (e.g.,F itzgibbonI99 0, HornockeIr9 70; Kruuka ndS nell
to be bom to a female of age x (reproductivev alue is I98I, SchallerI9 72) and is a moree ffectivwe ayo fs us-
usually only given forf emales).I ncluded in the calcula- tainingh arvestsa t higherl evels than age-selectiveh ar-
tion of the value is the probabilityt hat the female will vesting( CaughleyI 977). Thus, ifP iroh untersa re choos-
live to age x. In most mammalian species, reproductive ingp reyw ith limitedi mpact on preyp opulationg rowth,
value is moderate at birth, increases until reproduc- theyw ill selectivelyk ill males.
tive maturity,a nd subsequentlyd eclines. Reproductive
value is low for young animals because mortalityf or
this age-class is high among mammals and many do not
ForagingT heory and Intraspecific
attain adulthood (Fisher I958). For example,,amongt he
howler monkeys (A. palliata) at Barro Colorado, Pan- Prey Choice
ama, 88% of the males and 65% of the females died
betweenb irtha nd 5 yearso f age (Froehlich,T horington, The short-termgo al of an optimalf oragingst rategyc on-
and Otis I98I). Bissonette(i 982) reportms ortalityfi g- trastw ith the long-termg oal of conservationA. ccording
ures in the firsty ear for collared peccaries in Texas of to optimalf oragingt heory,h untersm ake decisions that
86%, 62.5%, IOO%, 9I.7%, 78%, 5o%, and ioo% for result in the highest returnr ate measured in terms of
a number of groups over a three-years pan. Harvesting resourcea cquiredp er unit time spentf oraging(S tephens
youngeri ndividualsh as less impact on subsequentp op- and Krebs I986). No considerationi s given to the long-
ulation growtht hanh arvestinga ge-categorielse ss likely term effectso f prey exploitation. From this point of
to sufferm ortality. view, selective harvestingi s based on the profitability
Prime reproductive-agien dividuals have a lower risk of harvestingd ifferentty pes of animals rathert han on
of mortalitya nd most of theire ntirer eproductiveli ves the effectso f theirr emoval on the sustainabilityo f the
ahead of them. Accordinglyr, eproductivev alue is often harvest. The best-knowno ptimal foragingm odels an-
the highest for these animals, and their removal from swer questions concerningp reyc hoice and time alloca-
the populationh as the greatestn egativei mpacto n popu- tion to resource patches. The classical prey-choice
lation growth.I ncreasingr isk of mortalityw ith age and model generates strict predictions concerningw hich
decreasingr eproductivep otential contributet o low re- preyt ypes of the suite of potential prey types encoun-
productivev alue fort he old. If Piro huntersa re choosing teredh unterss hould pursue (Pyke,P ulliam, and Char-
794 CURRENT ANTHROPOLOGY Volume 36, Number 5, December 1995
nov I977). Preyt ypesa re rankeda ccordingt o profitabil- sented with a choice among a number of types during
ity, calculated as the amount of energyh arvestedp er a group encounter,h owever,t he hunteri s predictedt o
time handlingt he preyt ype.P ursuit,k ill, and fieldp ro- pursue the most profitable type (Waddington and
cessing are consideredh andling;t he cost of searchingi s Holden I979).4 Assuming equal handlingt imes, adults
assumed to be shared among the preyt ypes.T he "opti- are largera nd morep rofitablet hany oungera nimals,a nd
mal diet" is the suite of preyp redictedb y the model to in sexually dimorphics pecies males are usually larger
be pursued by the hunteru pon encounter.A preyt ype and more profitablet han females. Piro hunters,w hose
is included in the optimal diet if the expected return goal is to maximize returns,a re expectedt o prefert hese
rate for that type is highert han the average expected largera ge/sex types.
returnr ate for continued search for all higher-ranked Although hunters should prefert hese larger types,
items. The hunter must decide between the expected they will not be the only types harvested.I f a smaller
benefitso f pursuinga n encountereda nimal and the ex- individualo f a certains pecies is encountereda lone and
pected benefits of looking for something better (Ste- it is large enough to be included in the optimal diet, it
phens and Krebs I986). will be expected to be pursued. Further,a n encounter
An optimal foraginga nalysis of interspecificp rey with a groupi s not necessarilya simultaneouse ncoun-
choice forP iro shotgunh unters( AlvardI 993b) indicates terw ith all its members.D ifferenta nimals oftenm ove
that theirb ehaviori s consistentw ith the predictionso f into and out of the hunter'sr ange durings uch encoun-
the model. While handlingt ime is technicallyi mportant ters. Hunters are ofteni nitiallyp resentedw ith the op-
forc alculating the profitabilityof prey,b ody size alone portunityo fp ursuinga smallera nimal withint he group,
was foundt o predictm uch oft he Piro optimald iet. (This although other largert ypes are presentn earby.I n this
is not an inevitable outcome of such as analysis.) Ani- situation,t he standardp rey-choicem odel applies. If the
mals consistentlyp ursued by Piro are those largert han smaller package obtained fort he handlingt ime is less
about 4-5 kg, includings pidera nd howler monkey,t a- than what is anticipated fromc ontinued search, then
pir, deer, collared peccary, and capybara. Prey types the animal should be ignored.I f the probabilityo f en-
weighing I.5-4 kg, including agoutis, game birds,a nd counteringa nother,p erhapsl argerp reyi tem is high,a s
medium-sizedm onkeyss uch as capuchins,w ere incon- is oftent he case duringg roupe ncounters,i t may be ef-
sistentlyp ursuedf orr easons relatedt o the specificc on- ficient for the hunter to wait. The prediction that
textso f individuale ncountersa nd the opportunityco sts emergesi s Piro huntersw ill prefert he largerm embers
involvedi n killings mallera nimals with a limitedn um- of encounteredg roups but will accept smaller types if
ber of shotguns hells (Alvard I993b). Preys maller than theyf all withint he optimal diet and if a shot at a larger
approximatelyi. 5 kg,s uch as squirrels,t amarins,s quir- is improbable.S maller individualsw ill also be taken if
rel monkeys, and small birds, were never taken with encountereda lone.
shotguns. With respectt o sex, only a few of the importantp rey
In theory,t he prey-choicem odel can be refinedt o pre- species are dimorphic( Alvard I993a). Howler monkey
dict hunterp reyc hoice at the intraspecificle vel. In this and capuchinm onkeym ales are significantlyla rgert han
case, it is age and sex rathert han species that defines females,a nd it is expectedt hatr ate-maximizingh unters
the prey types available to hunters. To calculate the will preferm ales if offereda choice. Adult male and
profitabilitfyo re ach type,s ufficiennt umberso fp ursuits female spider monkeys,c ollared peccaries, tapir,d eer,
for each age and sex categoryf ore ach species need to and capybaraa re the same size, and it is expectedt hat
be observed,a nd this requiresa much largers ample of rate maximizersw ill take each sex in proportiont o its
observedh untst han is available fort he Piro.A s an alter- abundance in the population.
native, we can assume that handling times are equal Immaturei ndividuals are smaller than older ones for
across age and sex categoriesw ithin species,3m aking all species, and thus adults should be preferreidn general
body size (number of calories) the primaryf actord e- over immatures.S ince the body size of same-agedi ndi-
terminingp reyc hoice. Furtherw, hereast he simplep rey- viduals of differenstp ecies varies, huntersm ay pursue
choice model assumes that hunterse ncounterp rey se- the young of one species while ignoringt he young of
quentially (Stephens and Krebs I986), this is oftenn ot another.F or example, juvenile tapirsa re largert han the
the case forn eotropicalp reys pecies. Whileh erds ofp ec- adults of many other,s maller species. If body size is an
caries or troopso f spiderm onkeysa re met sequentially,
it is commonplacef orh unterst o encounterm anym em-
bers of any particularg roups imultaneously.I n most cir- 4. An initiala nalysisb y Waddingtonan d Holden (I979) suggests
cumstances when the hunter is simultaneously pre- thatt hem ostp rofitablper eyt ypes houldb e takend urings imulta-
neous encountersE. ngena nd Stenseth(I 984; see also Stephensa nd
Krebsi 986) have notedt hati n situationsi n whicht wo preya ni-
3. Therei s some evidencet o suggestt hats ome age/sext ypesa re mals are encounteredsi multaneouslyan d one typei s morep rofit-
morev ulnerablet o certaint ypeso fp redationth ano thersa nd thus able but the otheri s a largerp ackageo f calories,t he encounter
havel owerh andlingt imesa ndp otentiallyg reatepr rofitabilitFyo.r ratesb ecomei mportanfto rd eterminintgh er ate-maximizindge ci-
example,M achiguengba ow hunterhs avel ongerp ursuitisn general sion.I n thiss ituationi,f e ncounterr atesa reh ighe nought, her ate-
than do Piro shotgunh untersM. achiguengah untersr eportt hat, maximizingc hoicem ayb e to taket hel argert houghle ss profitable
althoughb igm ale monkeysa rep referretda rgetsp, regnanfte males type;i f encounterr atesa re low, the most profitablhe uts maller
or femalesw ith youngo ftend ropl ower into the canopyd uring typei s the best choice (see Stephensa nd KrebsI 986 fora discus-
longp ursuitsa nd are easiert argets(A lvarda nd Kaplan i991). sion).
ALVARD IntraspecificP rey Choice in Amazonia 1795
importantm easure ofp reyp rofitabilitya ,g reaterp ropor- were asked to save the mandibleso fk ills,a nd with these
tion of youngera nimals will be found among the kills mandibles it was possible to age the animals in terms
of larger-bodiedp rey because their subadults are large of wear patterninga nd tooth eruption.I ndividualsw ere
and still within the optimal diet. For smaller-bodied classifieda s adult (primeo r old) and immature.E ruption
prey,w hose adults are withint he body-sizer angeo f the of the thirdp ermanentm olar distinguishedi mmatures
optimal diet but whose younga re not, a greaterp ropor- froma dults,a nd subjectivem easureso f "extreme"w ear
tion of adults will be representedin the kills. and tooth loss were used to identifyo ld adults. When
individualsw ere placed in the old-adultc ategoryt he de-
cision was almost always unambiguous.A nimals were
Tests oft heA lternativHe ypotheses placed within a system of two or three age categories
dependingo n the resolutiono f the comparatives ample
The two hypothesesg eneratea numbero f mutuallye x- and the available wear patterno r eruptiond ata available
clusive predictions concerning the intraspecificp rey for each species. Old capybara could not be distin-
choice ofP iro hunters.T he conservationh ypothesisp re- guished because this species has persistentlyg rowing
dicts that huntersw ill harvestt o minimize the impact crowns (Hillson I986).
on the reproductivep otential of the prey population. The frequencieso f age and sex typeso fh arvestedp rey
The optimal foragingh ypothesisp redictst hat hunters were examined to see if they deviated frome xpected
will choose accordingt o the net profitabilityof alterna- frequenciesa s determinedf rom the censused popula-
tive prey types. With respect to age, the conservation tions. A chi-square-testw as used to see if age and sex
hypothesisp redictst hat hunterss hould selectivelyk ill classes were takenr andomly( thati s, in the same propor-
individuals of lower reproductivev alue-younger and tion as in the censused populations)o r in a way consis-
oldera nimals. In contrast,t he optimalf oragingh ypothe- tentw ith eithert he optimalf oragingo r the conservation
sis predictst hat they will pursue prime-ageda nimals, hypothesis.
those that deliver the largesta nd highest-qualityp ack- The age-sex structureso f the preyp opulationsi n the
age. It also predictsa positive relationshipb etween im- Diamante area are unknown and must be estimated
matureb odys ize ofa given species and the proportiono f fromo therp opulations that have been studied. This is
kills ofi mmatures.W ithr espectt o sex, the conservation acceptable, however,b ecause although age-sex profiles
hypothesisp redictst hat hunterss hould selectivelyk ill varyb etween species and even withint hem,t he popula-
males, while optimal foragingt heoryp redictsa prefer- tion structureo f mammals is predictableb ecause of the
ence fort he largests ex, which fora numbero f species natureo f fecunditya nd survivorship(L yman I987). Re-
is male. The null hypothesisi s that the harvesti s pro- cruitmenti s into the youngests tage, and survivorship
portionalt o the frequencyo f the preyt ypesi n the popu- declines with increasing age; in populations that are
lation. growing the age distributioni s skewed toward the
Data to test these hypothesesw ere collected in Dia- youngerc ategoriesa nd the resultinga ge distributionis
mante during two field sessions amounting to I8 pyramidal( Caughley I977).
monthsb etweenA ugust I988 and May I99I. Direct ob- Preyc hoice by age. The age profileso f the preyk illed
servationa nd interviewm ethodsw ere designedt o deter- at Diamante are presentedi n tables 2 and 3. Table 4
mine the species, sex, and age of a large sample of prey summarizesd ata on the age profileso f censused popula-
killed by Piro hunters. During 79 directly observed tions of the importantP iro prey species. Many of the
hunts the species and sex of all the animals killed were species show considerable variability,b ut there does
determined.D ata fora nimals killed duringu nobserved tend to be some consistencyb etween differenpto pula-
hunts were collected duringv isits to households for a tions and betweent ime periodsa t the same site. Follow-
subsampleo ft he village. Households were visitede ither ing Sokal and Rohlf( I98I:7 IO), I used a chi-squaret est
everyd ay or everyt hirdd ay, dependingu pon the sched- of goodness of fitw ith a continuityc orrectiont o com-
ule of otherr esearcha ctivities.E ithert he huntero r an pare the age profileso f the censused populations with
adult familym emberw as interviewedP. reyc haracteris- the Diamante kills. (Fors amples less than 25, Sokal and
tics were ofg reati nterestt o the Piro,a nd sex and species Rohlf recommendw orkingo ut the exact probabilities
were reportedw ith confidence.M any reportsg iven by froma binomial distribution.I)n almost everyc ase, the
nonhunters were subsequently corroboratedb y the ratio of adults to immaturesk illed is eithers tatistically
hunters.A ge could not be reportedw ith equal precision. indistinguishablef romt hat fort he censused population
In the I988-89 season, informantws ere asked to classify or biased towarda dults (table 5). A greaterp roportiono f
the animals as adult or not-yet-adultE.r rorsi n judgment Piro capybarak ills is immaturesc omparedw ith either
and variationa mong huntersa s to what constituteda n of the censused populations,a lthought he differencies
adult were problemsh ere, and therew as no way to ob- not significantC. apybarai n Venezuela had a birthp eak
tain any indication of variabilityw ithin the two rather in the rainy season, and all the capybara kills in Dia-
broad age categories,p articularlyf or discriminatinga mante took place in that season. In any case, the young-
young adult froma n old one. To standardizea ge esti- dominatedp rofilep redictedb y the conservationh ypoth-
mates and to provide more detail, the method of de- esis is not apparent.
terminingt he age of kills was changedf ort he I990-9I The mandible collection indicates that old adults
field season. Hunters and women who processed game make up a small proportion(8 -I4%) of Piro kills. Data
796 CURRENT ANTHROPOLOGY Volume 36, Number 5, December 1995
TABLE 2
Age Profileso f PreyH arvested by Field Season
I988-89 9 90-9I BothS easons
Species Immature Adult N Immature Adult N Immature Adult N
Tapir .29 .7I 7 .50 .50 I0 .4I .59 I7
Peccary .26 .74 46 .27 .73 95 .27 .73 I4I
Deer .I7 .83 I2 .I9 .8I 2I .i8 .82 33
Capybara .20 .80 5 .62 .37 8 .46 .54 I3
Spiderm onkey .25 .75 8 .I0 .90 2I .I4 .86 29
Howlerm onkey .II .89 9 .I4 .86 35 .I4 .86 44
fine-grainede nough to distinguishw ith any precisior of a species should be predictive of their proportion
the proportiono f old adults to prime-ageda dults do nol among the kill. A species whose immaturesw eigh more
exist fora ny of the censused populations,b ut it can b( than 4-5 kg should be killed in higherp roportiont han
assumed that old adults are a small proportiono f an) one whose immaturesw eigh less than 4-5 kg.
population that is stable or growing.P lottingt he three- For selected prey,t able 6 presentsd ata on the birth
categorkyi llp rofilefsr omI9 90-9I on thet rianglger apt weights,t he age at which the species attains 5 kg, and
developedb y Stiner(i 99i) showst hatt heP iroh unters adult body size. The immatureso f the largeru ngulates
are not selectivelyk illingo ld adults (fig.2 ). (notablyt he tapira nd the capybara)r apidlyg rowi nto the
These results supportt he optimal foragingh ypothe. rangeo fb odys izes (> 5 kg) ofp reyc ommonlyp ursuedb y
sis; Piro hunters are targetingp rimarilyp rime-age( the Piro. The immatureso f some of the smallers pecies,
adults. Althought here is a bias towardy oung tapir (as the primatesi n particularg, rows lowly and do not attain
the conservationh ypothesisw ould predict),t herei s alsa a weighto f 5 kg fora numbero fy earsa fterb irth.I mma-
a bias towardy oungc apybara,w hich are much less vul. tureso f peccarya nd deer are of intermediates ize.
nerablet o depletion.T his interspecifivc ariabilityi n in- Figure 3 plots for each species the log percentageo f
traspecificp reyc hoice cannot be understoodi n termso i the kill that is immature against the log adult body
a conservationg oal. weight of that species. The relationshipi s positive and
The body size of animals included in the optimald iet statisticallys ignificantP. iro huntersa re more likely to
of Piro shotgunh untersi s 4-5 kg or more. Preya nimals pursue immatureso f species such as tapira nd capybara
weighingb etween i and 4-5 kg are the least profitablc whose young are large; with species whose immatures
of the optimal diet and the ones that the Piro inconsis- are small, they focus on adults. Althought he samples
tentlyp ursue. Assuming that body size is the primary are small, this resultp rovidess upportf ort he hypothesis
factora ffectingt he profitabilityof a preyt ype,s maller- that it is return-ratme aximizationt hat motives age se-
bodied immaturesa re less profitablet han adults of the lection by Piro hunters,a s predictedb y optimalf oraging
same species. The immatureso f some species, however, theory,r athert han a desire to choose age-categoriest o
are largert han the adults of others pecies. If return-rate conservep reyp opulations.
maximization rathert han conservationi s the motiva- Prey choice by sex. Tables 7 and 8 display the sex
tingf actorf orh unters,t hen body size of the immatures ratios forp rey species in the Piro sample, divided into
adult and immature animals because the sex ratios of
many species vary regularlyw ith age (Clutton-Brock,
TABLE 3 Harvey, and Rudder I977, Clutton-Brocka nd Albon
Age Profileosf P reyH arvestedin 1990-9I from I979). For example,i n manys pecies male-malec ompeti-
Mandibles tion results in increased mortalitya mong adult males,
and this may produce a bias of the sex ratio towardf e-
males at later ages. Humans follow this pattern( Daly
Species Immature PrimeA dult Old Adult N
and Wilson I983). For the howler monkeys( A. palliata)
at Barro Colorado, Froehlich, Thorington,a nd Otis
Tapir .50 .40 .IO Io (I98I) argue that high mortalitya mong subadult males
Peccary .27 .65 .o8 95 is the reason fort he observedb iased adult sex ratio in
Deer .2I .66 .14 2I favoro f females. Crocketta nd Eisenberg( I987) suggest
Capybara .62 .37 - 8
Spiderm onkey .IO .76 .14 2I just the oppositei n theirs tudyo fA . seniculus,i n which
Howler monkey .14 .74 .II 35 farf ewerf emales survivedt o age 7 than males' (though
thism ay reflectp roblemsi n identifyinegm igrants)T. his
ALVARD IntraspecificP rey Choice in Amazonia 17 97
TABLE 4
Age Profileso f Censused Populations of ImportantP iro Prey Species
Count Percent Percent
Speciesa nd Location Method AgingM ethod Immature Adult Sample Source
Collaredp eccary
Cocha Cashu, Peru Observation Bodys ize .3I .69 i8 groupsN, = 100 Kiltiea nd Terborgh
(i983)
Texas Long-term Knowna ges .24 .76 Estimatedf rom5 Bissonette(i 982)
observation groups
Arizona Survey Bodys ize .26 .74 N = between3 ,62o ArizonaG ame and
and 4I2 for2 6 Fish Surveyi,n
years Sowls (i984)
Venezuela Short-term Bodys ize .44 .56 Two groupsN, = 97 Castellanos( i983)
observation .44 .56
Arizona Trapping Tooth eruption .45 .55 N = .29 Sowls (i984:78)
Deer
(White-tailed)
Michigan Reconstructed Body size .39 .6i N = 2,1IO9 McCullough (i984)
(gamer eserve) drivec ounts
Capybara
Venezuela Trappinga nd Bodys ize and .30 .70 N -io6 for3 0 Ojasti (I973)
observation knowna ges months
Venezuela Observation Bodys ize .42 .58 Two groupsf or5 Herreraa nd Mac-
and trapping years,N -So Donald (i987)
Spiderm onkey
(A. paniscus)
Cocha Cashu, Peru Long-term Known ages, .47 .53 2 groups, N = 77 Symington (i988)
study behavior .46 *54
Surinam Long-term Knowna ges, .39 .6I N = I8 van Roosmalen
study behavior (I980)
Spiderm onkey
(A. belzebuth)
Colombia - .34 .66 N = I7 Klein (I972)
Howlerm onkey
(A. seniculus)
Venezuela Long-term Bodys ize, .53 .47 N = I63 (I972) Neville (I972)
observation femaleg enitalia .5 I .49 N = I46 (I970)
.5 I .49 N = 219 (I969-70)
Venezuela Long-term Bodys ize .54 .46 I78 Rudran( I979)
observation
TABLE 5
Comparison of Age Profileso f Piro Kills and Censused Populations
Censused
Piro Kill Sample Population
Species Immature Adult N Immature Adult Source X2adl P
Collaredp eccary .27 .73 I4I .3I .69 Kiltiea nd Terborgh(i 983) 0.37 .5426
.24 .76 Bissonette(i 982) O.I9 .6590
.45 *55 Sowls (i984) 9.I5 .002 5
.44 *56 Castellanos (i983) 8.I7 *??43*
.26 .74 ArizonaG ame and Fish in Sowls (i984) O.OI .9559
Deer .i8 .82 27 *39 .6i McCullough (i984) i.85 .1732
Capybara .46 .54 I3 .30 .70 Ojasti (I973) O.I9 .6260
.42 *58 Herreraa nd MacDonald (1I987) 0.14 .8496
Spiderm onkey .I4 .86 29 *47 .54 Symingtonl (1988) 5.84 *OI57*
*34 .66 Klein(1 972) 2.18 .I400
Howlerm onkey .1I4 .8 6 44 *.5I .49 Neville (1I972 12% .09 .0005*
*.54 .46 Rudran(1I 979) I13.95 .0002*
* Statisticallysi gnificant.