ANT 154B Course notes- Lecture _10

ANT 154B Course notes- Lecture _10 - ANT 154BN Course notes...

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Unformatted text preview: ANT 154BN Course notes Lecture #10: Predation ARTICLE5 3 February 2011 Evolutionary An smaller primates, if for son than that fewer spe to prey on them. It has a Outline gested that large groups ject to predation t h a n a which will be less effectiv 1. Predation on primates: general o r repelling predators. U m e a n feeding g r o u p si 2. Anti-predator strategies body s i z e covary a c r o Therefore, analyses that 3. Predation by primates: chimpanzees mated predation r a t e s a cannot reliably separate body size a n d group siz 1. Predation on primates: general Wrangham5 s h o w e d t h predation r a t e s t e n d t o d Presumed to be an important selective force, even if it occurs rarely (cf. infanticide) creasing body size amon mates (they excluded ap but hard to study Figure 1 . Possible routes to group living in primates. Pathway 1 : Predation favors group living as a analysis). Reanalysis of direct result of the benefits it provides in reducingthe risks of predation.Female philopatry (i.e.,site f fideiity) i not required.7,Iq Pathway 2: Resource competition favors the evolution o groups in conjunction with d a t a s Expect low predation ratesdin nse of resources; memberslow ilifeshistories nefits b y forming groups with relatives.16 a n d mean group ~ i z e efe primates because s gain nclu ive fitness be imply low extrinsic mortality Pathway 3: Predation favors the evolution and maintenance of female philopatry (site fidelity). body s i z e a n d ( l o g ) g r Resource competition combines with phiiopatty when the advantages of cooperative defense favor the evolution of groups. This combination results in the formation of kin groups because equally good predictors relatives live closer to each other than to nonrelatives. predation rate (see Fig. Major types of predators Great a p e s have larg smaller t h a n average gr less, no direct observations of preda- d u c t s t u d i e s t h a t g u a r a n t e e higher though pongids a r e not tion were e biogeographic ring the to p r e d a t i ~ n , he~ ,p~ t ~ ir Predation rates: differ among ver witnessed duregions productivity. study, even though the annual rate of are apparently quite l PATTERNS OF PREDATION O N p be quite high (e.g., stimated to be year). This may indicat can redation in 1987 was ered colobus) PRIMATES at least 45%, which is a n exceptionally fects of body size o n p hinfluence n for this p o p l a t i o n . ~ ~ can igh rate evedemographic~composition Given these problems, it is not sur- may b e stronger t h a n A final reason for t h e lack of docu- prising that there has been heavy reli- group size, if such comp mentation of predation o n primates is a n c e o n i n d i r e c t e v i d e n c e a n d species a r e legitimate. A that such studies a r e likely t o involve o p p o r t u n i s t i c r e p o r t s of p r e d a t i o n find a n association betw great investment of time and energy in events and attempts. A s the data have a n d estimated predatio return for relatively few d a t a points trickled in, however, interesting pat- groups within a populat 2. Anti-predator strategies because predators eat other animals terns have begun to emerge. failed to find a significan besides primates. Primatologists a r e 1 ) Large primates should be inher- 3b). The fact that evenla Group size understandably more eager to con- ently less vulnerable t o predation t h a n including ~himpanzees pZZi--l Key terms and concepts are indicated in blue Resource Corn peti t ion I Predation rates generally decrease with increased group size b h -t 0 301 a t r = -0.49, p < 0.01 -t 301 a t r = -0.40, p< c U m 0 t ! a a n 0 U 10 - .lFi c v) c .E c v) c W 0 0 1 2 3 4 5 oi 0 = I I 5 10 Ln ( G r o u p Size) B o d y size ( k g Figure 2. The reiationshipbetween estimated predation rate and group size (a) and body size (b), among Smaller primate species From data in Cheney and Wrangham?, Goodman et ai.30 and Clutton-Brockand Harvey.34 ANT 154B Lecture #10 course notes but, group size does not always confer a benefit page 2 of 7 No evidence that chimpanzee hunting -> larger colobus groups 50 - CHIMPS Jozani Abuko Tana + CHIMPS Gombe Tai Mahale Kanyawara Group size 40 30 20 10 0 0 G K T J M A T 100 200 300 Population density (per sq km) Thursday, February 4, 2010 19 Group composition More males in folivores preyed upon by eagles 4 # adult males Alouatta Colobus Presbytis 1 1 # adult females 10 van Schaik & Horstermann (1994) Thursday, February 4, 2010 21 ANT 154B Lecture #10 course notes Polyspecific associations page 3 of 7 America folivore predation males per female polyspecific associations 396 Birth synchrony xCensus of Adult Females Thursday, February 4, 2010 Africa colobus C-H eagle chimps high common Asia langurs (none) low absent howlers harpy eagle high common 24 I I I xxx xx o= F- I x xxx III II x I 10- x II III x x x ~ x x x x x x x~o( x 8 I IIIII I I 6 4 2 1 5 9 1 3 ' 1 7 21 2 5 2 9 4 February I ,11, ,lift I I I I ....... 8 12 16 2 0 2 4 2 8 1 5 9 13 17 21 2 5 2 9 April DATE I ........ I,, 0 March N (8) D (9) J (13) F (14) F i g . 2. D i s t r i b u t i o n o f 1984 b i r t h s in t h e m a i n s t u d y t r o o p ( P T ) a n d t w o a d j a c e n t t r o o p s ( M W a n d CT). T i m i n g o f births was only precisely d e t e r m i n e d for the P T t r o o p . Censuses o f the adult females in t h e o t h e r two t r o o p s were c o n d u c t e d at irregular intervals indicated by an • The last birth in the C T t r o o p o c c u r r e d a b o u t 15 J u n e a n d is n o t f i g u r e d Other correlates of predation pressure F i g . 3. t - t e s t s o f t h e m e a n reproductive and nonrepr r e p r o d u c t i v e female was o f f s p r i n g b o r n i n 1984. T n u m b e r o f females sampl o f P < 0 . 0 5 is i n d i c a t e d b y was c o n s i s t e n t w i size Predation rates decrease with increased body t h t h e Births in C o r c o v sharp increase in f o o sons. Predation rates decrease closer to human settlements (at least by most non-human predators) I n t h e PV t r o o p five b i r t h s o c c u r r e d i n t h e While births o c c u r r e d c o u r s e o f o n e n i g h t , 14 F e b r u a r y , a n d a t o t a l o f ly h i g h f o o d a b u n d a n 9 ( 6 4 % ) i n 8 d a y s (Fig. 2). T h r e e o f t h e 4 n e o n a t e s the April-June peak b o r n b e f o r e 13 F e b r u a r y d i e d b e f o r e t h a t d a t e , s u r ciation between birt viving a t m o s t 2 d a y s . T h o s e b o r n o n o r a f t e r t h e a n d the s u d d e n avai w a s n o t d e t e c t e d (Bo 13-17 F e b r u a r y b i r t h p e a k h a d a g r e a t e r i n d i v i d u a l p r o b a b i l i t y o f survival to t h e e n d o f t h e s t u d y stead the f o o d peak with the p e r i o d w h e (50%). S u r v i v o r s h i p t o t h e e n d o f t h e s t u d y a m o n g weaned and became these t w o g r o u p s o f n e o n a t e s , h o w e v e r , was n o t s e q u e n c e o f b r e e d i n g sea- ANT 154B Lecture #10 course notes influence on social system page 4 of 7 3. Predation by primates: chimpanzees Explaining chimpanzee hunting Explaining chimpanzee hunting Kanyawara 1996-2000. level of level of analysis is important (individual vs. the group) analysis is important • Probability that at least one male hunts onth as an indicator 0.85 0.8 0.75 0.7 NUMBER OF HUNTS PER 1000 HOURS 61 hunts, 9278 hours Probability that focal male hunted -level and focal-level 0.65 primary explanatory r2 = 0.90 0.6 simple (unadjusted) n = 11 0.55 P < 0.0001 come either ‘hunt 0.5 d’ (yes/no), as appro1 2 3 4 5 6 7 8 9 10 11 12 13 ntially confounding Number of adult male chimpanzees bles, we ran multiple Figure 1. Relation between the probability of hunting by at least one ith all interactions male and adult male party size. The regression line is from an unadr of swollen females Number of Adult + Adolescent Males justed logistic regr es sion. Error ba rs represent 95% confidence on type (woodland, intervals. Thursday, February 4, 2010 36 n. We refer to these ran four MLR models: ) focal-level hunting Social factors c2 1748:82, P ¼ 0.003; Fig. 1). The magnitude ratio ¼ 1.06, 1 ¼ A N I M A L B E H A V I O U R , 72, 1 cess (MLR 3) and (4) and significance of this association increased after controle reduced each model male ling for bonding social the confounding factors (see Methods; MLR 1: Gombe: Gilby et al. 2006 removing interaction odds ratio ¼ 1.12, P < 0.0001; Table 2). There was no asso(type III analysis).4, 2010 ciation between hunting by the focal male and adult male We Thursday, February 35 Table 4. Output from MLR 3: par 0.6 ignificance. For all foparty size (unadjusted odds ratio ¼ 0.96, c2 ¼ 3:49, 1 estimating equations Parameter Category P ¼ 0.06). This result became0.5 significant when we conrepeated observations trolled for the confounding factors (c2 ¼ 7:02, P ¼ 0.03), 1 on 9.1 (SAS Institute, 0.4 but only for parties with two or more swollen females alyses. Adult males (odds ratio ¼ 1.11, P ¼ 0.05; Table 3, Fig. 2). Explaining chimpanzee hunting male social bonding hunting probability/group 0.3 Swollen females th colobus monkeys. those encounters in nzee was present and ient detail to detered (1087 encounters; for which the focal it was clear whether set for all focal-level ratio ¼ 0.56, P ¼ 0.002) and 55% lower in parties with two Figure 2 Rela ¼ n betwe 0.0002), or more swollen females (odds. ratio tio0.45, P ¼ en the probability of hunting by the focal compared to parties male, no swollen females (MLR 1; with adult male party size and number of swollen females. The re- 0 Swollen females 0.1no association between 2+ Swollen females Unadjusted analysis revealed the number of swollen females and the probability of 0 party-level hunting (c2 ¼ 1:29, P ¼ 1 2However, multi-6 7 8 9 10 11 12 13 0.5). 345 2 ple regression showed that the odds of party-level hunting Number of adult male chimpanzees were 44% lower in parties with one swollen female (odds (including focal) Meat for Sex 0.2 2þ 1 0 Vegetation Leaf/pith Woodland Evergreen Output from a multiple logistic re by at least one male hunter on females, vegetation type and le 176 tion between 1999 and 2002, 94 enco monkeys were recorded. In this data s of hunting were 40% and 18% for p level hunts, respectively, indicating th page 5 of 7 in the long-term study may indeed h In general, however, this observer b affect the overall results with respect t tested. Consistent with the long-term 1999–2002 data showed that the pro 1 by at least one male was positive 0.9 adult male party size (odds ratio P ¼ 0.0007), but hunting by the fo 0.8 (c2 ¼ 1:62, P ¼ 0.2). In similar con 1 0.7 long-term study, Gilby’s data reve 0.6 monthly leaf/pith consumption on 0.5 hunting (party level: c2 ¼ 0:63, P 1 176 A N I M A L B E H A V I O U R , 72, 1 c2 ¼ 0:09, P ¼ 0.76). Regarding vegeta 0.4 1 results from the two data sets were s 0.3 Table 6. Summary of results, organized by hypothesis Gilby’s data set, focal males were mo Hypothesis Explanatory variable Dependent variable Predicted effect Observed 0.2 woodland effect in evergreen forest than 0 Swollen females 2 0.1 c1 ¼ 3:92, P ¼ 0.05), but there was no Male social bonding Adult males (few / many) Party hunt Swollen females þ þ 2+ Focal hunt þ type on 0* party-level hunting (c2 ¼ 1:2 0 1 Meat for sex À 1 Swollen females4(0 / many) 7 Party hunt9 10 11 12 13 þ 23 56 8 hunt long-term data set, there was a signific Focal þ À Party kill þ 0 tion type on party-level hunting bu Number of adult male chimpanzees Focal kill þ 0 hunting. t NutrientF factors do (low / explain of hunting by at least one shortfall Diet between high) Party hunt À Socialigure 3. Relationqualitynot the probabilitychimpanzee hunting sets 0While we cannot explain ch A N I M A L B E H A V I O U R , 72, 1 Focal hunt 0indicate that hunting by data male, adult male party size and number of swollen females. The re-À Party kill À 0 likely in 0woodland than in evergree Focal (ML gression lines are from a logistic regression kill R 1, Table 2). ErrorÀ Nutrient by hypothesis surplus Diet quality (low / high) Party hunt þ 0 Gilby’s data Table 6. Summary of results, organized bars represent 95% confidence intervals. Gombe: Gilby et al. 2006 set, there was no effect P ¼ 0.7; Table 5). Adult male party size was positively correlated with the number of monkeys killed per hunt (mixed effects linear regression: parameter estimate ¼ 0.12, F1,749 ¼ 91.58, ANT 154B Lecture #10 course notes P < 0.0001) and total kilograms of meat secured (parameter estimate ¼ 0.35, F1,749 ¼ 48.65, P < 0.0001). However, meat for sex the number of kilograms available per male decreased Explaining chimpanzee hunting meat for sex Probability that at least one male hunted hunting rate for group Adult males (few / many) Vegetation type Evergreen / Woodland Focal hunt Focal kill Hypothesis Explanatory variable Thursday, February 4, 2010 Male social bonding Meat for sex Dependent variable Party kill Predicted effect Party hunt Focal hunt Party hunt Focal hunt Party kill Focal kill þ Observed effect þ þ þþ þ 0* þ þ þ þ þ þ þ 0 0 0 42 Swollen females (0 / many) Cooperative hunting Party hunt Party kill Focal kill Adult males (few hunt Focal / many) Nutrient shortfall Diet quality (low / high) Party hunt þ 0 Focal hunt þ 0 swollen females and hunting (Stanford et al. 1994b), we the probability of hunting by at least one male, a result that Party kill þ 0 found that hunting was significantly less likely to occur agrees with previously published studies of chimpanzee Focal kill þ 0 Ecological factors behaviour (Stanford et al. 1994b; Mitani & Watts if swollen females were present. There are several possible hunting Vegetation type Evergreen / this seems þ þ reasons for such contrasting results. First, we examined 25 2001). At first, Woodland to supportPartymale social-bondthe hunt Focal hunting by speþ 0 years of data, compared to Stanford et al. (1994b), who ing hypothesis. However, examination ofhunt Nutrient shortfall/surplus Party kill þ þ examined a relatively short period of just 10 years of cific males (rather than by at least one male) casts doubt Focal kill þ þ Gombe: Gilby et al.y2006 data. Second, we restricted the definition of ‘swollen’ feon this idea. Adult male party size did not affect hunting Cooperative hunting Thursday, February (few / many) that there kill fewer than Adult focal male, Party were þ þ 46 males to those that were maximally tumescent, whereas by the males 4, 2010 provided Focal kill was a positive þ 0 Stanford et al. (1994b) included females that were only two swollen females present. Although there Number of hunting by prey þ þ Group partially swollen. Male chimpanzees rarely mate with association between adult male party size and Total prey mass (kg) þ þ females who are not maximally swollen (Goodall 1986; the focal male in parties with two or more swollen females, Per capita prey mass (kg) þ À Wallis 1997), so it seems unlikely that the decision to it is unlikely that the presence of swollen females served as hunt should be affected by the presence of partially swolan added incentive for males to seek meat to foster alliances *There was a positive effect of adult males on hunting by the focal male, but only in parties with 2þ swollen females. len females. Third, we used a more restrictive definition of with other males. At Ngogo, males form coalitions to guard yWoodland hunts by the focal male were more likely to succeed, but only in parties with fewer than two swollen females. hunting. Our examination of the Gombe narrative notes swollen females (Watts 1998), so it is possible that meat indicated that sometimes the observers used the term sharing may solidify such partnerships. However, coalitionary mate guarding is Gombe. females ‘hunt’ to describe a mere al. 1994b), we the probability of hunting by at least one male, rarely observed at swollen We dis- and hunting (Stanford etinterest in hunting, rather than a result that active pursuit. We therefore used climbing as the main cuss a possible alternative explanation for our results in found that hunting was significantly less likely Finally, we used logistic to occur agrees with previously published studies of chimpanzee criterion for defining hunting. the following section. if swollen females were present. There are several possible hunting behaviour (Stanford et al. 1994b; Mitani &of atts male social-bonding regressions, the appropriate analysis for modelling a binoAnother key assumption Wthe reasons for such mial response (Agresti 1996 examined 25 2001). At first, this seems hypothesis is that males use meat as ‘currency’ with which contrasting results. First, we). Stanford et al. (1994b) used to support the male social-bonding hypothesis. However,to establish and hunting by speexamination of maintain intrasexualyears of data, compared to Stanford et al. (1994b), who a linear regression model with the proportion of encounalliances. This ters short to hunts just response of notion was supported at Ngogo (Mitani & Watts 2001; examined a relatively that ledperiod of as the 10 years variable. A linear cific males (rather than by at least one male) casts doubt Watts & Mitani 2002a), hunting data. Second, we regression is definition of under these on this idea. Adult male party size did not affectwhere males share reciprocally restricted the not appropriate‘swollen’ fe- circumstances ( were 1996). with each other However, than males to those by the focal male, provided that there .were fewerGilby (2006) demonstrated thatAgresti maximally tumescent, whereas Our finding females were less only that at Gombe, adult males do not Stanford share two swollen females present. Although there was a positive preferentiallyet al. (1994b) includedthat males that were likely to hunt when with frequent grooming partners or partially swollen. swollen females were present does not support the meatMale chimpanzees rarely mate with association between adult male party size and hunting by associates. for-sex hypothesis and suggests that males face a trade-off females who are not maximally swollen (Goodall 1986; is a common the focal male in parties with two or more swollen females, between hunting and mating. ‘Possessiveness’ Wallis 1997), so itmating strategy in that the high-ranking male prevents seems unlikely which a decision to it is unlikely that the presence of swollen females served as Meat for sex hunt should be an added incentive for males to seek meat to foster alliancesGombe hunting data affected by the presence of partially swol- males (Tutin a swollen female from mating with other Although a previous analysis of we used a more restrictive definition temporarily loses with other males. At Ngogo, males positive associationguard found a form coalitions to between len females. Third, 1979; Goodall 1986). Ifetpossessive male of the presence of Gilby a al. 2006 Nutrient surplus Party hunt 0 Focal hunt À 0 Party kill *There was a positive effect of adult males on hunting by the focal male, but only inÀ parties with 2þ swollen 0 females. 0 yWoodland hunts by the focal male were Focal kill to succeed, but only in parties À more likely with fewer than two swollen females. Diet quality (low / high) Party kill þ Focal kill þ Number of prey þ Total prey mass (kg) À Per capita prey mass (kg) þ þ þ À À 0 0 þ 0 þ þy þ 0 þ þ À Explaining chimpanzee hunting Nutrient shortfall/surplus no effect on hunting rate for group swollen females (Watts 1998), so it is possible that meat Thursday, February 4, 2010 sharing may solidify such partnerships. However, coalitionary mate guarding is rarely observed at Gombe. We discuss a possible alternative explanation for our results in the following section. Another key assumption of the male social-bonding hypothesis is that males use meat as ‘currency’ with which hunting. Our examination of the Gombe narrative notes indicated that sometimes the observers used the 48 term ‘hunt’ to describe a mere interest in hunting, rather than active pursuit. We therefore used climbing as the main criterion for defining hunting. Finally, we used logistic regressions, the appropriate analysis for modelling a binomial response (Agresti 1996). Stanford et al. (1994b) used ANT 154B Lecture #10 course notes Focal male Explaining chimpanzee hunting page 6 of 7 Nutrient shortfall/surplus Explaining chimpanzee hunting no effect on hunting rate for focal Habitat type Habitat type Gilby et al. 2006 49 Thursday, February 4, 2010 more hunts/group in open habitat 176 Cooperation 176 A N I M A L B E H A V I O U R , 72, 1 Thursday, February 4, 2010 Gilby et al. 2006 50 Table 6. I O U R , 72, 1 A N I M A L B E H A V Summary Hypothesis of results, organized by hypothesis Explanatory variable Dependent variable Predicted effect Observed effect þ 0* À À 0 0 0 0 0 0 0 0 0 0 Table 6. Summary of results, organized by hypothesis 176 SummaryN I M A L B EMeat U R , 72, 1 A H A V I O for sex Male social bonding (few Party Hypothesis Explanatory variable Predicted hunting? Do ecologicalAdult malesvariable/ many) Dependenthunt factors explain chimpanzeeeffect þ Observed effect Focal hunt þ Male social bonding Adult males (few / many) Swollen females (0 / many) Party hunt Focal hunt Focal hunt Party hunt Party kill Meat for sex Swollen females (0 s Table 6. Summary of results, organized by hypothesi/ many) Nutrient shortfall Hypothesis Male socialshortfall Nutrient bonding Meat Nutrient surplus for sex Diet quality (low Explanatory variable / high) Adult males (few // high) Diet quality (low many) Party hunt kill Focal Focal hunt Party hunt Dependent variable Party kill Focal hunt Focal kill þþ 00 þþ 00 Party kill þ Vegetation Evergreen / Woodland Party huntParty hunt þ þ Nutrient shortfall type Diet quality (low / high) À 00 Focal kill Focal huntFocal hunt Àþ 00 þ 0 Party kill hunt kill Àþ 0þ Party þ þ Vegetation type Evergreen / Woodland Party Focal kill hunt kill Àþ 00 þ þy Focal Focal Nutrient surplus huntingDiet quality (low / high) / many) Party kill Party huntParty kill þþ 0þ Cooperative Adult males (few þ þ Focal kill Focal huntFocal kill þþ 0 þy þ 0 Party kill kill þþ 0þ Cooperative hunting Adult males (few / many) Party Number of prey þ þ Focal kill kill þþ 00 Focal Total prey mass (kg) þ þ Number of prey Vegetation type Evergreen / Woodland Party huntPer capita prey mass (kg) þ þ þþ þ À Total prey Focal hunt mass (kg) þþ 0þ Per capita prey mass (kg) Party kill þþ þÀ *There was a positive effect of adult males on hunting byFocal focal male, but only in parties with 2þ swollen females. the kill þ þy yWoodland hunts by the focal male were more likely to succeed, but only in parties with fewer than two swollen females. *There was a positive effect of adult males on / many) the focal male, but only in parties with 2þ swollen females. Cooperative hunting Adult males (few hunting by Party kill þ þ yWoodland hunts by the focal male were more likely to succeed, but kill in parties with fewer than two swollen females. 0 Focal only þ Number of prey þ þ swollen females and hunting (Stanford et al. 1994b), we the probability of hunting by at least one male, a result prey mass (kg) Total that þ þ Per þ À found swollen females and Gombe: (Gilby et al. 2006 likely hunting Stanford et al. less theagrees withof hunting bypublished studies result that capita prey mass (kg)that hunting was significantly1994b), we to occur probability previously at least one male, a of chimpanzee X X ! Party hunt Party hunt kill Focal Focal hunt Focal hunt Diet quality/ many) high) Party huntParty hunt (low / Party kill Swollen females (0 Focal kill Focal huntFocal hunt Party kill þ þ Predicted effect þ þ þÀ þÀ þÀ þÀ þ þ þ þ þ þ þ 0* À À effect 0 0 ÀObserved À À þ À þ þ þ þ Nutrient surplus Diet quality (low / high) Party kill hunt kill Party Party Focal kill hunt kill Focal Focal 0 0* 0 À0 À0 X found if swollen femalessignificantly less likely to several possible that hunting was were present. There are occur agrees with behaviour (published et al. 1994b; Mitani & Watts hunting previously Stanford studies of chimpanzee *There was ).February effect of adult males on hunting by the focal male, but swollen females were present. There are several possible Thursday,behaviour (Stanford et al. 1994b;the male social-bond- only in parties with such contrasting results. First, we examined 53 if hunting a positive 4, 2010 reasons for 2þ swollen females. 2001 At first, this seems to support Mitani & Watts yWoodland hunts by the focal male were more likely to succeed, but only in parties with fewer than two swollen females. 25 2001). At first, this seems to support the male social-bond- speyears of contrasting results. Stanford et al. (1994b), who ing hypothesis. However, examination of hunting by reasons for such data, compared to First, we examined 25 years of data, compared to Stanford et al. (1994b), who years of ingcific males (rather than by at leastof hunting by spe- doubt hypothesis. However, examination one male) casts examined a relatively short period of just 10 examined a Second, we restricted et definition of of cific males idea. Adult by at party one male) casts doubt swollen females relatively short periodthe al. 1994b), we ‘swollen’ fethe probability of huntingmale least one male, a result that hunting data. and hunting (Stanford of just 10 years on this (rather than by at least size did not affect ANT 154B Lecture #10 course notes page 7 of 7 Importance of “impact hunters” Take home messages 1. Predation is thought to be an important force shaping primate social systems; the evidence of this is limited– in part because studying predation is difficult. 2. Protection against predation is generally assumed to be a benefit of group living. 3. Primate prey species exhibit a variety of anti-predator strategies. 4. There is weak evidence in support of the hypothesis that social factors explain chimpanzee hunting. Among ecological factors examined to date, vegetation type appears to be the only consistent predictor of chimpanzee hunting (at least at Gombe). Question to ponder Plot the relationship between the costs of scramble competition and group size (specifics are not important, but think about whether the line has a positive or negative slope, and whether it is straight or curved). Explain your logic in one sentence. On the same axes, plot the benefits from anti-predator protection as a function of group size (again focusing on slope and shape of the line). Explain your logic in one sentence. Indicate on your graph the predicted optimum group size. Explain your logic in one or two sentences. Finally, assume that infanticide is an important selective force in this species. Will optimum group size be smaller or larger than what you would predict in the absence of infanticide? Why? ...
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