The meatscrap hypothesis small quantities of mea

Authors: Claudio Tennie Ian C Gilby Roger Mundry

Publish Date: 2008/10/14

Volume: 63, Issue: 3, Pages: 421-431

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Abstract

A common explanation for hunting in groups is that doing so yields a greater per capita caloric benefit than hunting solitarily This is logical for social carnivores which rely exclusively on meat for energy but arguably not for omnivores which obtain calories from either plant or animal matter The common chimpanzee Pan troglodytes is one of the few true omnivores that regularly hunts in groups Studies to date have yielded conflicting data regarding the payoffs of group hunting in chimpanzees Here we interpret chimpanzee hunting patterns using a new approach In contrast to the classical assumption that hunting with others maximizes per capita caloric intake we propose that group hunting is favored because it maximizes an individual’s likelihood of obtaining important micronutrients that may be found in small quantities of meat We describe a mathematical model demonstrating that group hunting may evolve when individuals can obtain micronutrients more frequently by hunting in groups than by hunting solitarily provided that group size is below a certain threshold Twenty five years of data from Gombe National Park Tanzania are consistent with this prediction We propose that our ‘meatscrap’ hypothesis is a unifying approach that may explain group hunting by chimpanzees and other social omnivoresGroup hunting presents an apparent paradox It would seem costly for an individual to incur the costs associated with hunting when it could exploit the actions of others Individuals should therefore refrain from participating Nevertheless group hunting is commonly observed eg African lions Panthera leo Scheel and Packer 1991 bottlenose dolphins Tursiops spp Gazda et al 2005 African wild dogs Crocuta crocuta Creel and Creel 1995 2002 One possible explanation for its widespread occurrence is that group hunting is ‘cooperative’ ie that an individual’s net payoff is higher when hunting with others than when hunting solitarily Packer and Ruttan 1988 MestertonGibbons and Dugatkin 1992 Clements and Stephens 1995 While there has been debate over exactly how to measure group hunting payoffs eg whether to consider reduced costs as well as increased benefits Creel 1997 Packer and Caro 1997 Creel and Creel 2002 most studies use calories to assess whether animals hunt cooperatively This is logical for social carnivores which obtain almost all of their energy from meat Indeed many species experience an increase in meat per capita and/or net caloric gain as hunting group size increases metaanalysis Creel and Creel 2002However the benefits of group hunting are likely to be different for omnivores which can choose to obtain calories from either plant or animal matter The common chimpanzee Pan troglodytes is one of the few true omnivores that regularly hunts in groups thus providing an important case for comparison with social carnivores Recently bonobos have been found to hunt other primates and there is also some evidence to suggest that they did so in a group Surbeck and Hohmann 2008 Chimpanzees prey most frequently upon red colobus monkeys Procolobus spp at all sites where the two species are present Uehara 1997 Boesch and BoeschAchermann 2000 Mitani and Watts 2001 To date the precise nutritional significance of meat for chimpanzees is unknown Nevertheless meat comprises a very small proportion of chimpanzee diet At Gombe National Park Tanzania chimpanzees spend less than 5 of their feeding time consuming meat Goodall 1986 McGrew 1992 This yields an estimated 55g/day for males and 7g/day for females Boesch and BoeschAchermann 2000 At Taï National Park Côte d’Ivoire males and females eat an average of 180 and 25g/day respectively However average values may be somewhat misleading as there is considerable individual and seasonal variation in meat consumption at both sites Additionally these values are difficult to interpret as the daily energy requirements of wild chimpanzees are likely to vary across sites and for the most part are unknownChimpanzees exhibit fission–fusion grouping in which members of the same community form subgroups ‘groups’ or ‘parties’ hereafter of changing size and composition Nishida 1968 Wrangham and Smuts 1980 Goodall 1986 Hunts are more likely to occur if many adult male chimpanzees are present in a group that encounters red colobus monkeys a pattern which appears to be ubiquitous across sites Mahale Mountains National Park Tanzania—Hosaka et al 2001 Kibale National Park Uganda Ngogo—Mitani and Watts 2001 Kanyawara—Gilby and Wrangham 2007 Gombe—Gilby et al 2006 However researchers remain divided over the explanation for this phenomenon At Taï group hunting appears to be calorically motivated There a ‘social mechanism’ of selective meat sharing apparently ensures that hunters obtain a positive net caloric payoff for participating while nonhunters rarely receive appreciable quantities of meat Boesch 1994b Boesch and BoeschAchermann 2000 However at Gombe individuals do not experience a net caloric energy gain by hunting in groups Boesch 1994b Meat per capita decreases with male chimpanzee hunting party size at Gombe Gilby et al 2006 and is not affected by party size at Ngogo Mitani and Watts 2001It might be argued that hunting in groups will be favored as long as the energy an individual obtains exceeds the energy expended to acquire it even if the magnitude of the benefit decreases as group size increases However if chimpanzees are motivated primarily by calories then one would expect an increase in hunting frequency when calories from alternative sources ie ripe fruit are scarce In contrast to this prediction several studies have shown the opposite pattern Hunts are more likely to occur when diet quality is high at Kanyawara Gilby and Wrangham 2007 and Gombe Gilby et al 2006 and when ripe fruit is plentiful at Ngogo Mitani and Watts 2001 2005 At Kanyawara this effect remains after controlling for party size which positively correlates with both diet quality and hunting probability Gilby and Wrangham 2007 These data indicate that obtaining calories is not a primary motivation for hunting at these sitesThe lack of universal support for a caloric explanation for group hunting among chimpanzees has prompted a recent focus on the potential social benefits of acquiring meat Specifically researchers have proposed that male chimpanzees seek to obtain meat in order to create opportunities for sharing in return for grooming coalitionary support Mitani and Watts 2001 and/or sex Stanford et al 1994 While there is still debate over why chimpanzees share meat for review see Muller and Mitani 2005 recent studies by Gilby et al 2006 2008 Gilby and Wrangham 2007 provide evidence against the hypothesis that chimpanzees base hunting decisions on the social benefits of meat sharing What then provides the selective pressure for chimpanzee social predation Here we explore an explanation that does not hinge upon caloric or social benefitsEvidence from many taxa illustrate the importance of acquiring micronutrients for which animals may invest considerable time and energy In doing so they may receive little or no caloric gain For example chimpanzees at Gombe spend hours licking areas that are presumably high in micronutrients including stones from cooking fires Goodall 1986 a board on which salt had previously been used to preserve a snake skin Wrangham 1975 and newly poured cement I Gilby personal observation Geophagy at habitual salt licks provides elephants Loxodonta africana with sodium in areas and seasons when dietary sodium is otherwise low Holdo et al 2002 Similarly sodium acquisition may serve as an important influence on aquatic foraging patterns of moose Alces alces Belovsky 1981 Belovsky and Jordan 1981 but see MacCracken et al 1993 and sodium calcium and phosphorous appear to influence the feeding selectivity of lowland gorillas Gorilla gorilla gorilla Magliocca and GautierHion 2002 Without the requisite levels of calcium vitamin A and phosphorous animals have been shown to suffer health and reproductive costs Tordoff 2001 Thus species may experience costly dietary deficiencies even in environments that abound with easily accessible calories Morrison 1983

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