Pontzer _ Wrangham 2006

Pontzer _ Wrangham 2006 - International Journal of...

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International Journal of Primatology, Vol. 27, No. 1, February 2006 ( C ± 2006) DOI: 10.1007/s10764-005-9011-2 Ontogeny of Ranging in Wild Chimpanzees Herman Pontzer 1 , 2 and Richard W. Wrangham 1 Received December 27, 2004; accepted February 5, 2005; Published Online March 21, 2006 We examined the relationship between juvenile age and distance traveled per day, or day range, in Kanyawara chimpanzees. Because the energy cost of locomotion is greater for small-bodied animals, we predict that day range is constrained by body size, i.e., younger individuals tend to have shorter day ranges. To test this hypothesis, we measured day range for 200 day-ranges of groups in which we recorded the age of the youngest juvenile present. As pre- dicted, day range correlated positively with age for juveniles. Comparisons of day range vs. estimated stature support the hypothesis that the increase in day range with age was a consequence of body size. To assess other sources of variation in day range, we also measured the effects of group size and the presence of a carried infant. While day range correlated significantly with group size, the presence of a carried infant had no effect on adult female day range. Our results suggest the size of a juvenile may constrain ranging for mothers and their offspring. KEY WORDS: chimpanzees; day range; maternal investment; ontogeny; primate locomo- tion. INTRODUCTION As large-bodied, ripe-fruit specialists, chimpanzees ( Pan troglodytes ) depend on widely dispersed food patches. Combined with other demands such as territorial defense, their dietary specialization results in long day ranges—distance traveled per day— i.e., means of 2–5 km (Gombe, Wrangham, 1977; Ta¨ı, Herbinger et al. , 2001; Kibale, Pontzer and Wrangham, 2004 ). Terrestrial travel accounts for 10–15% of total daily 1 Harvard University, Cambridge, Massachusetts. 2 To whom correspondence should be addressed; e-mail: [email protected] 295 0164-0291/06/0200-0295/0 C ± 2006 Springer Science + Business Media, Inc.
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296 Pontzer and Wrangham energy expenditure, equal to the estimated combined cost of feeding, climb- ing, socializing, and resting (Pontzer and Wrangham, 2004 ; Leonard and Robertson, 1997 ). Travel costs appear sufFciently high to in±uence the in- tensity of scramble competition and Fssion–fusion grouping pattern (Janson and Goldsmith, 1995 ; Wrangham, 2000 ), but for chimpanzees researchers previously have examined the effects of such costs only for adults. We consider the ontogeny of ranging costs among chimpanzees by ex- amining the impact of body size on day range. Because juveniles are smaller than adults and the energy mass-speciFc energy cost of locomotion tends to be inversely proportional to body size (Taylor et al. , 1970, 1982 ), we expect juveniles to use more mass-speciFc energy per km than adults. Therefore, if the energy budget of juveniles is constrained in the same way as it appears to be for adults, we expect juveniles to keep the total cost of locomotion low by maintaining shorter day-ranges than those of adults. To test this hy- pothesis, we assume that age closely predicts juvenile size. Accordingly, we
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Pontzer _ Wrangham 2006 - International Journal of...

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