Optimal group size in a highly social mammal.
Group size is an important trait of social animals, affecting how individuals allocate time and use space, and influencing both an individual's fitness and the collective, cooperative behaviors of the group as a whole. Here we tested predictions motivated by the ecological constraints model of group size, examining the effects of group size on ranging patterns and adult female glucocorticoid (stress hormone) concentrations in five social groups of wild baboons (Papio cynocephalus) over an 11-y period. Strikingly, we found evidence that intermediate-sized groups have energetically optimal space-use strategies; both large and small groups experience ranging disadvantages, in contrast to the commonly reported positive linear relationship between group size and home range area and daily travel distance, which depict a disadvantage only in large groups. Specifically, we observed a U-shaped relationship between group size and home range area, average daily distance traveled, evenness of space use within the home range, and glucocorticoid concentrations. We propose that a likely explanation for these U-shaped patterns is that large, socially dominant groups are constrained by within-group competition, whereas small, socially subordinate groups are constrained by between-group competition and predation pressures. Overall, our results provide testable hypotheses for evaluating group-size constraints in other group-living species, in which the costs of intra- and intergroup competition vary as a function of group size.
Department of Anthropology, Stony Brook University, Stony Brook, NY 11794; Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544; markham.catherine@gmail.com altj@princeton.edu.
A Catherine
AC
Markham
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544; Department of Biology, Duke University, Durham, NC 27708;
Laurence R
LR
Gesquiere
Department of Biology, Duke University, Durham, NC 27708; Institute for Primate Research, National Museums of Kenya, Nairobi, Kenya 00502.
Susan C
SC
Alberts
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544; Institute for Primate Research, National Museums of Kenya, Nairobi, Kenya 00502 markham.catherine@gmail.com altj@princeton.edu.
Jeanne
J
Altmann
D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D048548:Papio cynocephalus / Q000502:physiology*; D012919:Social Behavior
Papio cynocephalus; glucocorticoid hormones; group size; ranging patterns; savanna baboons