Gregariousness, foraging effort, and social interactions in lactating bonobos and chimpanzees

Fission-fusion dynamics have evolved in a broad range of animal taxa and
are thought to allow individuals to mitigate feeding competition. While
this is the principal benefit of fission-fusion, few studies have
evaluated its costs. We compared gregariousness, foraging budgets, and
social budgets between lactating bonobos and chimpanzees from wild
populations to evaluate such costs. Both species exhibit fission-fusion
dynamics, but chimpanzees, particularly in East African populations,
appear to experience higher feeding competition than bonobos. We expected
lactating chimpanzees to be less gregarious than lactating bonobos;
reduced gregariousness should allow lactating chimpanzees to mitigate
costs of higher feeding competition without requiring more foraging
effort. However, we expected the reduced gregariousness of lactating
chimpanzees to limit their time available for affiliative social
interactions. Using long-term data from LuiKotale bonobos and Gombe
chimpanzees, we found that lactating chimpanzees were indeed less
gregarious than lactating bonobos while feeding and travel time did not
differ between species. Contrary to our predictions, lactating females did
not differ in social interaction time, and lactating chimpanzees spent
proportionately more time interacting with individuals other than their
immature offspring. Our results indicate that lactating chimpanzees can
maintain social budgets comparable to lactating bonobos despite reduced
gregariousness and without incurring additional foraging costs. We discuss
potential explanations for why lactating bonobos are more gregarious.