Work

The Goldilocks effect: female geladas in mid-sized groups have higher fitness

Year: 2021

Type: article

Abstract: The cost-benefit ratio of group-living is thought to vary with group size: individuals in “optimally-sized” groups should have higher fitness than individuals in groups that are either too large or too small. However, the relationship between group size and individual fitness has been difficult to establish for long-lived species where the number of groups studied is typically quite low. Here we present evidence for optimal group size that maximizes female fitness in a population of geladas ( Theropithecus gelada ). Drawing on 14 years of demographic data, we found that females in small groups experienced the highest death rates, while females in mid-sized units exhibited the highest reproductive performance. This group-size effect on female reproductive performance was largely explained by variation in infant mortality (and, in particular, by infanticide from immigrant males) but not by variation in reproductive rates. Taken together, females in mid-sized units are projected to attain optimal fitness due to conspecific infanticide and, potentially, predation. Our findings provide insight into how and why group size shapes fitness in long-lived species.

Source: Proceedings of the Royal Society B Biological Sciences

DOI: 10.1098/rspb.2021.0820

Authors: Elizabeth Tinsley Johnson, Jacob A. Feder, Thore J. Bergman, Amy Lu, Noah Snyder-Mackler, Jacinta C. Beehner

Institutions: Michigan State University, Stony Brook University, University of Michigan, Arizona State University

Language: English

Topic: Primate Behavior and Ecology

Subfield: Social Psychology

Field: Psychology

Domain: Social Sciences

Open Access status: hybrid