PT  - JOURNAL ARTICLE
AU  - Elizabeth Tinsley Johnson
AU  - Jacob A. Feder
AU  - Thore J. Bergman
AU  - Amy Lu
AU  - Noah Snyder-Mackler
AU  - Jacinta C. Beehner
TI  - The Goldilocks Effect: Female geladas in mid-sized groups have higher fitness
AID  - 10.1101/348383
DP  - 2021 Jan 01
TA  - bioRxiv
PG  - 348383
4099  - http://biorxiv.org/content/early/2021/04/07/348383.short
4100  - http://biorxiv.org/content/early/2021/04/07/348383.full
AB  - 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.Competing Interest StatementThe authors have declared no competing interest.