TY - JOUR
T1 - The Goldilocks effect
T2 - Female geladas in mid-sized groups have higher fitness
AU - Tinsley Johnson, Elizabeth
AU - Feder, Jacob A.
AU - Bergman, Thore J.
AU - Lu, Amy
AU - Snyder-Mackler, Noah
AU - Beehner, Jacinta C.
N1 - Funding Information:
All authors gave final approval for publication and agreed to be held accountable for the work performed therein. Competing interests. Authors declare no competing interests. Funding. This work was supported by National Science Foundation (grant nos. BCS-0715179, IOS-1255974, BCS-1340911, IOS-1854359; BCS-1723237; BCS-17123228; BCS-2010309); Leakey Foundation (multiple grants); National Institutes of Health (grant no. R00-AG051764); National Geographic Society (grant nos. 8100–06, 8989– 11, 50409R-18; the Fulbright Scholar Program; Sigma Xi; American Society of Primatologists; Wildlife Conservation Society and the University of Michigan. Acknowledgements. We would like to thank the Ethiopian Wildlife Conservation Authority (EWCA) and the wardens and staff of the Simien Mountain National Park for their permission and ongoing support for our long-term research project. Additionally, we are grateful to our excellent field team across the years, most especially E. Jejaw, A. Fanta, S. Girmay, J. Jarvey and M. Gomery for their assistance with field data collection. We also owe thanks to A. Marshall, B. Dantzer, J. Mitani and two anonymous reviewers for providing valuable feedback on this manuscript.
Publisher Copyright:
© 2021 The Authors.
PY - 2021/6/9
Y1 - 2021/6/9
N2 - 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 groups 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 groups 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.
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 groups 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 groups 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.
KW - fitness
KW - folivore paradox
KW - infanticide
KW - optimal group size
KW - reproduction
KW - survival
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U2 - 10.1098/rspb.2021.0820
DO - 10.1098/rspb.2021.0820
M3 - Article
C2 - 34074124
AN - SCOPUS:85107412879
VL - 288
JO - Philosophical Transactions of the Royal Society B: Biological Sciences
JF - Philosophical Transactions of the Royal Society B: Biological Sciences
SN - 0800-4622
IS - 1952
M1 - 20210820
ER -