TY - JOUR
T1 - Climate fluctuations influence variation in group size in a cooperative bird
AU - Riehl, Christina
AU - Smart, Zachariah Fox
N1 - Funding Information:
We thank Meghan J. Strong for assistance with fieldwork, genetic analyses, and database management and Luke Carabbia, Amanda G. Savagian, and Maria G. Smith for field data collection. Funding for long-term data collection was provided to C.R. by the Smithsonian Tropical Research Institute , the Harvard Society of Fellows , the William F. Milton Fund at Harvard University, the Department of Ecology and Evolutionary Biology at Princeton University, the Program in Latin American Studies at Princeton University, the Grand Challenges Program of the High Meadows Environmental Institute at Princeton University, and the National Science Foundation ( IOS-1755279 and IOS-184543 ). Funding to Z.F.S. was provided by the High Meadows Environmental Institute’s Environmental Internship Program and the Smith-Newton Scholars Program . We thank Charles R. Brown, B. Rosemary Grant, Peter R. Grant, and Damien R. Farine for their insightful comments on earlier drafts of this manuscript.
Publisher Copyright:
© 2022 Elsevier Inc.
PY - 2022/10/10
Y1 - 2022/10/10
N2 - Variation in group size is ubiquitous in social animals, but explaining the range of group sizes seen in nature remains challenging.1–3 Group-living species occur most frequently in climatically unpredictable environments, such that the costs and benefits of sociality may change from year to year.4–6 It is, therefore, possible that variation in climate may help to maintain a range of group sizes, but this hypothesis is rarely tested empirically.7,8 Here, we examine selection on breeding group size in the greater ani (Crotophaga major), a tropical bird that nests in cooperative groups containing multiple co-breeders and non-breeding helpers.9 We found that larger groups experience lower nest predation (due to cooperative nest defense) but suffer higher nestling starvation (due to intra-clutch competition). Long-term data revealed that the relative magnitude of these costs and benefits depends on climate, with frequent changes across years in the strength and direction of selection on group size. In wet years, individual reproductive success was higher in large groups than in small groups, whereas the opposite was true in dry years. This was partly a consequence of competition among nestlings in large clutches, which suffered significantly higher mortality in dry years than in wet years. Averaged over the 13-year study period, annual reproductive success was approximately equal for females in small and large groups. These results suggest that temporal changes in the direction of selection may help explain the persistence of a range of group sizes and that a full understanding of the selective pressures shaping sociality requires long-term fitness data.
AB - Variation in group size is ubiquitous in social animals, but explaining the range of group sizes seen in nature remains challenging.1–3 Group-living species occur most frequently in climatically unpredictable environments, such that the costs and benefits of sociality may change from year to year.4–6 It is, therefore, possible that variation in climate may help to maintain a range of group sizes, but this hypothesis is rarely tested empirically.7,8 Here, we examine selection on breeding group size in the greater ani (Crotophaga major), a tropical bird that nests in cooperative groups containing multiple co-breeders and non-breeding helpers.9 We found that larger groups experience lower nest predation (due to cooperative nest defense) but suffer higher nestling starvation (due to intra-clutch competition). Long-term data revealed that the relative magnitude of these costs and benefits depends on climate, with frequent changes across years in the strength and direction of selection on group size. In wet years, individual reproductive success was higher in large groups than in small groups, whereas the opposite was true in dry years. This was partly a consequence of competition among nestlings in large clutches, which suffered significantly higher mortality in dry years than in wet years. Averaged over the 13-year study period, annual reproductive success was approximately equal for females in small and large groups. These results suggest that temporal changes in the direction of selection may help explain the persistence of a range of group sizes and that a full understanding of the selective pressures shaping sociality requires long-term fitness data.
KW - El Niño-Southern Oscillation
KW - climate unpredictability
KW - cooperative breeding
KW - fluctuating selection
KW - sociality
KW - tropical
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U2 - 10.1016/j.cub.2022.07.057
DO - 10.1016/j.cub.2022.07.057
M3 - Article
C2 - 35998636
AN - SCOPUS:85139323625
SN - 0960-9822
VL - 32
SP - 4264-4269.e3
JO - Current Biology
JF - Current Biology
IS - 19
ER -