Lyanne Brouwer, Andrew Cockburn, and Martijn van de Pol (Feb 2020)
Integrating fitness components reveals the importance of survival in shaping the costs and benefits of group living
“Integrating fitness components reveals that survival costs outweigh other benefits and costs of group living in two closely related species”
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Group sizes vary enormously across the animal kingdom, but does it always pay to be in a large group, and why?
Studies on group living animals have shown that living in groups maybe beneficial, for example because it increases foraging success, provides protection against predators, or even increases reproduction because group members assist in raising each other’s offspring. However, group living can also be costly because group members compete for food, space, or mating opportunities. Despite the enormous attention on the costs and benefits of group living, a limitation is that most studies have focused on a single cost or benefit—for example, the cost on survival or the benefit for reproduction. This means that it is unclear what the overall costs and benefits of group living are. In this study, Brouwer and collaborators investigate how six different fitness components vary with group size and subsequently integrate these to determine the overall costs and benefits of group living in two closely related fairy-wrens, family-living songbirds from Australia.
They find that despite the differences between the species, the overall costs and benefits of group living are very similar, suggesting that the same behavioral mechanisms are important. For both species, the costs for additional group members on survival are most important for integrated fitness and this is amplified through carry-over effects of group size between years (i.e. large groups suffer survival costs, and are likely to do so the next year as well). In both species, integrated fitness of most group members was highest in small groups (size 2-3), and larger group sizes reduced fitness. How group size affects integrated fitness varied among different types of individuals, suggesting that group members potentially have a conflict of interest over optimal group size. This study provides a quantitative framework for future studies that aim to understand what demographic and behavioral mechanisms favor the evolution of cooperation or cause intra-group conflict.
Abstract
Group living can be beneficial when individuals reproduce or survive better in the presence of others, but simultaneously there might be costs due to competition for resources. Positive and negative effects on various fitness components might thus counteract each other, so integration is essential to determine their overall effect. Here, we investigated how an integrated fitness measure (reproductive values; RV) based on six fitness components varied with group size among group members in cooperatively-breeding red-winged and superb fairy-wrens (Malurus elegans and M. cyaneus). Despite life history differences between the species, patterns of RVs were similar, suggesting that the same behavioral mechanisms are important. Group living reduced RVs for dominant males, but for other group members this was only true in large groups. Decomposition analyses showed that our integrated fitness proxy was most strongly affected by group size effects on survival, which was amplified through carry-over effects between years. Our study shows that integrative consideration of fitness components and subsequent decomposition analysis provide much needed insights into the key behavioral mechanisms shaping the costs and benefits of group living. Such attribution is crucial if we are to synthesize the relative importance of the myriad group size costs and benefits currently reported in the literature.