Group augmentation underlies the evolution of complex sociality in the face of environmental instability

Abstract

Although kin selection is assumed to underlie the evolution of sociality, many vertebrates—including nearly half of all cooperatively breeding birds—form groups that also include unrelated individuals. Theory predicts that despite reducing kin structure, immigration of unrelated individuals into groups can provide direct, group augmentation benefits, particularly when offspring recruitment is insufficient for group persistence. Using population dynamic modeling and analysis of long-term data, we provide clear empirical evidence of group augmentation benefits favoring the evolution and maintenance of complex societies with low kin structure and multiple reproductives. We show that in the superb starling ( Lamprotornis superbus )—a plural cooperative breeder that forms large groups with multiple breeding pairs, and related and unrelated nonbreeders of both sexes—offspring recruitment alone cannot prevent group extinction, especially in smaller groups. Further, smaller groups, which stand to benefit more from immigration, exhibit lower reproductive skew for immigrants, suggesting that reproductive opportunities as joining incentives lead to plural breeding. Yet, despite a greater likelihood of becoming a breeder in smaller groups, immigrants are more likely to join larger groups where they experience increased survivorship and greater reproductive success as breeders. Moreover, immigrants form additional breeding pairs, increasing future offspring recruitment into the group and guarding against complete reproductive failure in the face of environmental instability and high nest predation. Thus, plural breeding likely evolves because the benefits of group augmentation by immigrants generate a positive feedback loop that maintains societies with low and mixed kinship, large group sizes, and multiple reproductives.

Harsh environmental conditions promote cooperative behavior in an epiphytic fern

“… 3 For example, cooperatively breeding birds are more prevalent in regions that experience harsh, unpredictable environments. 1 , 2 , 4 Results from this study indicate that similar relationships may also occur in plants.…”

Conditional reciprocity in invertebrates is robust to disturbances

“…To induce external disturbance, we removed the entire clutch, an event that might occur in natural condition if nest predation occurs. As these worms are marine, interstitial, and almost microscopic, there is no information available about their natural predators, but predation and, more generally, environmental instability, are likely ubiquitous and powerful selective pressure shaping morphological, life-history, and behavioral traits across species, including cooperation (Groenewoud et al, 2016;Shah & Rubenstein, 2023;Wheatcroft & Price, 2008).…”