Opposing Responses to Scarcity Emerge from Functionally Unique Sociality Drivers

Abstract

From biofilms to whale pods, organisms across taxalive in groups, thereby accruing numerous diverse benefits of soci-ality. All social organisms, however, pay the inherent cost of in-creased resource competition. One expects that when resources be-come scarce, this cost will increase, causing group sizes to decrease.Indeed, this occurs in some species, but there are also species forwhich group sizes remain stable or even increase under scarcity.What accounts for these opposing responses? We present a concep-tual framework, literature review, and theoretical model demon-strating that differing responses to sudden resource shifts can beexplained by which sociality benefit exerts the strongest selectionpressure on a particular species. We categorize resource-relatedbenefits of sociality into six functionally distinct classes and modeltheir effect on the survival of individuals foraging in groups underdifferent resource conditions. Wefind that whether, and to what de-gree, the optimal group size (or correlates thereof) increases, de-creases, or remains constant when resource abundance declinesdepends strongly on the dominant sociality mechanism. Existingdata, although limited, support our model predictions. Overall,we show that across a wide diversity of taxa, differences in howgroup size shifts in response to resource declines can be driven bydifferences in the primary benefits of sociality.