The authors have declared that no competing interests exist.
Cooperative breeding, in which more than a pair of conspecifics cooperate to raise young at a single nest or brood, is widespread among vertebrates but highly variable in its geographic distribution. Particularly vexing has been identifying the ecological correlates of this phenomenon, which has been suggested to be favored in populations inhabiting both relatively stable, productive environments and in populations living under highly variable and unpredictable conditions. Griesser et al. provide a novel approach to this problem, performing a phylogenetic analysis indicating that family living is an intermediate step between nonsocial and cooperative breeding birds. They then examine the ecological and climatic conditions associated with these different social systems, concluding that cooperative breeding emerges when family living is favored in highly productive environments, followed secondarily by selection for cooperative breeding when environmental conditions deteriorate and within-year variability increases. Combined with recent work addressing the fitness consequences of cooperative breeding, Griesser et al.’s contribution stands to move the field forward by demonstrating that the evolution of complex adaptations such as cooperative breeding may only be understood when each of the steps leading to it are identified and carefully integrated.
Soon after W. D. Hamilton revolutionized behavioral ecology with his ground-breaking papers formalizing the theory of inclusive fitness [
What drives the evolution of cooperative breeding? One clearly important factor is Hamiltonian kin selection, although the precise role that genetic relatedness plays is to some extent controversial. In support of the hypothesis that kin selection plays a key role in providing fitness benefits to helpers, the majority of cooperative breeders are indeed composed of family groups. An intuitively pleasing extension of this hypothesis is the idea that monogamy, because of its presumed role in enhancing genetic relatedness within family groups, has been foundational to the evolution of cooperative breeding and eusociality [
Beyond the importance, both real and potential, of kinship and inclusive fitness, the one thing that almost all workers agree on is that ecological factors play a key role in driving cooperative breeding. The earliest and most widespread ecological hypothesis for cooperative breeding focuses on what are now generally known as “ecological constraints” or “habitat saturation.” Based originally on a proposal by Robert Selander to explain cooperative breeding in
But, alas, such optimism was short-lived. Not only were flaws in the logic of habitat saturation pointed out—including the fact that many or even most species are ecologically constrained in some way but do not delay dispersal or breed cooperatively [
The field of cooperative breeding has progressed in many ways since these early studies [
One relevant observation is that both these concepts, despite being virtual opposites, involve constraints—the first on obtaining a reproductive position and the second on successful breeding once a position is obtained [
Although early papers distinguished delayed dispersal from helping behavior, particularly when considering their fitness consequences [
Griesser et al. [
They find that the best-fitting model includes transitions between all pairs of the 3 categories, but that the transition rate from nonfamily living directly to cooperative breeding is rare compared to the transition rate from nonfamily living to cooperative breeding via the intermediate stage of family living without cooperative breeding. In other words, cooperative breeders almost always evolve from family-living but noncooperative-breeding ancestors. This insight sets the stage for a multinomial analysis investigating the ecological and climatic correlates of the 3 major categories of species, thus focusing on the potential drivers of family living separate from that of cooperative breeding.
And here’s where it gets exciting. Griesser et al. find that the apparent ecological factors distinguishing nonfamily-living species from both family-living and cooperative-breeding species are generally similar, with both of the latter species tending to occur in habitats in which rainfall is greater and growing seasons are longer. Cooperative breeders, however, are more likely to be found in environments with higher within-year variability in environmental productivity compared to family-living species that are not cooperative breeders.
These results suggest a novel resolution to the conundrum of how 2 apparently contradictory environmental conditions appear to drive cooperative breeding. Relatively stable, productive conditions favor the transition from nonfamily living to the intermediate stage of family living, whereas subsequent evolution of cooperative breeding is favored when conditions subsequently deteriorate, becoming less productive and more variable. Such a scenario fits in well with the current geographic distribution of cooperative breeding, which occurs disproportionately in Australia, southern Africa, and northern South America—places that have undergone dramatic climatic changes from past geological epochs, resulting in less productive and more variable conditions that may have favored the evolution of cooperative breeding from family-living ancestors.
Although Griesser et al.’s paper is focused primarily at the level of evolutionary origins, their hypothesis also has implications for the fitness benefits associated with family living and cooperative breeding. As such, it dovetails with recent work by Shen et al. [
Thus, both papers are interested in the ultimate drivers of cooperative breeding and attempt to explain how very different environmental conditions appear to drive cooperative breeding but at complementary time scales and levels of analysis. It is the longer temporal scale combined with the 2-step evolutionary progression leading to cooperative breeding envisioned by Griesser et al. that allow their analysis to potentially explain the ecological and climatic factors leading to the highly heterogeneous incidence of cooperative breeding observed on a continental scale that has until now gone largely unexplained.
The ultimate goal of these, as well as other recent broad-scale investigations of social behavior [
I thank my colleagues Janis Dickinson, Steve Emlen, Michael Griesser, Sheng-Feng Shen, and Dustin Rubenstein for their insights and comments.