Climate predicts which sex acts as helpers among cooperatively breeding bird species

Abstract

Among avian cooperative breeders, help in raising offspring is usually provided by males or by both sexes. Sex bias in helping should evolve in response to sex-specific ecological constraints on independent reproduction, with mate shortage for males and breeding vacancy shortage for each sex. Given that male-biased adult sex ratios are prevalent among birds, we predict that maleonly helping mainly occurs in temperate species where fast population turnovers deriving from low adult annual survival allow all adult females to hold breeding vacancies, whereas some males overflow as helpers, and bothsex helping in tropical species where saturated habitats prevent not only males, but also females from breeding themselves. As expected, we found that across species, adult survival increased towards tropical zones and warmer climates, and higher adult survival tended to be associated with both-sex helping. Furthermore, sex bias in helping was predicted by latitude and ambient temperature. Our findings of demographic response of species to climate as a potential determinant of bias in helper sex uncover how ecological constraints operate to limit independent reproduction in sex-specific ways.

Intersexual Resource Competition and the Evolution of Sex-Biased Dispersal

“…At present, efforts to evaluate this are hampered by the fact that broad patterns of sex-biased dispersal tend to be evaluated with respect to dispersal probability and/or dispersal distance (Clarke et al, 1997;Mabry et al, 2013;Trochet et al, 2016;Li and Kokko, 2019). We have shown that timing can also be kin-selected, but the comparative studies available to evaluate this factor are rather focused on delayed dispersal in cooperative breeding, where philopatry also offers individuals the opportunity to become helpers (Zhang et al, 2017). These additional kin-selected opportunities are interesting in their own right (Johnstone and Cant, 2008), but our model shows that interesting phenomena can arise even if the only form of helping and harming is the choice between dispersal and continuing to deplete local resources, with no other layers of social complexity.…”

Social organization, demography and genetic mating system of a Tibetan cooperative breeder

“…A recent comparative analysis showed that mate limitation is more likely to operate in temperate regions and leads to male-only helping, whereas breeding-vacancy limitation tends to apply to subtropical and tropical regions and promotes helping by both sexes (Zhang et al 2017). The former case should be true for the Ground Tits where the two conditions required for the mate constraint hypothesis are met.…”

Reply to Engelhardt et al.: Inclusive fitness does maintain a heritable altruism polymorphism in Tibetan ground tits

“…This was also the case for territory quality, which was a poor predictor of a helper's presence (2). Compared with the only previous estimate for the heritability of helping behavior in the wild (8), ours (2) was based on a more accurate pedigree, and the result (0.47) represents a relatively high value that indicates the role of genes (9), although helping behavior can also be promoted by environments (10). We encourage future studies to take into account more factors such as maternal and dominant effects to obtain further details about the genetic component of this important social behavior.…”