Infanticide and within-clutch competition select for reproductive synchrony in a cooperative bird

Abstract

Reproduction among members of social animal groups is often highly synchronized, but neither the selective advantages nor the proximate causes of synchrony are fully understood. Here I investigate the evolution of hatching synchrony in the Greater Ani (Crotophaga major), a communally nesting bird in which several unrelated females contribute eggs to a large, shared clutch. Hatching synchrony is variable, ranging from complete synchrony to moderate asynchrony, and is determined by the onset of incubation of the communal clutch. Data from a 10-year field study indicate that individual reproductive success is highest in synchronous groups, and that nestlings that hatch in the middle of the hatching sequence are most likely to survive. Nestling mortality is high in asynchronous clutches because early-hatching nestlings are more likely to be killed by adult group members, whereas late-hatching nestlings are more likely to starve due competition with their older nest-mates. Therefore, the timing of hatching appears to be under stabilizing selection from infanticide and resource competition acting in concert. These results provide empirical support for models predicting that synchrony may evolve as an adaptive counter-strategy to infanticide, and they highlight the importance of competition in shaping the timing of reproduction in social groups.

Conditional female strategies influence hatching success in a communally nesting iguana

“…In fact, competition among females appears to be a major driver of reproductive synchrony—a recurrent pattern associated with group formation across diverse taxa (reviewed in Ims, ). This temporal clustering of reproductive events is consistent with stabilizing selection, as early parturition elevates the risk of infanticide by other gravid females, while late parturition leaves offspring vulnerable to starvation in competitions with older littermates (Ebensperger, Hurtado, & León, ; Hodge, Bell, & Cant, ; Poikonen, Koskela, Mappes, & Mills, ; Riehl, ).…”

Embryo movement is more frequent in avian brood parasites than birds with parental reproductive strategies

“…For example, if this embryonic trait has an adaptive benefit for brood parasites, then we would expect increased embryo movement to increase the parasitic chick's efficiency at evicting or killing host offspring. Additionally, it would be informative to measure embryo movement in other species that experience challenging nestling social environments, such as nest-sharing colonial breeders or species with large asynchronous clutches and/or high rates of siblicide [ 42 , 43 ]. Further to their relevance for brood-parasitic species, our findings suggest that embryo movement may be a generally overlooked process in the evolution of the diverse life histories, forms and behaviours observed in birds.…”

Can the morality of a group whose members are interdependent be extended to all humanity and beyond?

“…But egg ejection costs. Although three or four pairs defend a nest better than two, any asynchrony in readiness to lay causes so many eggs to be ejected from 3-and especially 4-pair nests that the anis abandon the nest before all have laid, perhaps forfeiting that season's reproduction (Riehl 2010(Riehl , 2011(Riehl , 2016).…”