Sex differences in the drivers of reproductive skew in a cooperative breeder

Abstract

Many cooperatively breeding societies are characterized by high reproductive skew, such that some socially dominant individuals breed, while socially subordinate individuals provide help. Inbreeding avoidance serves as a source of reproductive skew in many high-skew societies, but few empirical studies have examined sources of skew operating alongside inbreeding avoidance or compared individual attempts to reproduce (reproductive competition) with individual reproductive success. Here, we use long-term genetic and observational data to examine factors affecting reproductive skew in the high-skew cooperatively breeding southern pied babbler (Turdoides bicolor). When subordinates can breed, skew remains high, suggesting factors additional to inbreeding avoidance drive skew. Subordinate females are more likely to compete to breed when older or when ecological constraints on dispersal are high, but heavy subordinate females are more likely to successfully breed. Subordinate males are more likely to compete when they are older, during high ecological constraints, or when they are related to the dominant male, but only the presence of within-group unrelated subordinate females predicts subordinate male breeding success. Reproductive skew is not driven by reproductive effort, but by forces such as intrinsic physical limitations and intrasexual conflict (for females) or female mate choice, male mate-guarding and potentially reproductive restraint (for males). Ecological conditions or "outside options" affect the occurrence of reproductive conflict, supporting predictions of recent synthetic skew models. Inbreeding avoidance together with competition for access to reproduction may generate high skew in animal societies, and disparate processes may be operating to maintain male vs. female reproductive skew in the same species.

Population level consequences of facultatively cooperative behaviour in a stochastic environment

“…Therefore, fitness is determined by the combined patterns of survival and reproduction at each stage and age, st(age), of an individual. Helpers typically do not reproduce or have very low reproductive success (co-breeding, Kaiser et al, 2019;Li & Brown, 2002;Nelson-Flower et al, 2018), while subordinate non-helpers do not reproduce (Clayton & Emery, 2007).…”

Kidnapping intergroup young: an alternative strategy to maintain group size in the group-living pied babbler (Turdoides bicolor)

“…The answer to this may lie in imperfect kin recognition systems: if young learn to recognize individuals based on who fed them during their developmental period [ 22 , 45 , 46 ], then provisioning behaviour by kidnappers could suffice to establish a relationship between kidnapper and abductee similar to the bonds observed in kin relationships [ 46 ]. Abductees may then become helpers in their new group and provide future benefits for their kidnappers, without incurring costs via intragroup reproductive competition, as has previously been recorded for this species [ 47 , 48 ]. This future helping behaviour has been observed in both pied babblers and white-winged choughs [ 25 ], suggesting that kidnapping behaviour can result in multiple benefits and may indeed be an adaptive, facultative response when intragroup breeding attempts fail.…”

The benefits of pair bond tenure in the cooperatively breeding pied babbler (Turdoides bicolor)

“…Individuals typically must be part of the dominant breeding pair (one such pair per group) in order to breed (Nelson‐Flower et al., ). Although subordinate reproduction does occur, it is very rare (Nelson‐Flower, Flower, et al., ). In each group, the dominant pair enforce their dominance through agonistic displays and physical attacks on subordinates (Raihani, ).…”

“…Pied babbler pairs form monogamous long‐term pair bonds within groups, with very low extrapair parentage: 92.3% of offspring are progeny of the dominant pair (Nelson‐Flower, Flower, & Ridley, ). Pied babblers are a long‐lived passerine, with some individuals reaching more than 10 years of age in the wild (Ridley, ); thus, pair bonds can persist for many years.…”