Male-male competition is not costly to dominant males in a cooperatively breeding bird

Abstract

Male-male reproductive competition occurs in many animal societies and can be costly, both through aggression or energy expenditure prior to mating and lost paternity. In most cooperative breeders, socially dominant males breed more often than do subordinates, but the costs of pre-copulatory subordinate male reproductive competition (including unsuccessful competition) have rarely been investigated in these systems. Here, we examine the association between such competition and the fitness of dominant males in the cooperatively breeding southern pied babbler (Turdoides bicolor). Babbler groups comprise a dominant breeding pair with adult subordinates of both sexes. Roughly 35 % of adult subordinate males live in groups with unrelated adult females, but despite their common engagement in active reproductive competition, these subordinate males rarely successfully breed (extra-group mating has never been detected). Overall, active reproductive competition with subordinate males is not costly to dominant males, notwithstanding that rare episodes of breeding by subordinate males cause large paternity losses (50-100 %). Specifically, the presence of an actively competing subordinate male does not affect the proportion of successful nests, the number of fledglings fathered by the dominant male or dominant male retention of dominance to the next breeding season. Instead, dominant male breeding success is likely to be more affected by factors such as food availability and group size. In the social context of cooperative breeding, a competing subordinate male may provide benefits of a larger group size or increased help; these may offset the low risk of paternity loss, possibly promoting tolerance of reproductive competitors by dominant males.

Nepotism and subordinate tenure in a cooperative breeder

“…However, a quantitative comparison of the length of subordinate tenure of males and females, especially with respect to the presence of same-versus opposite-sex close kin, has not been conducted. Direct comparison of males and females is important because, in most species, sexes vary in dispersal distance, timing or tactics, and the strength of nepotism or the cost of sexual competition may vary with subordinate sex or that of the unrelated dominant [5,6,9,[11][12][13].…”

“…When subordinates attempt to breed, intrasexual competition with dominant individuals results; such competition is costly to the reproductive success of dominant females (competitive subordinate females destroy eggs), though not to dominant males [9,13]. Subordinates benefit from delaying dispersal: floaters fare badly compared with when they reside in groups [15], and older subordinates are more likely to attain dominance [12].…”

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

“…For example, competition by subordinate females causes many breeding attempts to fail before incubation begins (Nelson‐Flower et al., ). Previous work has identified subordinate nest‐building and initiation of courtship as behaviours that indicate reproductive competition (and not cooperation) in both sexes; adult subordinates demonstrate these behaviours from the age of 11 months old only when their groups contain unrelated potential breeding partners (Nelson‐Flower & Ridley, ; Nelson‐Flower et al., ). We excluded nests built during winter months (seven of 215 nests observed) because for most years, observers were not present during winter months, and winter conditions vary markedly from breeding season conditions.…”

“…Some subordinates are unrelated to both dominants, which can occur when the original dominant pair of the group have both been replaced or if the subordinate is itself an immigrant (although subordinate immigration is comparatively rare; Nelson‐Flower et al., ). Subordinates with unrelated potential breeding partners in the group often attempt to breed, sometimes gaining a proportion of parentage in broods initiated by the socially dominant pair (Nelson‐Flower & Ridley, ; Nelson‐Flower et al., , ). Southern pied babbler groups build a single nest per breeding attempt, and subordinate female babblers breed only by laying fertilized eggs in the clutch of the dominant pair, while subordinate males breed with a group dominant or subordinate female that subsequently successfully lays in this nest.…”

“…Female competition for reproduction within groups is costly and can involve the repeated destruction of eggs by competitors, resulting in a decrease in group productivity (Nelson‐Flower et al., ). In contrast to females, competition by subordinate males does not affect group productivity, although subordinate males gain large shares of paternity on the rare occasions when they do successfully breed (Nelson‐Flower & Ridley, ). Within groups, age‐based hierarchies or queues exist for each sex, such that the eldest subordinate inherits the dominance position in the group when it becomes vacant and if the current opposite‐sex dominant is unrelated (Nelson‐Flower et al., ).…”