Gender Differences in the Costs that Subordinate Group Members Impose on Dominant Males in a Cooperative Breeder

Abstract

In group‐living species, a dominant male’s ability to monopolize reproduction, and the cost of doing so, are expected to vary with a group’s gender composition. We used spawning observations of a group‐living cichlid, Neolamprologus pulcher, to test this expectation. We constructed groups that contained a dominant breeding pair and either two male subordinates, one male and one female subordinate or two female subordinates. Parasitic spawning by male subordinates was more common in groups with two male subordinates than in groups with one male and one female subordinate. Female subordinates were never observed laying eggs in dominant females’ clutches, but three female subordinates laid independent clutches. During spawning, frequencies of dominant male aggression towards male and female subordinates were similar. Dominant males were less aggressive during non‐reproductive periods. The declines were greater for female subordinates, such that, during non‐reproductive periods, dominant males were more aggressive towards male subordinates. Aggression towards each subordinate was also affected by the second subordinate’s gender; the direction of that effect differed for large and small subordinates. Male subordinates approached breeding shelters less often than female subordinates, and both male and female subordinates approached shelters more frequently when the second subordinate was male. Collectively, these patterns suggest: (1) that male subordinates impose higher costs on dominant males than female subordinates do and (2) that the presence of a second male subordinate imposes additional costs beyond those of the first male subordinate. We discuss the implications of these effects for dominant and subordinate group members.

Smell or vision? The use of different sensory modalities in predator discrimination

“…Several alternative mechanisms might be responsible for this result, including sex-specific differences in personality (e.g. Schurch and Heg 2010 ), metabolic differences and ensuing differences in hunger state between sexes, or the different sex-specific roles and behaviours of male and female helpers (Mitchell et al 2009 ) and breeders (Taborsky and Limberger 1981 ; Heg and Taborsky 2010 ).…”

Group size adjustment to ecological demand in a cooperative breeder

“…Instead, regardless of the sex of the subordinates, individuals that are only slightly smaller than female breeders might induce higher potential costs to the pair, including for example an increased risk of egg cannibalism, as the submissive status of large helpers is an important precondition for them to care for eggs instead of eating them [79]. Unlike in cooperatively breeding mammals, where eviction is under female control [33,77], in N. pulcher both males and females participate in regulating group size [80]. Generally, the potential costs caused to breeders by opposite-sex subordinates are presently not well understood in cooperatively breeding vertebrates.…”

Helper Response to Experimentally Manipulated Predation Risk in the Cooperatively Breeding Cichlid Neolamprologus pulcher

“…Combining these facts, we conclude subordinates provide active, costly help and therefore they are usually called ‘helpers’. Nevertheless, helpers also impose costs on the breeders, because large mature male helpers engage in parasitic spawning [20] – [22] , [49] , large mature female helpers may try to breed with the breeder male [33] , [37] – [38] also in a separate breeding shelter [38] , which may draw away breeder male or helper assistance from the primary female's brood [45] , and both helper males and females may compete for the breeding position with the breeder male and female, respectively [25] . These effects are also apparent in sex-dependent dominant-subordinate interactions [50] .…”

“…decrease the level of aggressive interactions with neighbouring groups [51] . Second, the group composition might affect the level of helping and social behaviours [49] , [50] . To test for both these effects, we first constructed GEEs with treatment, helper size and their interaction as independent variables; helper feeding rate and total food intake (normal distributions) or helper behaviour (digging, territory defence, within-group aggression or within-group social contacts; all frequencies with poisson distributions and log-link) were the dependent variables in each GEE ( n = 50 groups).…”