Sex ratio variation in a eusocial mammal, the Damaraland mole-rat, Fukomys damarensis

Abstract

Sex allocation theory predicts that mothers benefit from adjusting the sex ratio of their offspring in relation to their offspring's future reproductive success. In cooperative breeders, parents are expected to bias the sex ratio in relation to their current need for help and the benefit received from helpers of each sex as proposed by the local resource enhancement (LRE) and helper repayment hypotheses (HR). Consequently, as group size increases, sex ratios are expected to be biased towards the sex that is more likely to disperse to avoid competition as proposed in the local resource competition hypothesis (LRC). The current study aimed to investigate helper effects on breeder fecundity and offspring sex ratio adjustments in a eusocial mammal the Damaraland mole-rat Fukomys damarensis. Both sexes equally contribute to helping in this species, but breeding dispersal is male biased. We found no evidence for helper effects on maternal body mass and litter size. Offspring sex ratio was not affected by maternal mass or litter size. However, the probability of male pups to be born increased with the number of female helpers and decreased with the number of male helpers. In addition, the number of male pups in a litter increased significantly with the number of female but not male helpers, while no helper effects were apparent for the number of female pups born. Hence, our data suggest that the mechanisms underlying the LRE, HR and LRC may operate simultaneously in the study species.