Reproductive effort in relation to maternal social rank in reindeer (Rangifer tarandus)

Abstract

In polygynous mammals, high-quality females may increase their fitness by providing superior care to their offspring. Based on the agonistic interactions of female reindeer in an experimental herd during two consecutive years (1997 and 1998), we tested whether maternal social rank influenced: (1) winter body-mass change of females, (2) preparturition reproductive effort (measured as fecundity, the birth mass and the birth date of their calves), (3) preweaning maternal effort (measured as calves’ preweaning mortality, early preweaning and late preweaning growth rate and September body mass of calves), and (4) postweaning maternal effort (measured as calves’ body-mass change during their first winter). In the models, we included September females’ body mass as a covariate to separate the effects of maternal rank and body mass. We also tested whether the effect of social rank on maternal efforts was dependent on offspring sex. High-ranked females gained body mass whereas low-ranked females lost weight during the winter. Fecundity was higher and date of birth was earlier in high-ranked females than in subordinates, whereas no effect of females’ rank on birth mass of calves was found. Early preweaning growth rate and September body mass of calves increased with increasing females’ social rank, whereas late preweaning daily growth rate of calves was not influenced by females’ rank. Calves’ preweaning mortality was only influenced by year, which also explained most of the variance in the winter body-mass change of calves. The effects of females’ rank on the reproductive-efforts parameters studied were not specific to offspring sex. These findings suggest that females’ rank influences reproductive effort during the preparturition, as well as the preweaning, period, the effect being sex independent.