EVIDENCE OF EQUAL MATERNAL INVESTMENT IN THE SEXES IN THE POLYGYNOUS AND SEXUALLY DIMORPHIC GREY SEAL (HALICHOERUS-GRYPUS)

In polygynous and sexually dimorphic mammals, parents may be expected to bias their investment towards sons because variation in reproductiv e success is usually higher among males than among females. Moreover, male reproductive success often depends on adult body size, which, in turn, may depend on the level of parental investment. We therefore pre dicted that in the grey seal (Halichoerus grypus), a polygynous and se xually dimorphic phocid seal, females should invest more in individual sons than in individual daughters. We found that male pups were born heavier than female pups, but that the growth rates and suckling behav iour were similar for the two sexes. The growth rates and the birth we ights were not correlated for the pups of either sex. Mothers did not behave differentially towards offspring of the two sexes, except that mothers of male pups spent more time in visual contact with their pups . Male and female pups had similar activity levels and begged at simil ar rates. We argue that reports of equal expenditure on the two sexes can be accepted as evidence of equal investment, provided that three a ssumptions are fulfilled. First, parental care must be costly to the p arent. Second, energy expenditure must be the most important component of parental investment. Third, there must be no negative correlation between maternal body condition and the ratio of sons to daughters pro duced. We argue that these assumptions are met in our study, and that our results provide evidence of equal maternal investment in the sexes in grey seals.