MATERNAL INVESTMENT IN ANTARCTIC FUR SEALS - EVIDENCE FOR EQUALITY INTHE SEXES

Studies of the otariids (fur seals and sea lions), a highly sexually d imorphic group, have provided conflicting evidence of differential mat ernal expenditure in male and female offspring and, thus, suggestions that they conform to predictions of investment theory are equivocal. S ince the mid-1970s, a diversity of research on Antarctic fur seals (Ar ctocephalus gazella) including studies of their reproductive ecology, lactation energetics, and foraging behaviour have been conducted at Bi rd Island, South Georgia that have resulted in one of the more complet e and diverse data sets for any species of otariid. These long-term da ta were reviewed to determine whether there was any evidence to suppor t that differential maternal expenditure occurred in Antarctic fur sea ls. Most of the data examined were collected during five consecutive a ustral summers from 1988 through 1992 and included years in which loca l food resources were abundant and scarce. We were unable to detect di fferences in the sex ratios of pups at birth or sex-biased differences in growth rates estimated from serial data, the number of foraging tr ips made, the duration of attendance ashore, diving behaviour, sucklin g behaviour, or milk consumption in any year and in the duration of fo raging trips or age at weaning in 2 of 3 years. In addition, we found no evidence of greater reproductive costs between mothers with sons or daughters relative to their reproductive performance the following ye ar. In contrast, sex-biased differences were only found in the duratio n of foraging trips in 1990, the age at weaning in 1988, and consisten tly in growth rates estimated from cross-sectional data. We suggest th at differential maternal expenditure does not occur in Antarctic fur s eals because male pups probably do not gain greater benefit from addit ional maternal expenditure than female pups. After weaning, males expe rience a period of rapid juvenile growth over 3-4 years during which t ime body mass nearly trebles. This growth will almost certainly be dep endent upon available food resources then rather than on any maternal expenditure received over the first 4 months of life and, thus, the as sumptions of the Trivers and Willard hypothesis are probably invalid f or Antarctic fur seals.