Nj. Lunn et Jpy. Arnould, MATERNAL INVESTMENT IN ANTARCTIC FUR SEALS - EVIDENCE FOR EQUALITY INTHE SEXES, Behavioral ecology and sociobiology, 40(6), 1997, pp. 351-362
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.