INTRASEASONAL VARIATION IN THE DEVELOPMENT OF SEXUAL SIZE DIMORPHISM IN A PRECOCIAL BIRD - EVIDENCE FROM THE LESSER SNOW GOOSE

1. Among species which feed their young, particularly those with large size dimorphism, parental investment trade-offs between growth and su rvival of male and female offspring, and parental fitness, may be sign ificant (sensu Trivers & Willard 1973). In contrast, little is known a bout the effects of variation in food supply on sex-differential growt h and survival in species with precocial young. In such cases, where p arental investment is generally smaller, the trade-off is more proxima te; how should individual offspring allocate resources to maximize the ir fitness. We examined this question by assessing the effects of seas onal variation in feeding conditions on growth and survival of male an d female offspring of an obligate avian herbivore with precocial young , the lesser snow goose (Anser caerulescens caerulscens L.), using lon g-term observational data from 1969 to present. 2. Snow geese show lim ited sexual size dimorphism, with males being 2-6% larger at all ages post-hatching. Growth of snow goose goslings has been previously shown to be extremely sensitive to variation in food supply, and previous a nalysis of this species indicated even small differences in growth rat es may significantly affect the probability of survival. 3. We found a highly significant difference in the relative body mass, but not stru ctural size, of male and female goslings at hedging in response to sea sonal declines in food supply, with males showing a greater proportion al fledging mass decrease than females. Despite growth differences the re was no detectable seasonal variation in gosling survival between th e sexes, and no difference between male and female goslings in surviva l to fledging overall. 4. Since the analysis involved only goslings wh ich had survived to hedging, it can be difficult to determine if the s easonal decrease in dimorphism reflects either (i) seasonal difference s in relative growth of male and female goslings, or (ii) increasing m ortality of larger male goslings later in the season. However, the fai lure to find any sex-specific differences in mortality generally favou rs the hypothesis of proportionately slower growth of males hatched la ter in the season. The lack of sexual differences in survival despite measurable differences in sex-specific growth may reflect some level o f adaptation to maximize the probability of survival of each sex. Howe ver, the possibility that the proportionately greater seasonal decline s exhibited by males may reflect a simple constraint, rather than an a daptation, cannot presently be ruled out.