Intraseasonal Variation in the Development of Sexual Size Dimorphism in a Precocial Bird: Evidence from the Lesser Snow Goose

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Among species which feed their young, particularly those with large size dimorphism, parental investment trade-offs between growth and survival of male and female offspring, and parental fitness, may be significant (sensu Trivers & Willard 1973). In contrast, little is known about the effects of variation in food supply on sex-differential growth and survival in species with precocial young. In such cases, where parental investment is generally smaller, the trade-off is more proximate ; how should individual offspring allocate resources to maximize their fitness. We examined this question by assessing the effects of seasonal variation in feeding conditions on growth and survival of male and female offspring of an obligate avian herbivore with precocial young, the lesser snow goose (Anser caerulescens caerulescens L.), using long-term observational data from 1969 to present. Snow geese show limited 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 analysis of this species indicated even small differences in growth rates may significantly affect the probability of survival. We found a highly significant difference in the relative body mass, but not structural size, of male and female goslings at fledging in response to seasonal declines in food supply, with males showing a greater proportional fledging mass decrease than females. Despite growth differences there was no detectable seasonal variation in gosling survival between the sexes, and no difference between male and female goslings in survival to fledging overall. Since the analysis involved only goslings which had survived to fledging, it can be difficult to determine if the seasonal decrease in dimorphism reflects either (i) seasonal differences in relative growth of male and female goslings, or (ii) increasing mortality of larger male goslings later in the season. However, the failure to find any sex-specific differences in mortality generally favours the hypothesis of proportionately slower growth of males hatched later in the season. The lack of sexual differences in survival despite measurable differences in sex-specific growth may reflect some level of adaptation to maximize the probability of survival of each sex. However, the possibility that the proportionately greater seasonal declines exhibited by males may reflect a simple constraint, rather than an adaptation, cannot presently be ruled out.