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Abstract Large mammals at northern latitudes show annual cycles of body mass gain in summer and body mass loss in winter. The amplitude and seasonal timing of these cycles may vary through ontogeny depending on trade‐offs toward investments in structural growth versus fat storage, reproduction, and future survival. Despite this knowledge, there is no detailed model of how the seasonality of body mass develops through ontogeny. Here, we define a new seasonal growth model that accounts for shifting seasonality through two sine components: one included in the growth coefficient and the other on the asymptotic size. We fitted 12 candidate models to longitudinal data on body mass of captive male and female red deer (Cervus elaphus) in Norway, with different baseline growth structures (von Bertalanffy, Gompertz, and Brody) and including zero, one, or both of the seasonal components. The best fitting model was the Brody model with both seasonal components included, allowing the annual peak to shift through ontogeny: The annual peak occurred in December for calves, shifting to November in yearlings, and October in 2‐ and 3‐year‐olds, ending with September for adults. All age classes showed an annual minimum at the end of winter around March. Males and females showed similar seasonal peaks through ontogeny, although males grow bigger and have larger seasonal amplitudes. Our new growth model provides a flexible framework to model seasonal growth changing through ontogeny, applicable to different species. |
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