Age and sex specific allometric growth of antlers in Rangifer tarandus: variability in the pattern of resource allocation

Sex-specific tradeoffs between body mass (linked to survival) and costly antlers (linked to reproductive success) are expected in sexually dimorphic ungulates. Unique among cervidae, female reindeer (Rangifer tarandus) possess antlers, providing an opportunity to investigate sex- and age-specific patterns of these tradeoffs. Using long-term (1996-2011) individual-based data (e.g. age, body mass, antler length) from a semi-domestic reindeer population in northern Finland, we established antler and body mass growth patterns and assessed the trends of resource allocations towards antlers relative to body mass, using allometric analysis. Sexual dimorphism was apparent in antler and body mass growth curves. Antler growth of males exhibited a steep incline without a plateau by five years, while body mass growth had a less steep incline with a slight decline of growth by five years of age. After an initial incline, female antler and body mass growth patterns showed a plateau near the age of three. All age and sex categories, apart from yearling males and mature females, exhibited positive allometry of antlers, meaning the rate of increase in antler length was higher than that of body mass. Relative allocation of resources towards antlers was highest in female calves, with both sexes exhibiting high allometric exponents as calves and decreasing allocations in yearlings. Male adults increased allocations, while females tended to decrease allocations further with age. Our results exhibit a varying allocation strategy towards antlers, relative to body mass, in reindeer, with clear age-specific sexual dimorphism in relative allocations