A SUBSTANTIAL ENERGETIC COST TO MALE REPRODUCTION IN A SEXUALLY DIMORPHIC UNGULATE

Abstract

Whereas the energetic costs of reproduction for female ungulates are well documented, those of males are not. We investigated age- and sex-specific changes in the kidney fat reserves of a large sample of Himalayan tahr (Hemitragus jemlahicus), a highly sexually dimorphic and polygynous mountain ungulate, during and following the mating period. Subadult male tahr (2–4 years old) obtain copulations opportunistically by “coursing,” whereas adult males (>4 years old) spend more time traveling and searching for females, and obtain copulations after engaging in more time-consuming “tending” and “blocking” displays. If there is a cost to these reproductive behaviors, we predicted that (1) the kidney fat index (KFI) of reproductive males (≥2 years old) should decline during the mating period relative to females and nonreproductive males, and (2) that the KFI of adult males should decline more than that of subadult males. We formulated our predictions into a series of candidate models, including three covariates also likely to affect fat reserves, and compared support for these models using Akaike's Information Criterion (AIC). The best-fitting model revealed that fat reserves in females and nonreproductive males changed similarly: Fat reserves increased during the mating period, in late autumn and early winter, reached a peak in mid-winter, and then declined throughout the remainder of winter and early spring. In contrast, the fat reserves of reproductive males declined during the mating period, as predicted, and remained low throughout winter. The extent of decline in fat reserves was greater in adult males than in subadult males, also as predicted. Our results strongly imply that male tahr incur a substantial energetic cost for engaging in reproduction and that they trade off reproductive success with reduced body condition during winter, which may lower survival.