Androgens, antlers, and sexual selection: testosterone's relationship to reproductive success and associated morphological characteristics in white-tailed deer

While hormones such as testosterone are known to drive reproduction and sexually selected traits in many species, research demonstrating a relationship between testosterone and annual or overall reproductive success is sparse. We sought to directly measure how circulating testosterone relates to sexually selected characteristics and reproductive success in a freely breeding population of white-tailed deer (Odocoileus virginanus). We captured individuals during September-March annually from 2007 to 2017 and investigated the association between testosterone and antler size, body size, and annual reproductive success for individuals. We also assessed lifetime patterns from individuals captured multiple years. We found a positive relationship between testosterone and body size, and between testosterone and antler size. However, we found a significant interaction between testosterone and age for both body size and antler size response variables, indicating that as age increases, the aforementioned positive relationships with testosterone diminish. Thus, for deer >= 6.5 years of age, testosterone is negatively related to body and antler size. In contrast, annual reproductive success, measured by number of offspring sired and recruited into the breeding population, was positively associated with antler size and body size, but not testosterone, age, or an interaction between testosterone and age. Instead, while an individual's average lifetime testosterone had no relationship with overall reproductive success, greater variation in lifetime testosterone was positively related to overall reproductive success. Thus, while we found no significant association between testosterone and annual reproductive success directly, testosterone may be indirectly related to reproductive success through its positive association with antler and body size. Furthermore, understanding how testosterone may fluctuate throughout an individual's lifetime may offer new perspectives in understanding variation in individual reproductive success.