Sexually-selected weapons have evolved to maximize individual reproductive success of males in polygynous breeding species that physically compete for access to mates. Yet, it remains poorly understood how secondary functions of weapons have also affected the evolution of specific traits. Here, we used 13 years of observational data from the long-term study of wolves and ungulates in Yellowstone National Park to evaluate how the threat of predation by a widespread, coursing predator has influenced weapon traits (e.g., size, antler retention time). We specifically evaluated how the presence or absence of antlers during the period of antler casting (i.e., March) affected whether individual adult male elk were killed by wolves. Moreover, we also assessed how the predatory escalation of wolf-adult male elk encounters was affected by whether adult male elk had antlers. We place our results in a broad context by comparing the traits of weapons across ungulate species.
Wolves strongly selected for adult male elk that had cast their antlers. The likelihood that an adult male elk killed by wolves was a pedicle individual (i.e., had cast its antlers) was affected by day of the month, elk population abundance, its age, and its nutritional condition. Interestingly, we found that wolves selected for pedicle adult male elk despite these individuals being in better nutritional condition. This finding is fundamentally opposed to previous work describing how coursing predators such as wolves select for individuals in poorer nutritional condition. Our results suggest that sexually-selected weapons may also be critical predatory deterrents that enhance survival and improve lifetime reproductive success. As such, species most preferred by predators should retain antlers for a longer period of time, despite the energetic costs associated with longer retention. This hypothesis is supported by considering that elk (the most strongly selected for species by wolves in northern Yellowstone) retain their antlers for ~1.5 and 2.5 months longer than deer and moose, respectively. Ultimately, our results suggest that ecological forces such as predation may be an underappreciated selective force on the specific traits of sexually-selected animal weapons.