Wolves preferentially target male deer that lack antlers, finds a study published online this week in Nature Ecology & Evolution. This presents a problem for deer, which shed their antlers early to improve future mating opportunities once they grow back, at the risk of losing their defence against predators.

Male elk (a North American species of deer) shed their antlers over two-three months following their mating period. New antlers begin to regrow immediately, so that they are ready in time to use as weapons against rival males in the next mating season. Males lose their antlers at different times, however - even though early shedding allows for larger antlers to grow by the next mating season. This leads to greater success over rivals and thus more mating opportunities.

Matthew Metz and colleagues investigate whether there may be hidden costs to this early shedding by observing male elk over a period of 13 years in Yellowstone National Park, USA. They find that groups of males were ten times more likely to be attacked by wolves if they contained at least one individual without antlers. This led to a greater risk of dying for individuals within the elk population without antlers, as wolves strongly preferred antlerless individuals, despite being in better health.

These findings suggest that, alongside serving as a weapon when competing for mates, antlers also perform a secondary function in deterring predators. However, the need to regrow antlers results in a trade-off between these two functions - one that is likely to have influenced the elk’s evolution.