Sexually selected weapons can wear out, decreasing their effectiveness in combat.

Sexual selection has resulted in some of the most elaborate traits seen in animals, many of which are used as weapons. These weapons can be incredibly diverse, even within species. Such morphological variation has largely been attributed to the environment in which individuals are reared and their genetics. However, variation in weapon form could also be the result of a weapon wearing out from usage. This mechanism has received relatively little attention. In this study, we demonstrate that sexually selected weapons can wear out from repeated use, providing experimental evidence that weapon usage can contribute to the diversity of weapon shapes observed within species. In a second experiment, we demonstrate that having a worn-out weapon decreases an individual's fighting ability. This finding illustrates that the shape of a weapon can have an important role in determining contest outcomes. Overall, these results suggest that individuals are limited in the number of times they can effectively use their weapons, which may be one factor (among others) influencing how frequently an animal engages in a fight.