Fighting scars: heavier gladiator frogs bear more injuries than lighter frogs

Animal contests are energetically costly, but injuries are said to be rare. In gladiator frogs, the males possess a spine beneath their pollex (i.e., prepollex) that can be used as weapons and frequently leave scars during contests over spawning areas. Knowing how scars are made, and how scars are distributed among individuals, might prove valuable to address the costs and benefits of fighting. Here, we studied the gladiator frog Boana curupi in a Neotropical stream to (1) assess the rarity of the contested resource, (2) assess how spines are used during contests, and (3) test how the quantity of scars on males’ backs correlate to their morphology. We found only four spawning areas in ~ 1 km of stream. And when males engaged in physical fights, they stroke the rival on their backs with the prepollex. Scars on males’ back are thus caused by the prepollex. Both results explain the high frequency of injuries: 90.7% (29 of 32 males) presented scars. Furthermore, the amount of injuries on a male’s back increased with the weight of the male, but not its body size. Therefore, heavier individuals are sustaining more injuries than lighter individuals, hinting that heavier individuals fight more frequently and value reproduction more than lighter individuals.