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.

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We thank Dr. Marcelo M. Dalosto for helping with the formal descriptions of the contest, and Dr. Danilo Muniz for the title idea. We also thank the anonymous reviewers for their comments that improved the quality of the manuscript.


AVP thanks FAPESP for the post-doctoral research grant (process no. 2016/22679-3).

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Correspondence to Alexandre V. Palaoro.

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All applicable international, national, and institutional guidelines for the care and use of animals were followed. This article does not contain any studies with human participants performed by any of the authors. For the frogs, we sampled and released frogs according to state and federal laws under the following licenses: SEMA license number 598 and IBAMA license number 59978-1.

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Candaten, A., Possenti, A.G., Mainardi, Á.A. et al. Fighting scars: heavier gladiator frogs bear more injuries than lighter frogs. acta ethol 23, 39–44 (2020).

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  • Animal contests
  • Animal weapons
  • Damage
  • Lethal weapons
  • Amphibian contest