The hidden links between animal weapons, fighting style and their expression on contest resolution

In many species that fight over resources, individuals use specialized structures to overpower their rivals (i.e. weapons). Despite their similar roles for contest settlement (i.e. affecting the winning chances), weapons are highly diverse morphological structures across species. However, the comprehension on how this diversity evolved is still open for debate. Unfortunately, most studies on how weapons are used during contests focus on size asymmetries between winners and losers. Although such information is important to understand, it does not provide much information regarding the mechanisms promoting weapon diversification. In addition, in many situations, the effect of weapon size on contest resolution may be confounded with the effect of body size due to allometric correlations. One way to decompose the relative importance of weapon and body size on contest outcome is to focus on the functional role of weapons. That is, investigate the function of the weapon during contests, and how size should affect its function. We propose two hypotheses that explicitly associate how weapon function may determine the relative importance of weapon and body size in affecting rivals during contests. One based on the internal morphology of the weapon — whether muscles are within the weapon or not — and the other based on fighting style — whether the weapon is used to injure the rival or not. To test our hypotheses, we developed a meta-analysis spanning 31 genera across the animal kingdom. We found that internal morphology and fighting style are important to determine the relative importance of weapon and body size on contest resolution. Weapons that do not have muscles within them (e.g., horns) affect more the chance of winning than weapons that have muscles within them (e.g., claws). Similarly, weapons used to throw/push rivals away (e.g., oversized mandibles) affect more the chance of winning than those used to make the rival flee (e.g., jaws). Overall, we have shown that weapon usage (i.e. weapon function) may affect weapon evolution in predictable ways: weapons without muscles and using for pushing may be selected for size, while muscled weapons used for injuring may be selected for strength.