Defensive structures influence fighting outcomes

Abstract

In many animal species, individuals engage in fights with conspecifics over access to limited resources (e.g. mates, food, or shelter). Most theory about these intraspecific fights assumes that damage has an important role in determining the contest winner. Thus, defensive structures that reduce the amount of damage an individual accrues during intraspecific competition should provide a fighting advantage. Examples of such damage‐reducing structures include the dermal shields of goats, the dorsal osteoderms of crocodiles, and the armoured telsons of mantis shrimps. Although numerous studies have identified these defensive structures, no study has investigated whether they influence the outcomes of intraspecific fights. Here we investigated whether inhibiting damage by enhancing an individual's armour influenced fighting behaviour and success in the giant mesquite bug, Thasus neocalifornicus (Insecta: Hemiptera: Coreidae). We found that experimentally manipulated individuals (i.e. those provided with additional armour) were 1.6 times more likely to win a fight when compared to the control. These results demonstrate that damage, and damage‐reducing structures, can influence fighting success. The implications of these results are twofold. First, our results experimentally support a fundamental assumption of most theoretical fighting models: that damage is a fighting cost that can influence contest outcomes. Second, these results highlight the importance of an individual's defensive capacity, and why defence should not be ignored. A free Plain Language Summary can be found within the Supporting Information of this article.

Weapon performance drives weapon evolution

“…relatively wide femurs) inflict more severe damage than others. Both theoretical [12][13][14][15][16][17] and empirical evidence [18,19] suggests that damage can have an important role in determining the contest winner. Thus, our results support the fighting-advantage hypothesis.…”

“…There are several ways that weapon modifications could provide a fighting advantage. For weapons that inflict injuries during fights, both theoretical [12][13][14][15][16][17] and empirical evidence [18,19] suggests that damage can have an important role in determining the contest winner. Thus, one way a fighting advantage could manifest itself is through an increase in the amount of damage that a weapon causes.…”

“…First, in contrast with most other systems, the frequency and severity of injury from male-male combat can readily be quantified (figures 1 and 2), as explained below. Moreover, experimental manipulations have demonstrated that this damage can influence fighting outcomes [19]. Second, there is considerable diversity in weapon form across the clade (figure 3), and similar weapons have evolved repeatedly across the group, providing statistical power for comparative analyses [21].…”

“…Thus, the wing damage observed here may have implications for an individual's ability to find mates, forage and evade predators. Such costs probably explain why wing damage can influence fighting outcomes in this clade [19].…”

“…The fighting-advantage hypothesis leads to clear macroevolutionary predictions in coreids. One way that a weapon modification may provide a fighting advantage is by increasing the amount of damage that the weapon can induce [12][13][14][15][16][17][18][19]. Therefore, this hypothesis would be supported if there is a positive relationship between particular weapon modifications (e.g.…”