Contests over reproductive resources in female roller beetles: Outcome predictors and sharing as an option

<div><p>Fights among females are frequent, although less attention has been placed on them than on male fights. They arise when females compete for food, oviposition, mates, brooding sites, or access to resources which increase offspring survival. It has been shown that the outcome of female fights may be less predictable by asymmetries in resource holding power, than in male fights. Male roller beetles fight over food resources, food balls, needed for mating and nesting, and it has been show in some species that asymmetries in reproductive experience and resource holding power in terms of size predict fight outcome, including ties in which contenders cut and split the food ball. In this study, we tested the influence of asymmetries in reproductive status (experience) and body size on female fight outcome in the carrion roller beetle <i>Canthon cyanellus cyanellus</i>. As predicted, and as previously found for males of the same species, female reproductive status of both contenders and relative size predict fight outcome. Larger and reproductively experienced contenders have a higher probability of winning. Furthermore, ties are more likely in fights involving opposing asymmetries (vgr. Large reproductively naïve owner versus small reproductively experienced intruder). Also as predicted, food ball splitting is more likely to be started by the predicted loser. This mode of resource sharing may be the result of a fighting strategy in which the costs of continuing to fight are greater than the benefits of not splitting, if a fraction of the disputed resource is more than the minimum needed for the present reproductive needs, and reduces costs associated to a longer fight.</p></div

Estimated probabilities of being the contender that started splitting the food ball on tied contests.

<p>(A) Effect of owner reproductive status; (B) effect of relative body size. Probabilities and 95% confidence intervals (whiskers) were estimated using a logistic model on the odds ratio between number of intruder and owners females of <i>C</i>. <i>c</i>. <i>cyanellus</i> that started splitting the food ball. Notice that even though the model reports a significant P value for the effect of owner reproductive status, the confidence intervals overlap. This is due to the fact that although non-overlapping confidence intervals do imply a significant effect, overlapping confidence intervals do not necessarily imply a non-significant effect (see [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0182931#pone.0182931.ref045" target="_blank">45</a>]).</p

Polytomous logistic regression minimal model (LR Chisq likelihood ratio chi square) of the effects of relative body size (RBS: large-small, small-large and size-matched) and reproductive status of the owner and the intruder (RS: experienced and naïve) on the outcome of fights between <i>C</i>. <i>c</i>. <i>cyanellus</i> females.

<p>Polytomous logistic regression minimal model (LR Chisq likelihood ratio chi square) of the effects of relative body size (RBS: large-small, small-large and size-matched) and reproductive status of the owner and the intruder (RS: experienced and naïve) on the outcome of fights between <i>C</i>. <i>c</i>. <i>cyanellus</i> females.</p

Logistic regression (LR Chisq likelihood ratio chi square) of the effects of relative body size (RBS: large-small, small-large and size-matched) and reproductive status of the owner and the intruder (RS: experienced and naïve) on the probability of being the contender that started splitting the food ball in tied fights between <i>C</i>. <i>c</i>. <i>cyanellus</i> females.

<p>Logistic regression (LR Chisq likelihood ratio chi square) of the effects of relative body size (RBS: large-small, small-large and size-matched) and reproductive status of the owner and the intruder (RS: experienced and naïve) on the probability of being the contender that started splitting the food ball in tied fights between <i>C</i>. <i>c</i>. <i>cyanellus</i> females.</p

Estimated probabilities on the outcomes of fights between females of <i>C</i>. <i>c</i>. <i>cyanellus</i>.

<p>(A) Effect of the interaction between owner reproductive status asymmetry and relative body size (owner larger than intruder in black, size-matched in gray and owner smaller than intruder in white); (B) owner reproductive status (eexperiencedin gray and naïve in white); (C) effect of intruder reproductive status (experienced in gray and naïve in white); and (D) effect of relative body size (owner larger than intruder in black, size-matched in gray and owner smaller than intruder in white). Probabilities and 95% confidence intervals (whiskers) were estimated using a logistic polytomous model.</p