Abstract
Theoretical models of animal contests such as the Hawk-Dove game predict that variation in fighting behavior will persist due to mixed evolutionarily stable strategies (ESS) under certain conditions. However, the genetic basis for this variation is poorly understood and a mixed ESS for fighting can be interpreted in more than one way. Specifically, we do not know whether variation in aggression within a population arises from among-individual differences in fixed strategy (determined by an individual’s genotype—direct genetic effects [DGEs]), or from within-individual variation in strategy across contests. Furthermore, as suggested by developments of the original Hawk-Dove model, within-individual variation in strategy may be dependent on the phenotype and thus genotype of the opponent (indirect genetic effects—IGEs). Here we test for the effect of DGEs and IGEs during fights in the beadlet sea anemone Actinia equina. By exploiting the unusual reproductive system of sea anemones, combined with new molecular data, we investigate the role of both additive (DGE + IGE) and non-additive (DGE × IGE) genetic effects on fighting parameters, the latter of which have been hypothesized but never tested for explicitly. We find evidence for heritable variation in fighting ability and that fight duration increases with relatedness. Fighting success is influenced additively by DGEs and IGEs but we found no evidence for non-additive IGEs. These results indicate that variation in fighting behavior is driven by additive indirect genetic effects (DGE + IGE), and support a core assumption of contest theory that strategies are fixed by DGEs.
Highlights
In many species, the acquisition of resources necessary for survival and reproduction is reliant upon engaging in and winning agonistic contests
We do not know whether variation in aggression within a population arises from among-individual differences in strategy, or from within-individual variation in strategy across contests
We investigated the role of IGEs in determining the contest behavior of juvenile beadlet anemones A. equina
Summary
The acquisition of resources necessary for survival and reproduction (i.e., mates, territory, and food) is reliant upon engaging in and winning agonistic contests. Any trait that increases an individual’s ability to win a contest (resource holding potential—RHP) will enhance its fitness and be subject to positive directional selection Under such selection, we might naively expect to see among-individual variation in these traits diminish; studies of agonistic behavioral traits have found evidence for high levels of among- and within-individual variation (e.g., Wilson et al 2011; Jennings et al 2013; Santostefano et al 2016). The HawkDove game, for example, demonstrates how negative-frequency dependent selection can lead to a stable mix of strategies showing different levels of aggression (Maynard Smith and Parker 1976; Maynard Smith 1982) Under this model, Hawks always fight whereas Doves never do, relying only on displays. The original model was extended to incorporate a third strategy,
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