Abstract
Abstract
Combat behavior is an interacting phenotype wherein individuals modulate their expression of aggression based on their partner’s behavior and their own prior experience. We hypothesize that interacting phenotypes generate winner and loser effects via carryover and second-order effects on behavior. Among many hypotheses for the mechanisms generating winner and loser effects, two consider carryover and second-order effects arising through the cumulative assessment of prior experience (self-assessment hypothesis) or the perception of a partner’s prior experience (social cue hypothesis). We tested predictions of these two non-exclusive hypotheses using forked fungus beetles, a long-lived insect with male-male competition for mates. Focal males aged 1 or 3 years were paired with a much larger or smaller partner to induce a win or loss, then re-tested 24 hours later with a size- and age-matched partner. Individuals exhibited strong loser effects but no winner effects regardless of age, contradicting predictions of the self-assessment hypothesis. Recent losers were less aggressive in contests and evaded conspecifics more. New social partners of recent losers evaded them much less frequently than those fighting recent winners, demonstrating cascading effects of a single loss consistent with predictions of the social cue hypothesis. These results suggest that individuals adjust behavior based on both their own and their partner’s recent combat outcomes but not their cumulative lifetime experience. Our findings reveal strengths and limitations of the current behavioral explanations of winner and loser effects and provide evidence that interacting phenotypes extend beyond the current partner, including the prior experience of both partners.