Mass, Residency Duration, and Previous Experience Shape Post-Escalation but Not Pre-Escalation Outcomes in Experimental Territorial Contests of Atlantic Mudskippers (Periophthalmus barbarus)

Defending territories increases access to food and shelter, enhancing survival and reproductive success, but it also incurs costs such as energy expenditure, injury risk, and reduced opportunities for other vital behaviors. We investigated predictors of dyadic contest outcomes in the Atlantic mudskipper (Periophthalmus barbarus) using a modified resident-intruder design that physically separated the contestants before physical contact, allowing separate investigation of pre-escalation (resident’s decision to engage) and post-escalation (contest resolution) complementary outcomes. This setup disentangled the effects of intrinsic and extrinsic factors influencing the resident’s decision to escalate from those affecting the outcome of a physical confrontation. A modified tournament design and Bayesian mixed-effects logistic models incorporated random effects to avoid pseudoreplication and yield generalized probabilistic inferences about parameters and uncertainty. First-order Markov chain analysis tested the effects of previous fighting experience on outcome probabilities. A binomial generalized linear model with logit link estimated the probability of resident escalation as a time-dependent, predictor-driven decision process, while generalized linear models examined how predictors influenced the resident’s total assessment duration in escalated contests and the post-escalation contest duration. We hypothesized that (1) heavier residents with longer residency durations would be more likely to escalate and win, (2) prior outcomes would influence subsequent contests, and (3) total assessment and post-escalation contest durations would increase as asymmetries in mass and residency duration decreased. Across 51 dyadic contests, residents were more likely to initiate fights, and winning probability was consistently predicted by residency status, relative mass and, to a lesser extent, residency duration. Recent contest experience had significant, role-specific effects only in the post-escalation phase: stronger winner and loser effects in intruders, and a weaker winner effect in residents. Duration analyses revealed directionally consistent but nonsignificant trends, likely reflecting sample-size limitations. These findings provide valuable insight into the determinants of agonistic contest in P. barbarus, supporting the use of mudskippers as model species for experimental studies of assessment strategies and territorial defense.