Male Xiphophorus multilineatus behavioral, brain weight, and testes weight data

Behavioural plasticity may require energetically expensive sensory and neural adaptations to detect, process, and respond to social cues. These costs could lead to selection against behavioural plasticity and its eventual loss. We show that males from the behavioural plastic alternative reproductive tactic (ART) in the swordtail fish Xiphophorus multilineatus have relatively larger brains, in addition to a trade off with testes size, that is not detected in the males from the behaviourally fixed ART. Given these costs, we consider the hypothesis that plasticity in mating behaviours is maintained due to intralocus tactical conflict, where a shared genome can constrain one or both ARTs from evolving to their optima. When we reduced any potential for intralocus tactical conflict by removing the behaviourally fixed ART from long-term breeding mesocosms, the males from the behaviourally plastic ART were less plastic and had smaller brains as compared to their counterpart from control mesocosms (both male ARTs). We also detected evidence for a genetic correlation between the ARTs for behaviour, which is required for intralocus conflict. Our findings suggest that intralocus tactical conflict could be maintaining behavioural plasticity, in which case behavioural plasticity may not be adaptative in some cases.