Physiological costs to behavioural plasticity in a swordtail fish: clues to its evolutionary maintenance and loss

Abstract

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 found that males from the behaviourally plastic alternative reproductive tactic (ART) in the swordtail fish Xiphophorus multilineatus had relatively larger brains, in addition to a trade-off with testes size, that was 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, males from the behaviourally plastic ART were less plastic and had smaller brains compared to their counterparts 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.