(behaviourally) from disturbance created by the experimental protocol prior to meeting an 26 opponent were more likely to initiate contests; however, they were not more likely to win 27 and, contrary to expectations, had higher pre-contest cortisol levels than their opponents. 28They also showed greater physiological stress responses to the experiment as determined 29 from the difference between pre-and post-contest cortisol levels. Moreover, stress 30 response was independent of whether a contest escalated. In contradiction to evidence 31 found in other taxa and fish systems, the suite of traits that we measured were not 32 correlated in a manner that allowed classification of the animals into the usual reactive and 33 proactive stress-coping styles. Our results suggest that coping style may play a key role in 34 determining the individual initiates a contest, but that other factors govern contest

Reception of pheromone cues can elicit significant physiological (e.g. steroid hormone levels) changes in the recipient. These pheromone-induced physiological changes have been well documented for male-female interactions, but scarcely in same-sex interactions (male-male and female-female). We sought to address this dearth in the current literature and examine whether mangrove rivulus fish (Kryptolebias marmoratus) could detect and, ultimately, mount a physiological response to the pheromone signature of a potential, same-sex competitor. We examined steroid hormone levels in mangrove rivulus exposed to one of three treatments: 1) isolation, 2) exposure to pheromones of a size-matched partner, and 3) pheromone exposure to a size-matched opponent followed by a physical encounter with the opponent. We found that exposure to a competitor's pheromone cues elicited a significant increase in testosterone levels. Increases in testosterone were similar across genetically distinct lineages derived from geographically distinct populations. Further, testosterone levels were similar between individuals only exposed to pheromone cues and individuals exposed to both pheromone cues and a subsequent physical encounter. Our findings led us to generate a number of testable predictions regarding how mangrove rivulus utilize pheromone signals in social interactions, the molecular mechanisms linking social stimuli and hormonal responses, and the possible adaptive benefits of hormonal responsiveness to receiving a potential competitor's pheromone cues.