Social interactions can generate rapid and dramatic changes in behaviour and neuroendocrine activity. We investigated the effects of a changing social environment on aggressive behaviour and brain aromatase activity (bAA) in a sex-changing fish, Lythrypnus dalli. Aromatase is responsible for the conversion of androgen into oestradiol. Male removal from a socially stable group resulted in rapid and dramatic (> or =200%) increases in aggression in the dominant female, which will become male usually 7-10 days later. These dominant females and recently sex-changed individuals had lower bAA but similar gonadal aromatase activity (gAA) compared to control females, while established males had lower bAA than all groups and lower gAA than all groups except dominant females. Within hours of male removal, dominant females' aggressive behaviour was inversely related to bAA but not gAA. These results are novel because they are the first to: (i) demonstrate socially induced decreases in bAA levels corresponding with increased aggression, (ii) identify this process as a possible neurochemical mechanism regulating the induction of behavioural, and subsequently gonadal, sex change and (iii) show differential regulation of bAA versus gAA resulting from social manipulations. Combined with other studies, this suggests that aromatase activity may modulate fast changes in vertebrate social behaviour.

The neurohypophyseal peptides are evolutionarily conserved and their expression can be socially modulated. Our question was what effect will socially induced sex change have on forebrain isotocin, an oxytocin homologue? We removed males from social groups to induce dominant females to change sex and become males in Lythrypnus dalli. Fish in the late stages of sex change had fewer forebrain isotocin-immunoreactive (-ir) cells than early stage and unchanged females. When groups were consolidated into unequivocal males (control males and sex-changed new males) and unequivocal females (fish prior to courtship as a male), females had significantly more isotocin-ir cells than control males and recently sex-changed fish. This is the first study demonstrating the social regulation of forebrain isotocin.

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In the bluebanded goby, Lythrypnus dalli, removal of the male from a social group results in a rapid behavioral response where one female becomes dominant and changes sex to male. In a previous study, within hours of male removal, aromatase activity in the brain (bAA) of dominant females was almost 50% lower than that of control females from a group in which the male had not been removed. For those females that displayed increased aggressive behavior after the male was removed, the larger the increase in aggressive behavior, the greater the reduction in bAA. To investigate whether decreased bAA leads to increased aggression, the present study used a more rapid time course of behavioral profiling and bAA assay, looking within minutes of male removal from the group. There were no significant differences in bAA between control females (large females from groups with the male still present), females that doubled their aggressive behavior by 10 or 20 minutes after male removal, or females that did not double their aggressive behavior within 30 minutes after male removal. Further, individual variation in bAA and aggressive behavior were not correlated in these fish. Whole brain decreases in aromatase activity thus appear to follow, rather than precede, rapid increases in aggressive behavior, which provides one potential mechanism underlying the rapid increase in androgens that follows aggressive interactions in many vertebrate species. For fish species that change sex from female to male, this increase in androgens could subsequently facilitate sex change.

Agonistic encounters featuring ritualized displays precede the establishment of dominance relationships in many animals. We investigated the predictive value of the amount of display behaviour (number and duration of displays) vs. quickness to display (latency to express a display behaviour) in determining the outcome of aggressive interactions and establishment of dominant vs. subordinate status in male green anole lizards, Anolis carolinensis. Similar-sized males were paired and observed for ninety minutes as they established social status. We recorded the number, duration (where appropriate) and latencies to first expression for multiple behavioural display components (head bob A, B and C, dewlap displays, open mouth displays, pushups, lateral displays) and colour changes (eyespot presence and body colour changes). Males that eventually won and became the dominant male had significantly higher counts and durations with the exception of Bob C counts, pushup counts and open mouth duration. Future dominants also maintained a green body colour longer than future subordinates, which had brown shades more often and for longer durations. Latency to first express a display component was shorter in future dominants for head bobs A and C, dewlap and open mouth displays when all data were considered. However, all significant latency differences disappeared when data only from pairs in which both males displayed a behaviour were included in the analysis. Counts, durations, and latencies were highly correlated with each other within individuals. The results indicate that behavioural display patterns during an initial display contest predict the outcome of the interaction, with the amount of display behaviour being the best predictor of whether a male will win or lose the contest and hence become dominant or subordinate. These results are consistent with the idea that displays are honest signals of a male’s physiological capacity or stamina, and hence fighting and resource holding ability.