Sexual phenotype drives variation in endocrine responses to social challenge in a quasi-clonal animal

Abstract

In many species, males tend to behave more aggressively than females and female aggression often occurs during particular life stages such as maternal defence of offspring. Though many studies have revealed differences in aggression between the sexes, few studies have compared the sexes in terms of their neuroendocrine responses to contest experience. We investigated sex differences in the endocrine response to social challenge using mangrove rivulus fish, Kryptolebias marmoratus. In this species, sex is determined environmentally, allowing us to produce males and hermaphrodites with identical genotypes. We hypothesized that males would show elevated androgen levels (testosterone and 11-ketotestosterone) following social challenge but that hermaphrodite responses might be constrained by having to maintain both testicular and ovarian tissue. To test this hypothesis, we staged fights between males and between hermaphrodites, and then compared contest behaviour and hormone responses between the sexes. Hermaphrodites had significantly higher oestradiol but lower 11-ketotestosterone than males before contests. Males took longer to initiate contests but tended to fight more aggressively and sustain longer fights than hermaphrodites. Males showed a dramatic post-fight increase in 11-ketotestosterone but hermaphrodites did not. Thus, despite being genetically identical, males and hermaphrodites exhibit dramatically different fighting strategies and endocrine responses to contests.

Hormone levels increase in response to video playback in the Siamese fighting fish

“…However, evidence has been accumulating that the loser effect may be mediated by a different physiological mechanism. Winners and losers have been shown to have a similar increase in post-fight androgen levels (e.g., [52]; unpublished data for B. splendens ) and androgen administration to losers failed to reverse the loser effect [50]. Losing also seems to be a more salient experience in zebrafish as losers show a stronger change in brain transcriptome as compared with winners, and different genomic pathways were implicated in the winner and loser experience [53].…”