Why do winners keep winning? Androgen mediation of winner but not loser effects in cichlid fish

Abstract

Animal conflicts are influenced by social experience such that a previous winning experience increases the probability of winning the next agonistic interaction, whereas a previous losing experience has the opposite effect. Since androgens respond to social interactions, increasing in winners and decreasing in losers, we hypothesized that socially induced transient changes in androgen levels could be a causal mediator of winner/loser effects. To test this hypothesis, we staged fights between dyads of size-matched males of the Mozambique tilapia (Oreochromis mossambicus). After the first contest, winners were treated with the anti-androgen cyproterone acetate and losers were supplemented with 11-ketotestosterone. Two hours after the end of the first fight, two contests were staged simultaneously between the winner of the first fight and a naive male and between the loser of first fight and another naive male. The majority (88%) of control winners also won the second interaction, whereas the majority of control losers (87%) lost their second fight, thus confirming the presence of winner/loser effects in this species. As predicted, the success of anti-androgen-treated winners in the second fight decreased significantly to chance levels (44%), but the success of androgenized losers (19%) did not show a significant increase. In summary, the treatment with anti-androgen blocks the winner effect, whereas androgen administration fails to reverse the loser effect, suggesting an involvement of androgens on the winner but not on the loser effect.

Figure 1

Experimental procedure to assess the presence of winner/loser effects in tilapia. (a) Four fish matched for size are placed in individual compartments separated by removable opaque partitions (social isolation); (b) after a period of social isolation, the central divider is removed and a fight between the two fish placed in the central compartments—the focal fish—is promoted (first interaction); (c) after assessing the winner and loser, the divider is put back in place and the winner and the loser of this first fight are back in their initial compartments (interval between the first and second interactions); (d) 2 hours after the end of the first fight, the lateral dividers are removed and two simultaneous fights are promoted between the winner of the previous fight and a neutral male and between the loser of the previous fight and another neutral male (second interactions).

Figure 2

Validation of the hormonal treatments. Levels of 11-KT in fish subjected to different intraperitoneal injection treatments: high dose (2 μg g⁻¹ body weight) of KT (large dot bars); low dose (0.2 μg g⁻¹ body weight) of KT (small dot bars); CA (1 mg g⁻¹ body weight) (hatched bars); and saline solution (control, white bars). Different letters indicate significantly different groups (p<0.05) within each sampling point.

Figure 3

Winner/loser effects in tilapia as assessed by the percentage of victories in the second fights of previous winners and losers. The treated winners were injected with a high dose of KT and the treated losers were injected with CA 2 hours before the second fight. The dashed lines indicate the cut-off values to consider the occurrence of winner/loser effects according to Bégin et al. (1996) (black bars, control; white bars, treatment).