Hormonal Responses to Noncontact Aggression in Convict Cichlid Fish

Abstract

This study explored whether convict cichlid fish mount a hormonal response to aggressive encounters where dominance status remains unresolved. Hormone samples were collected at two time points before an aggressive interaction to obtain confinement-induced and baseline measures, and at one time point following a contest across a clear partition (experimental) or exposure to an opaque partition with an opponent on the opposite side (control). There was no overall significant effect of treatment (control vs. experimental) on hormone release rates but there were trends for cortisol and testosterone (T). A priori linear contrasts showed that individuals that engaged in aggressive interactions had lower postfight cortisol and T release rates than controls, suggesting that aggression, in this context, might attenuate the synthesis of both hormones. Cortisol decreased significantly between initial confinement and baseline, indicating that individuals habituate to the water-borne hormone collection procedure. Contrary to expectation, individuals with higher baseline T and 11-ketotestosterone (KT) release rates took longer to initiate conflict. None of the other measures of behavior were predicted by baseline hormone release rates, and contest behavior did not predict postfight hormone release rates. There was a significant positive relationship between KT and T at all time points. As with studies that employ mirror image stimulation, we found no hormonal response to unresolved contests despite high levels of aggressive behavior. Our study is unique because we demonstrate that animals engaged in conflict with live opponents also do not mount a significant hormonal response when clear dominance relationships are not established.

Audience Effects in Territorial Defense of Male Cichlid Fish Are Associated with Differential Patterns of Activation of the Brain Social Decision-Making Network

“…These studies may explain why T levels were lower for intrusions in the presence of an audience, since in our experimental setup the territorial male also interacted with the intruder across a clear partition, and therefore no definite resolution of the social challenge could be achieved. Furthermore, the lower F levels in the treatment with the territorial intrusion in the presence of an audience, when compared to focal males that had an audience present but were not challenged by an intruder also corroborates this interpretation and replicates the findings of Scarsella et al (2016). It is also reasonable to assume that in our experiment the effects of the female audience and of the inability to resolve the agonistic interaction on lowering the T levels were cumulative, since the comparison between the treatments without territorial intrusions also showed that T levels were lower when the female audience was present.…”

“…A recent experiment with convict cichlids ( Amatitlania nigrofasciata ; Scarsella et al, 2016), in which animals had an agonistic encounter across a clear partition, found a decrease in T and F levels in response to the interaction. This result is in line with previous research in tilapia that had shown that unresolved interactions fail to trigger an androgen response (Oliveira et al, 2005).…”

Heterospecific aggression differs predominately by species, rather than sex, in Lake Malawi cichlid fish

“…Signaling peptides also play a role in aggression in cichlids; additional studies using the A. burtoni model found no association of aggression with plasma levels of testosterone or 11-ketotestoterone, but found that a reduction in the regulatory peptide somatostatin increases aggressive acts in males 32,33 . Also, lower levels of the stress hormone cortisol have been associated with higher levels of aggression; less aggressive, non-territorial A. burtoni males had higher plasma cortisol levels 34 , and aggressive convict cichlids had lower post-assay excreted cortisol levels 35 . Both of the experiments evaluated conspecific aggression, so it is possible that cortisol is only associated with conspecific aggression or mediation of social dominance, rather than aggression per se.…”