Risk assessment and contest behaviour in the Cichlid fish Nannacara anomala

Abstract: The risk of predation is an important factor for our understanding of animal behaviour in general. This thesis investigates the influence of predation risk on fighting behaviour in the cichlid fish Nannacara anomala. According to game theoretical models, a fighting animal should base its decisions during a fight on its fighting ability in relation to the opponent and on the value of the contested resource. The result of the present thesis suggests that fighting N. anomala males engaged in escalated fighting are likely to be vulnerable to predation due to impaired vigilance. The presence of a model fish predator made the males reluctant to perform mouth wrestling and the intensity of mouth wrestling decreased significantly compared to undisturbed fights. When I used predation risk as a cost of fighting, the variability among fights increased. In some fights, the use of low intensity behaviour elements increased in the presence of a model fish predator in accordance with the previous findings on vigilance. Other fights escalated to mouth wrestling in a way resembling undisturbed fights, suggesting a difference among contestants in what behaviour they prefer to use when there is an increased risk of predation. The study showed that fish classified as 'bold' towards a model fish predator escalated significantly faster to mouth wrestling and performed significantly more agonistic behaviour in the presence of a model fish predator compared with fish that had been classified as 'cautious'. 'Bold' pairs also escalated significantly faster to mouth wrestling compared to 'cautious' pairs when the model fish predator was absent. In agreement with theoretical predictions, fights between N. anomala males contained significantly more costly behavioural acts in the presence of a female compared to fights in the absence of a female. Despite the presence of a female, the variability among fights increased significantly when predation risk increased, suggesting that increased risk of predation is not accepted even in fights about a valuable resource in this species. The study also showed that the probability for the lighter fish to win increased in the presence of a female and when predation risk was high. In fights over a valuable resource, other asymmetries besides relative body weight are likely to become important and increased risk of predation may impair assessment of relative fighting ability or the contestants may differ in risk assessment. A difference among contestants in what behaviour they prefer to use, clearly expressed when predation risk increase, may be an additional factor explaining some of the variability among fights found in many studies.