Effect of environmental enrichment upon resource holding power in fish in prior residence situations.

Resource holding power (RHP), as expressed by gaining dominance, can be affected by extrinsic and intrinsic factors. Extrinsic factors that increase the RHP include e.g. prior exposure to the contest area. The pay-off asymmetry hypothesis was tested according to which there is an asymmetry in the value the resident's territory has for the resident and intruders, i.e. the resident loses more than intruder opponents when losing the contest over a territory in which it has invested much energy during the exploration of food and shelter resources, during the settlement of conflicts with territorial neighbours, etc. This asymmetry will increase the resident's chance to win the fight over intruders. If the resident-intruder asymmetry is enhanced by presenting resource-rich territories to e.g. fish (structurally diverse or 'rich' aquariums), the hypothesis predicts that the resident's dominance chance (probability to win the fight) will be higher than in structurally 'poor' aquariums. It was found that residents were proportionately more often dominant (fight winners) than intruders in the rich compared to the poor aquariums in all seven tested fish species. This demonstrates that a high territory resource value (aquarium enrichment) significantly facilitates the expression of the resident's dominance advantage (prior residence effect). In contrast to dominance, aggressive behaviour before the dominance settlement did not generally differ between residents and intruders when compared in rich and poor aquariums. This suggests that dominance in itself and aggression prior to dominance settlement are at least partially guided by different motivational systems.