Habitat preference increases territorial defence in brown trout (Salmo trutta)

The striking ability of territory owners to repel intruders has generated a number of theoretical explanations as well as experimental studies in many animal species. However, effects of individual habitat preferences on terr itorial defence have rarely been studied. From the territory value hypothes is, we predicted that owners of preferred habitats should invest more resou rces in defence than owners of non-preferred habitats. We tested this predi ction with young territorial brown trout in a two-stage experiment. First, trout were allowed to choose individually between gravel and a uniform brig ht substrate. As expected, they showed a significant (79%) preference for g ravel. However, there was considerable variation between individuals in sub strate preference, with a few fish preferring the bright substrate. Half of the tested fish were then transferred to a gravel substrate and the rest t o a bright substrate, manipulating habitat type in relation to preference. Territory owners were then staged against size-matched intruders whereupon contest aggression was observed and the winner of each contest determined. Overall, owners won most of the contests. Satisfied owners won 86% and owne rs of less preferred territories, 74% of the contests. Furthermore, more sa tisfied owners attacked sooner and were more aggressive relative to the int ruders. We conclude that brown trout show individual variation in habitat p reference, which appears to be linked with their investment in territorial defence. These results suggest that understanding and modelling of animal c ontests could benefit from considering how territorial defence is influence d by individual habitat preference and specialisation.