Metabolic rate and aggressiveness between Brown Trout populations

1. Studies focusing on the physiological variation between populations and its connection to fitness-related traits are rare, even though integrating these fields would increase knowledge on the evolution of traits. Standard metabolic rate (SMR) has been suggested to influence an individual's social status and the level of aggressiveness, as dominant individuals tend to have higher SMR than subordinate individuals. 2. The SMR of juvenile Brown Trout (Salmo trutta) from four populations was measured, and the population-level relationship between SMR and aggressiveness, which is a fitness-related behavioural trait, was investigated. 3. SMR differed between the populations. but no differences were found in the amount of aggression. Nevertheless, a significant positive correlation between aggressiveness and SMR between the populations was found. 4. Unlike many previous studies on geographical variation of metabolic rate, SMR correlated negatively with the latitude of origin of the populations. 5. The results suggest that SMR and aggressiveness are correlated not only at the individual level as shown by previous studies, but also at the Population level. The costs and benefits of high metabolic rate depend largely on the environment, and local differences in environmental conditions. as for example in food availability, may select for local differences in SMR.