Why do some fish fight more than others?

Reversible changes in how readily animals fight can be explained in terms of adaptive responses to differences in the costs and benefits of fighting. In contrast, long-term differences in aggressiveness raise a number of questions, including why animals are consistent with respect to this trait, why aggressiveness is often linked to general risk taking, and why aggressive and nonaggressive animals often coexist within a population. In fish, different levels of aggressiveness bring several direct fitness-related consequences, such as when aggressive individuals monopolize a limited food supply and grow fast. They also bring indirect consequences, such as when aggressive fish are more susceptible to predation and when they require a larger respiratory surface to service a higher metabolic rate. Fitness consequences of aggressiveness are often context dependent, with aggressive fish tending to do well in simple, predictable conditions but not in complex, less predictable conditions. The diverse, context-dependent consequences of aggression mean that aggressive and nonaggressive fish flourish in different conditions and explain in general terms why these behavioral phenotypes often coexist. There are a number of candidate evolutionary frameworks for explaining why individual differences in aggressiveness are often, but not always, consistent over time and often, but not always, linked to differences in general risk taking

Respiratory function in common carp with different stress coping styles: a hidden cost of personality traits?

The purpose of this study was to compare investment in structures for gas exchange in common carp, Cyprinus carpio, with different stress coping styles, which are known to differ in resting metabolic rate. Common carp were classified as proactive or reactive on the basis of rate of emergence from cover into a novel, potentially dangerous environment in three successive tests. The fish were then killed and their gill arches removed. Length-independent estimates of the size of the respiratory structures were derived by multivariate analysis of arch length and the number and length of gill filaments and lamellae for all gill arches. Overall gill area was also estimated. Filaments from the second gill arch were sampled for histological estimation of the extent to which the respiratory surface is covered by epithelial cells (hyperplasia). Proactive carp had longer gill filaments and more, longer lamellae, and consequently a larger gill surface area, than reactive carp. In contrast, the extent of hyperplasia was higher in reactive than proactive carp. Thus, compared to reactive fish, proactive carp had a larger respiratory surface, more of which was exposed to the surrounding water rather than being covered by epithelial cells. We suggest that the higher metabolic rate of proactive fish requires greater investment in, and greater exposure of, the respiratory structures. This is likely to make oxygen uptake more effective, but may also impose hidden costs of a proactive, aggressive lifestyle