Why Does Testosterone Influence Morphology, Behaviour and Physiology?

Several decades of research have produced a large number of studies that examine the effects of hormones on physiology, behaviour and morphology. In the last fifteen years there has been considerable interest from evolutionary bi- ologists on the impact of hormones, especially testosterone, on aspects of physiology in particular immune function. Inter- estingly, especially given the recent attention from evolutionary biologists, the primary focus has been on determining the existence of links between hormones and other aspects of physiology or behaviour, with an emphasis on the understand- ing of mechanism. Typically though evolutionary biology focuses not on mechanism but on function - i.e. the evolution- ary explanation for why a given trait or relationship between traits exists. Evolutionary biologists would expect that if two parts of an organism's physiology were both affected by a hormone then there should be some adaptive reason why such a link exists. The fact that a hormone simultaneously influences aspects of physiology, behaviour and morphology suggests that individuals linking these traits typically benefit in someway from doing so. This paper attempts to provide some func- tional explanations for such links and proposes that testosterone may be the hormone that tips animals between 'hare-like' and 'tortoise-like' life-history strategies, with testosterone pushing individuals towards 'hare-like' strategies. If links be- tween physiological, behavioural and morphological traits exist because they benefit the organisms concerned, then we might expect different species with different ecologies to arrive at different adaptive solutions. The lack of consistency be- tween the results of similar studies in different taxa may be informing us that different optimal strategies are arrived at by different species.