Berthold's classic study of domesticated roosters in 1849 demonstrated that testicular secretions are necessary for the normal expression of aggressive behaviour. Although this conclusion is undoubtedly correct, field studies of wild songbirds have yielded important modifications and limitations of Berthold's original hypothesis. For example, studies of the North American song sparrow (Melospiza melodia) during the breeding season reveal that not only does testosterone increase aggression, but aggressive interactions also increase plasma testosterone levels. Furthermore, in winter, nonbreeding song sparrows have low plasma testosterone levels but are very aggressive, and castration of nonbreeding song sparrows does not decrease aggression. Interestingly, an aromatase inhibitor (fadrozole) does decrease male aggression in the nonbreeding season, and the effects of fadrozole can be rescued with oestradiol. In winter, dehydroepiandrosterone (DHEA) from the periphery can be metabolised within the brain to supply oestradiol to specific neural circuits. Additionally, oestradiol might be synthesised de novo from cholesterol entirely within the brain. These mechanisms may have evolved to avoid the 'costs' of circulating testosterone in the nonbreeding season. Recent studies in tropical birds, hamsters, and humans suggest that these neuroendocrine mechanisms are important for the control of aggression in many vertebrate species. Keywords 3β-HSD; aromatase; AVT; bird; BNST; brain; c-fos; dehydroepiandrosterone; DHEA; oestradiol; oestrogen; neurosteroid; season; songbird; stress; vasotocin; ZENK oestradiol; oestrogen Studies in birds have contributed greatly to understanding the neuroendocrine regulation of aggression (1-7). The first study of hormones and aggressive behaviour was conducted in roosters in the 19th Century. More recently, field studies of wild songbirds have been central in unravelling hormone-behaviour relationships across seasons and habitats. Free-living animals are often more aggressive and generally have higher circulating hormone levels than captive animals (8-10). In addition, testing conditions in the laboratory can have large unexpected effects on behaviour, particularly when animals are forced to interact in constrained spaces. For these reasons, field studies can be an important complement to laboratory studies. Combined field-laboratory approaches in birds have produced novel
CIHR Author Manuscript
CIHR Author Manuscript
CIHR Author Manuscriptinsights into the social regulation of testosterone levels, the expression of aggression during the nonbreeding season, and the 'costs' of elevated circulating testosterone levels (4).
Berthold's caponsThe study of hormones and aggression, and of hormones in general, can be traced back to the classic experiment of Arnold Berthold in 1849 (11). He removed the testes of immature male chickens (the castrated animals are called capons) and found a decrease in some secondary sex characteristics (comb, wattles) and male-typical behaviours (11). Capons di...