Introduction Walter D Koenig and Janis L Dickinson 1 Evolutionary origins J David Ligon and D Brent Burt 2 Delayed dispersal Jan Ekman, Janis L Dickinson, Ben J Hatchwell and Michael Griesser 3 Fitness consequences of helping Janis L Dickinson and Ben J Hatchwell 4 Parental care, load-lightening and costs Robert G Heinsohn 5 Matings systems and sexual conflict Andrew Cockburn 6 Sex-ratio manipulation Jan Komdeur 7 Physiological ecology Morne Du Plessis 8 Endocrinology Steven J Schoech, S James Reynolds and Raoul K Boughton 9 Incest and incest avoidance Walter D Koenig and Joseph Haydock 10 Reproductive skew Robert D Magrath, Rufus A Johnstone and Robert G Heinsohn 11 Joint-laying systems Sandra L Vehrencamp and James S Quinn 12 Conservation biology Jeffrey R Walters, Caren B Cooper, Susan J Daniels, Gilberto Pasinelli and Karen Schiegg 13 Mammalian contrasts and comparisons Andrew F Russell Summary Steven J Pruett-Jones Names of bird and mammal species mentioned in the text References Index

Ecology and evolution of cooperative breeding in birds

A combination of laboratory and field investigations of birds has shown that expression of behavior such as territorial aggression can occur throughout the year in many species and in different life h

Avoiding the 'costs' of testosterone: ecological bases of hormone-behavior interactions.

Phenotypic plasticity is the extent to which an organism can change its physiology, behaviour, morphology and/or development in response to environmental cues. Environmentally induced differences in the endocrine system are among the underlying causes of phenotypic plasticity. For example, maternal and other environmental influences on developing young can affect the range of physiological and behavioural responses available to them as adults. The mechanisms underlying phenotypic plasticity can be elucidated using multidisciplinary approaches, in which the dynamic interactions among developmental, hormonal and environmental factors are considered. Such studies point to the importance of considering the overall developmental trajectory of an organism when assessing the adaptive value of phenotypic variation, rather than simply evaluating the individual at a single point in time.

Hormones, developmental plasticity and adaptation

Individual and seasonal variation in fecal testosterone and cortisol levels of wild male tufted capuchin monkeys, Cebus apella nigritus.

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.

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Testosterone and Aggression: Berthold, Birds and Beyond

Many studies have demonstrated that male aggression is regulated by testosterone. The conversion of testosterone to estradiol by brain aromatase is also known to regulate male aggression in the breeding season. Male song sparrows (Melospiza melodia morphna) are territorial not only in the breeding season, but also in the nonbreeding season, when plasma testosterone and estradiol levels are basal. Castration has no effect on nonbreeding aggression. In contrast, chronic (10 day) aromatase inhibitor (fadrozole) treatment decreases nonbreeding aggression, indicating a role for estrogens. Here, we show that acute (1 day) fadrozole treatment decreases nonbreeding territoriality, suggesting relatively rapid estrogen effects. In spring, fadrozole decreases brain aromatase activity, but acute and chronic fadrozole treatments do not significantly decrease aggression, although trends for some behaviors approach significance. In gonadally intact birds, fadrozole may be less effective at reducing aggression in the spring. This might occur because fadrozole causes a large increase in plasma testosterone in intact breeding males. Alternatively, estradiol may be more important for territoriality in winter than spring. We hypothesize that sex steroids regulate male aggression in spring and winter, but the endocrine mechanisms vary seasonally.

Kim Sullivan

Papers

Acute and chronic effects of an aromatase inhibitor on territorial aggression in breeding and nonbreeding male song sparrows

Combined aromatase inhibitor and antiandrogen treatment decreases territorial aggression in a wild songbird during the nonbreeding season.

The design of animal communication