Proximate perspectives on the evolution of female aggression: good for the gander, good for the goose?

Abstract

Female–female aggression often functions in competition over reproductive or social benefits, but the proximate mechanisms of this apparently adaptive behaviour are not well understood. The sex steroid hormone testosterone (T) and its metabolites are well-established mediators of male–male aggression, and several lines of evidence suggest that T-mediated mechanisms may apply to females as well. However, a key question is whether mechanisms of female aggression primarily reflect correlated evolutionary responses to selection acting on males, or whether direct selection acting on females has made modifications to these mechanisms that are adaptive in light of female life history. Here, I examine the degree to which female aggression is mediated at the level of T production, target tissue sensitivity to T, or downstream genomic responses in order to test the hypothesis that selection favours mechanisms that facilitate female aggression while minimizing the costs of systemically elevated T. I draw heavily from avian systems, including the dark-eyed junco ( Junco hyemalis ), as well as other organisms in which these mechanisms have been well studied from an evolutionary/ecological perspective in both sexes. Findings reveal that the sexes share many behavioural and hormonal mechanisms, though several patterns also suggest sex-specific adaptation. I argue that greater attention to multiple levels of analysis—from hormone to receptor to gene network, including analyses of individual variation that represents the raw material of evolutionary change—will be a fruitful path for understanding mechanisms of behavioural regulation and intersexual coevolution.

Female competition and aggression: interdisciplinary perspectives

“…As is clear from several contributions to the current issue [80][81][82][83][84][85]86,90,121], females may often invest flexibly in competitive effort according to immediate need. Hence aggression may be heightened at specific reproductive or life-history stages when competition for mates, resources or the need to defend offspring is particularly acute [57,68,85,102,109,110,143].…”

“…Such diversity reveals that our understanding of the role of prenatal androgen exposure in mediating female aggression is still far from complete. In adulthood, female T levels are typically lower than males, even in species where females dominate males or are unusually aggressive ( [84]; see also Rosvall [121] and Campbell [102] this issue). This suggests that T level alone is unlikely to fully explain sex differences in aggression.…”

“…This might be achieved by reducing T sensitivity in some tissues (such as neural sites associated with maternal care), while increasing sensitivity in others (those involved in competition). One route to sex-specific effects of T examined by Rosvall in the current issue [121] is differential gene expression. Experimental elevation of T in dark-eyed juncos (Junco hyemalis) affected the expression (transcription) of genes in the medial amygdala and hypothalamus, areas of the brain linked with aggressive and social behaviours.…”

“…To minimize reproductive costs, females need a way of directing T to only those processes and behaviours where it offers benefits [121]. This might be achieved by reducing T sensitivity in some tissues (such as neural sites associated with maternal care), while increasing sensitivity in others (those involved in competition).…”

“…T appears to mediate female aggression to some extent in more typical vertebrate species also [121], and since females with elevated T levels are likely to incur reproductive costs such as decreased fertility or maternal care of offspring, the physiological mechanisms by which aggression is mediated to minimize associated costs are also of interest from an evolutionary perspective [84,121]. To minimize reproductive costs, females need a way of directing T to only those processes and behaviours where it offers benefits [121].…”