Male sexual behavior and hormones in non-mammalian vertebrates

In most vertebrate species, male sexual behavior is activated by the action of testosterone on the preoptic area and in many cases this behavioral activation requires transformation of the steroid into estradiol and 5a-dihydrotestosterone. These metabolites, either alone or in synergy, act at the cellular level to produce changes in neuronal function that mediate behavioral changes. Variation in plasma testosterone concentrations does not simply correlate in a positive manner with the degree of behavioral activation. For example, in many species, testosterone cannot activate male-typical sexual behaviors in females. This sex difference in the behavioral consequences of testosterone results from organizational effects of sex steroids that irreversibly differentiate the brain during ontogeny so that male brains will respond in adulthood to testosterone while female brains will not. Behavioral effects of testosterone are largely mediated by changes in the transcription of specific proteins, including enzymes that synthesize or catabolize neurotransmitters and their receptors. These changes in protein expression in turn produce changes in neurotransmission in a defined neuronal circuitry that supports behavior expression. There are more recently described effects of steroids that do not involve protein transcription but rather involve actions at the neuronal membrane or direct changes in intracellular signaling cascades that also seem to contribute to the control of male sexual behavior. These principles seem to apply to a wide range of non-mammalian vertebrate species though a number of specialized reproductive strategies that could be associated with exceptions to these general rules have also been described.