In one large qualitative study of men who have sex with men MSM , roughly half of participants cited age of a potential partner as a specific draw, alongside other physical features. How such modular control is transformed into cohesive internal states that correspond to sexually dimorphic behavior is poorly understood. The publisher's final edited version of this article is available at Neuron See other articles in PMC that cite the published article. Such external and internal control mechanisms ensure that these social behaviors are displayed in the appropriate context.
The literature on sexually dimorphic behaviors in other organisms has been reviewed elsewhere Baum, ; Cahill, ; Crews and Moore, ; Dickson, ; Manoli et al. Our review discusses the mechanisms that regulate sexually dimorphic behaviors in mammals with a specific focus on mice and the assumption that similar mechanisms are likely to operate in humans. Any reactive tests were confirmed with enzyme immunoassay EIA and Western blot. We focus largely on sex differences in mating and aggression because the underlying neural pathways have been studied in some detail. Recent studies show that these over-arching control mechanisms regulate distinct genes and neurons that in turn specify the display of such behaviors in a modular manner. Sensory cues and sex hormones control the entire repertoire of sexually dimorphic behaviors, including those commonly thought to be charged with emotion such as courtship and aggression. How such modular control is transformed into cohesive internal states that correspond to sexually dimorphic behavior is poorly understood. The activation of the underlying neural circuits is controlled by sensory cues as well as by physiological signals such as sex hormones. Sex differences at the level of sensory input can also drive sexually dimorphic behaviors. A framework to understand how the brain can generate sexually dimorphic behaviors Males and females transform sensory input into sexually dimorphic behaviors, suggesting that such behaviors are generated by neural circuits that differ between the sexes. Introduction Men and women exhibit sex differences in behaviors that immediately enhance reproductive success as well as in tasks that involve higher cognitive function. We do not list all known cellular or molecular sexual dimorphisms in the nervous system because these have been documented extensively Cahill, ; Cooke et al. Sex steroid hormones secreted by the gonads are the critical internal signals that control these behaviors in vertebrates McEwen, Such external and internal control mechanisms ensure that these social behaviors are displayed in the appropriate context. We summarize current understanding of the neural circuit control of sexually dimorphic behaviors from several perspectives, including how neural circuits in general, and sexually dimorphic neurons in particular, can generate sex differences in behavior, and how molecular mechanisms and evolutionary constraints shape these behaviors. We propose that emergent themes such as the modular genetic and neural control of dimorphic behavior are broadly applicable to the neural control of other behaviors. Absent the complete delineation of such a neural circuit, we envision several mutually non-exclusive neural circuit wiring diagrams that enable sexually dimorphic output Figure 1. One example of a neural circuit with well-defined sensory sex differences is that underlying female pheromone-elicited chemo tactic flight in the male moth Bombyx mori Nakagawa et al. One example may be the circuit for penile muscles involved in coitus, which are controlled by motor neurons in the spinal nucleus of the bulbo cavernosus SNB , a population of neurons largely absent in females Breedlove and Arnold, In the most extreme case, such a neural circuit is unique to one sex. Given that wild type females of many species can display some male-type mounting behavior see later , if the neural pathway controlling these penile muscles is one component of a singular male sexual behavior circuit then at least some neural centers pre-synaptic to the SNB are likely to be shared between the sexes. In one large qualitative study of men who have sex with men MSM , roughly half of participants cited age of a potential partner as a specific draw, alongside other physical features. However, the chemo taxis elicited by the presence of the female pheromone engages output pathways that control flight, a behavior shared between the two sexes. Where instructive, we discuss findings in other model organisms, especially flies, that provide insight into the neurobiological basis of sex differences in behavior. The factors driving selection of sex partners are complex. In this scenario, the gender dimorphism in SNB neurons may represent an example of a shared neural circuit that differs between the sexes at the level of motor neurons.
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