Hormonal mechanisms for variation in female mate choice
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Although female mate choice has been the subject of many investigations, the causes of variation in mate choice are less understood. Theoretical models predict that individual females should show variable mate choices as a consequence of intrinsic and extrinsic factors. Empirical support for these predictions demonstrates that females show flexible mate choices over their lifetime, the breeding season and a reproductive cycle. The objective of the research presented here is to examine two intrinsic factors, reproductive and hormonal state, to determine how these contribute to individual variation in female mate choice over a reproductive cycle. I examine the link between flexibility in mate choice behavior, changes in gonadal hormones and hormonal modulation of sensory systems involved in mate choice behavior. Flexibility in mate choice was examined in female túngara frogs (Physalaemus pustulosus), a Neotropical species in which mate choice has been well studied. Acoustic based phonotaxis tests were used to assay mate choice behaviors such as receptivity, permissiveness and discrimination. These behaviors are respectively defined as a response to conspecific mate signals, a response to unattractive mate signals and the ability to discern the v difference between mate signals. The expression of receptive and permissive mate choices significantly fluctuates throughout different reproductive stages. The concentration of gonadal steroids, such as estrogen, progesterone and androgens, also significantly fluctuate throughout the same reproductive stages. Furthermore, hormone concentration was manipulated using human chorionic gonadotropin (HCG) and such manipulation induces flexible mate choice. Finally, I investigate whether hormones modulate a central auditory nucleus involved in phonotaxis behavior. Immediate early gene (IEG) induction, specifically egr-1, was used to mark neuron activity. Females were treated with either HCG or saline and exposed to either mate choruses or silence. Egr-1 expression was quantified in an auditory midbrain nucleus, the torus semicircularis (TS). The region within the TS responsible for auditory-motor integration showed a near significant elevation in egr-1 expression in response to acoustic exposure and a significant elevation in egr-1 expression in response to hormone treatment, suggesting that hormones can play a role in phonotaxis response by modulating midbrain neurons that act as an auditory-motor interface.