Neuroendocrine regulation of female mate recognition behavior in túngara frogs

An enduring question in sexual selection is how females choose mates to increase their reproductive success. Therefore, understanding the proximate mechanisms underpinning female choice is essential to understanding speciation and evolution. An important mechanism underlying female mate choice is modulation of neural pathways by sex steroid hormones. My dissertation explores how steroid hormones influence female sexual behavior and auditory processing of species-specific signals in túngara frogs. To determine which hormonal conditions promote sexual behavior in female túngara frogs, I assessed the effect of hormone manipulations on phonotaxis behavior toward conspecific calls in post-reproductive females. I found that estradiol is sufficient for the expression of phonotaxis behavior. Steroid hormones exert their effects by acting through steroid receptors in the brain. I found expression of androgen receptor (AR), estrogen receptors alpha and beta (ERalpha and ERbeta mRNA in parts of the auditory system and the forebrain auditory targets. I identified new putative sites of steroid action within the pallium, posterior tuberculum, locus coeruleus, and the principal nucleus of the torus semicircularis. Females had higher ERalpha and ERbeta expression than males in the auditory midbrain, whereas males had higher AR expression than females, indicating that sex steroid hormones are likely to have sexually dimorphic effects on auditory processing. Neural representation of species-specific signals is thought to emerge at higher levels of processing. I measured expression of the immediate early gene egr-1 in response to conspecific, heterospecific, or no sound stimuli in parts of the ascending auditory system and the primary forebrain targets. With three exceptions, all auditory nuclei showed greater responses to the conspecific call than the heterospecific call, suggesting that the auditory system responds preferentially to conspecific stimuli. Finally, I measured expression of egr-1 after estradiol injections in parts of the ascending auditory system and its forebrain targets and the nucleus accumbens. Both estradiol and conspecific calls together induced greater neural responses than either alone in most auditory nuclei and the nucleus accumbens, suggesting an additive effect on egr-1 induction. I conclude that estradiol is an important neuromodulator and may influence mate recognition systems that are critical for mate choice