Estradiol, reproductive cycle and preference behavior in a northern swordtail.

Estrogen is associated with female sexual behaviors, particularly receptive behaviors during the reproductive cycle. Less is known about the relationship between estrogen and female preference behaviors that may precede receptivity and copulation. Separating the mechanisms underlying preference from receptivity is often confounded by the tightly coupled cycle- or estrogen-dependent expression of female sexual behaviors. Here we utilize a live-bearing poeciliid (Xiphophorus nigrensis), a model species for studying the evolution of female mate choice that can store sperm over multiple brood cycles. We assayed estradiol along with preference, receptivity and locomotor behaviors in gestating females and then re-tested these females on days 1, 7, 14, 21, and 28 post-parturition. With a posteriori reproductive cycle assessment, we asked whether reproductive state predicts differences in (i) estradiol levels, and (ii) behaviors (preference, receptivity, and general locomotor activity). We then examined if estradiol levels (independent of reproductive state) explain any variation in these behaviors. We found that endogenous estradiol levels vary across the reproductive cycle: gestating females had lower estradiol levels than those undergoing vitellogenesis/fertilization. In contrast, receptivity and preference behaviors did not vary over the reproductive cycle. Estradiol levels did not predict variation in receptive behavior, but were associated with increased locomotion. While individual female preference behaviors were consistent across the reproductive cycle, there was a small negative relationship between estradiol and preference behaviors explaining between 3% and 10% of the inter-female variation in preference behavior. Our data indicate X. nigrensis females may exhibit a facultatively dissociated reproductive system.