Estradiol, reproductive cycle and preference behavior in a northern swordtail Mary E. Ramsey ⇑ , Ryan Y. Wong, Molly E. Cummings Section of Integrative Biology, University of Texas at Austin, USA article info Article history: Received 18 June 2010 Revised 12 October 2010 Accepted 18 October 2010 Available online 25 October 2010 Keywords: Estradiol Reproductive cycle Preference behavior Receptivity Xiphophorus nigrensis Sexual selection Mate choice Sperm storage Dissociated reproduction abstract Estrogen is associated with female sexual behaviors, particularly receptive behaviors during the repro- ductive 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 pref- erence 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 pref- erence behavior. Our data indicate X. nigrensis females may exhibit a facultatively dissociated reproduc- tive system. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Female reproductive cycles exist to coordinate the expression of sexual behavior and can be regulated by a variety of external and internal cues (reviewed [36]). In many taxa, steroid hormones are associated with a reproductive cycle as a mechanism to coordi- nate the timing of female reproductive effort with a maximum chance for reproductive success. This classic paradigm is known as an associated reproductive system wherein ova production, ste- roid hormones and sexual behaviors cycle together [10]. For in- stance, in the rat, female sexual behaviors are strongly associated with reproductive cycle status, and behavioral estrus (i.e. the will- ingness to perform proceptive/solicitation and receptive behaviors towards males) is expressed primarily during the proestrus/estrus stages of the ovarian cycle when estrogen levels are high, ovulation is imminent, and the female is willing to accept copulation (re- viewed in [15]). In other systems such as the red-sided garter snake, sexual behaviors and steroid hormones are decoupled. These systems display a dissociated reproductive pattern wherein steroid hormone levels and ova production do not coincide with sexual behavior, and mating behavior can occur with very low lev- els of circulating steroid hormones (e.g. [10]). The relationship between hormones and other critical behaviors leading to reproduction has been under-studied. Specifically, the role that hormones play in female mate discrimination or prefer- ences for specific male phenotypes has only recently received attention [1]. In associative reproductive systems, changes in hor- mone levels have been shown to correlate with changes in recep- tivity, permissiveness and discrimination behaviors in female anurans [28,29], while in mice, peptide hormones may play a role in governing female response to dominant males [30]. However, in these systems the role of steroid hormones in female preferences may act primarily in a motivational aspect of mate choice: driving females to prefer to associate with opposite sex partners rather than mediating a discriminatory function amongst different males [1,7,37]. Little is known about the social and physiological influences underlying female preference in a dissociated reproductive system. The de-synchronization of reproductive behavior and gamete pro- duction can arise from physical constraints (e.g. in garter snakes, [16]; and sea perch, [54]) or social environment (e.g. sperm compe- tition, see [4]). In many poeciliid fish species, a single mating bout 0016-6480/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.ygcen.2010.10.012 ⇑ Corresponding author. Address: University of Texas at Austin, 1 University Station, C0930 Austin, TX 78712, USA. Fax: +1 512 471 3878. E-mail address: mramsey@mail.utexas.edu (M.E. Ramsey). General and Comparative Endocrinology 170 (2011) 381–390 Contents lists available at ScienceDirect General and Comparative Endocrinology journal homepage: www.elsevier.com/locate/ygcen