Paternal responsiveness in biparental dwarf hamsters (Phodopus campbelli) does not require estradiol Johanna M. Hume, Katherine E. Wynne-Edwards ⁎ Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received 12 October 2005; revised 24 November 2005; accepted 25 November 2005 Available online 3 February 2006 Abstract Males of the biparental hamster species Phodopus campbelli act as midwives and are responsive to an experimentally displaced pup. Males also have peripheral estradiol concentrations that are similar to conspecific females. Castration reduces peripheral estradiol, yet does not affect paternal responsiveness despite the known role of estradiol in maternal behavior. Synthesis of estradiol within the central nervous system, however, might not be affected by castration. Males received implants of osmotic pumps containing the aromatase inhibitor letrozole to reduce both peripheral and central estradiol concentrations. Though estradiol was effectively reduced, it had no effect on paternal responsiveness or reproductive success. Neither testosterone nor aggression directed towards an intruder was altered. Results support the emerging conclusion that estradiol is not required for the exceptional paternal behavior of male P. campbelli. © 2005 Elsevier Inc. All rights reserved. Keywords: Paternal care; Intruder aggression; Estradiol; Testosterone; Phodopus campbelli; Djungarian hamster; Aromatase inhibition; Letrozole; Biparental care Few studies have examined the role of estradiol in the regulation of parental behavior in males, yet such an association is both plausible and generally supported by the limited data available. Male rats primed with progesterone and estradiol and then stimulated with estradiol exhibit levels of maternal care similar to that displayed by recently parturient females (Rosenblatt and Ceus, 1998; Rosenblatt et al., 1996). Further- more, if the medial preoptic area (MPOA) of the hypothalamus is lesioned, males, like females, can no longer be induced to exhibit maternal behavior (Miceli et al., 1983; Rosenblatt and Ceus, 1998). Castrated biparental males of the California mouse, Peromyscus californicus, express affiliative behavior towards their young litter when an aromatizeable androgen or estradiol is replaced, but not when an aromatase inhibitor, fadrozole, or a non-aromatizeable androgen is replaced (Trainor and Marler, 2001, 2002). Finally, men becoming fathers for the first time have higher salivary estradiol concentrations than control men (Berg and Wynne-Edwards, 2001). Phodopus campbelli is an obligately biparental hamster species native to the arid semi-desert regions of Siberia (Wynne-Edwards, 1987, 1995, 1998, 2003). Phodopus sungorus is a closely related species, but unlike P. campbelli, it is only facultatively biparental. Males of both species have circulating estradiol concentrations similar to those found in reproductive conspecific females (Schum and Wynne- Edwards, 2005). However, over the transition from sexually naïve, infanticidal adult to responsive parent (Vella et al., 2005), uniparental P. sungorus had significant up-regulation of estradiol as the birth approached and passed, whereas estradiol concentrations in biparental P. campbelli males remained constant through all stages (Schum and Wynne- Edwards, 2005). This result was in marked contrast to the a priori prediction that highly paternal P. campbelli males would experience increases in estradiol as the birth of their young approached, and P. sungorus would not. As such, those data were the first to suggest that there might not be a functional relationship between estradiol and paternal respon- siveness in this animal model of paternal behavior. As a further test of this conclusion, Hume and Wynne- Edwards (2005) castrated male P. campbelli and quantified the Hormones and Behavior 49 (2006) 538 – 544 www.elsevier.com/locate/yhbeh ⁎ Corresponding author. Fax: +1 613 533 6617. E-mail address: wynneedw@biology.queensu.ca (K.E. Wynne-Edwards). URL: http://biology.queensu.ca/~wynneedw (K.E. Wynne-Edwards). 0018-506X/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2005.11.005