Castration reduces male testosterone, estradiol, and territorial aggression, but not paternal behavior in biparental dwarf hamsters (Phodopus campbelli ) Johanna M. Hume, Katherine E. Wynne-Edwards * Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received 4 February 2005; revised 3 April 2005; accepted 4 April 2005 Available online 31 May 2005 Abstract Biparental male hamsters, Phodopus campbelli , act as midwives during the birth of their litter and are highly responsive to an experimentally displaced pup. They also have high peripheral concentrations of estradiol, a hormone with known roles in maternal behavior. Surgical castration during the gestation of their first litter was used to investigate the source of that estradiol and the functional role of testicular sex steroids in paternal responsiveness. In Experiment I, castration reduced both testosterone and estradiol concentrations, confirming that the testes were the primary source of estradiol. However, neither paternal responsiveness nor multiple measures of reproductive success were altered by the castration. Aggression directed towards an intruder, however, was reduced by castration. In Experiment II, removal of prior experience with birth or pups also failed to alter paternal responsiveness in castrated males. Although the present results do not preclude a role for local estradiol synthesis in the brain, results do not support an association between high circulating estradiol in males and their paternal behavior. D 2005 Elsevier Inc. All rights reserved. Keywords: Paternal care; Intruder aggression; Estradiol; Testosterone; Phodopus campbelli ; Djungarian hamster; Castration; Biparental care Estradiol has long been recognized as an important mediator of maternal behavior (Rosenblatt et al., 1996, 1998; Wynne-Edwards and Reburn, 2000), yet until recently, it was largely ignored in the regulation of paternal behavior. However, male rats exhibit maternal behavior when they are primed with estradiol and progesterone, then stimulated with estradiol (Rosenblatt and Ceus, 1998; Rosenblatt et al., 1996). In addition, men becoming fathers have higher estradiol concentrations than control men (Berg and Wynne-Edwards, 2001), and aromatization of andro- gens to estradiol is important for paternal behavior in the biparental California mouse (Peromyscus californicus ; Trainor and Marler, 2002). Thus, it is possible that paternal behavior involves the estradiol-sensitive circuitry for mater- nal behavior (Wynne-Edwards and Reburn, 2000). Phodopus campbelli and Phodopus sungorus are two closely related species of dwarf hamsters that have high peripheral estradiol concentrations in males (Schum and Wynne-Edwards, 2005), but that differ in their paternal behavior. P. campbelli is obligately biparental whereas P. sungorus is only facultatively biparental (Wynne-Edwards, 1987, 1995, 1998, 2003). Based on the role for estradiol in maternal behavior, Schum and Wynne-Edwards (2005) hypothesized that P. campbelli , but not P. sungorus , would show dynamic regulation of male estradiol concentration as the birth approached and after experience with the young pups. In contrast to that prediction, results challenged the emerging role of estradiol in male paternal responsiveness. Peripheral estradiol was stable in biparental males as they became fathers, but elevated before the birth, and later during pup development in the non-paternal P. sungorus males (Schum and Wynne-Edwards, 2005). 0018-506X/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2005.04.001 * 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). Hormones and Behavior 48 (2005) 303 – 310 www.elsevier.com/locate/yhbeh