Sex and seasonal differences in aggression and steroid secretion in Lemur catta: Are socially dominant females hormonally ‘masculinized’? Christine M. Drea ⁎ Department of Biological Anthropology and Anatomy, Duke University, 08 Biological Sciences Bldg., Box 90383, Durham, NC 27708-0383, USA Received 22 June 2006; revised 12 February 2007; accepted 14 February 2007 Available online 23 February 2007 Abstract Female social dominance characterizes many strepsirrhine primates endemic to Madagascar, but currently there is no comprehensive explanation for how or why female lemurs routinely dominate males. Reconstructing the evolutionary pressures that may have shaped female dominance depends on better understanding the mechanism of inheritance, variation in trait expression, and correlating variables. Indeed, relative to males, many female lemurs also display delayed puberty, size monomorphism, and ‘masculinized’ external genitalia. As in the spotted hyena (Crocuta crocuta), a species characterized by extreme masculinization of the female, this array of traits focuses attention on the role of androgens in female development. Consequently, I examined endocrine profiles and social interaction in the ringtailed lemur (Lemur catta) to search for a potential source of circulating androgen in adult females and an endocrine correlate of female dominance or its proxy, aggression. I measured serum androstenedione (A 4 ), testosterone (T), and estradiol (E 2 ) in reproductively intact, adult lemurs (10 females; 12 males) over four annual cycles. Whereas T concentrations in males far exceeded those in females, A 4 concentrations were only slightly greater in males than in females. In both sexes, A 4 and T were positively correlated, implicating the Δ 4 -biosynthetic pathway. Moreover, seasonal changes in reproductive function in both sexes coincided with seasonal changes in behavior, with A 4 and T in males versus A 4 and E 2 in females increasing during periods marked by heightened aggression. Therefore, A 4 and/or E 2 may be potentially important steroidal sources in female lemurs that could modulate aggression and underlie a suite of masculinized features. © 2007 Elsevier Inc. All rights reserved. Keywords: Female dominance; Aggression; Masculinization; Androstenedione; Testosterone; Estradiol; Seasonality; Lemur; Spotted hyena Introduction Unlike most mammals (Ralls, 1976), many strepsirrhine primates display a social organization that is characterized by female dominance over males (Jolly, 1966; Richard, 1987). This unusual trait has received considerable attention and its behavioral regulation has engendered significant debate: Some researchers suggest that males defer to females only in feeding contexts, via ‘male deference’ (Hrdy, 1981) or ‘female feeding priority’ (Jolly, 1984), whereas others suggest that females maintain elevated status through overt aggression against males, via ‘female dominance’ (Kappeler, 1990a; Pereira et al., 1990). Regardless of the gradations in intersexual social relationships, functional explanations for the evolution of this trait, that invoke benefits to the female, are often linked to reproductive energetics, maternal investment, or nutritional intake (Jolly, 1984; Richard and Nicoll, 1987; Tilden and Oftedal, 1995; Young et al., 1990); nevertheless, the proximate mechanism remains a mystery. Insight into a potential mechanism of female social dominance may derive from other unusual features of strepsirrhines, including sexual size monomorphism (Kappe- ler, 1990b), absence of bimaturation (Leigh and Terranova, 1998), and ambiguous (Hill, 1953; Ioannou, 1971; Petter- Rousseaux, 1964) or moderately ‘masculinized’ (Drea and Weil, submitted for publication) external genitalia. This array of male-like behavioral, physiological, and morphological traits calls attention to the possible role of androgens in female lemur development. Although female animals display aggres- sion in a variety of social contexts, the neuroendocrine mechanisms that govern female competitive behavior remain Hormones and Behavior 51 (2007) 555 – 567 www.elsevier.com/locate/yhbeh ⁎ Fax: +1 919 660 7348. E-mail address: cdrea@duke.edu. 0018-506X/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2007.02.006