Effects of Testosterone Treatment and Season on the Frequency of Dewlap Extensions during Male–Male Interactions in the Lizard Anolis sagrei Richard R. Tokarz, 1, * Stephen McMann,² Linda C. Smith,‡ and Henry John-Alder§ *Department of Biology, University of Miami, Coral Gables, Florida 33124; †Department of Natural Sciences, Williams Baptist College, Walnut Ridge, Arkansas 72476; ‡Stockton College of New Jersey, P.O. Box 195, Pomona, New Jersey, 082403; and §Graduate Program in Ecology and Evolution and Department of Animal Sciences, Rutgers University, 84 Lipman Drive, New Brunswick, New Jersey 08901 Received June 25, 2001; revised August 6, 2001; accepted August 7, 2001 Lizards of the genus Anolis extend and retract a large and often brightly colored throat fan called a dewlap. The dewlap in most anoles is a sexually dimorphic struc- ture. It is larger in males than females and males use the dewlap more frequently and in more contexts than do females. In the present study we investigated whether plasma testosterone (T) levels and season affect the frequency of dewlap use in male–male interactions in the brown anole, Anolis sagrei. We manipulated plasma T levels by implanting adult, castrated males with pellets that delivered no T, a moderate dose of T, or a high dose of T. In tests with stimulus males, castrated males that were treated with a moderate or with a high dose of T had a significantly higher frequency of dewlap exten- sions than did castrated males that received no T. How- ever, the frequency of dewlap extensions in castrated males that received the high dose of T did not differ significantly from castrated males that received the moderate dose of T or from noncastrated control males. Males captured during the breeding season and tested in the laboratory had a significantly higher frequency of dewlap extensions than did males captured in the post- breeding season and tested in the same manner. These results suggest that plasma T levels affect how fre- quently males of A. sagrei extend their dewlaps in male– male interactions and that seasonal changes in male dewlap use may be due to seasonal differences in plasma T levels. © 2002 Elsevier Science (USA) Key Words: androgen; Anolis sagrei; dewlaps; lizards; male display behavior; sexually dimorphic traits; testosterone. An important question in behavioral endocrinology is how hormones affect the development and use of sexually dimorphic traits (Kelley, 1988). The impor- tance of this question is reflected in the large number of studies that have examined, for example, the role of hormones in the development and activation of bird- song, a sexually dimorphic trait in many species of passerine birds (e.g., Nottebohm, Nottebohm, Crane, and Wingfield, 1987; Harding, Walters, Collado, and Sheridan, 1988; Balthazart and Ball, 1995; Arnold, 1997; Smith, Brenowitz, and Wingfield, 1997a; Smith, Brenowitz, Beecher, and Wingfield, 1997b). The throat fan or dewlap of Anolis lizards is another interesting sexually dimorphic trait that is now beginning to be examined in terms of its possible hormonal regulation (see Wade, 1999). The dewlap is a relatively large and often brightly colored throat skin that can be extended and retracted in a variety of visual displays. Interest- ingly, West-Eberhard (1983) has suggested that the dewlap of male Anolis lizards might be considered to be the saurian equivalent of the plumage and song of male birds. The dewlap is considered to be a sexually dimor- phic trait in anoles because males in most species have a larger dewlap than females (Orrell and Jenssen, 1998; Jenssen, Orrell, and Lovern, 2000). Moreover, there is evidence that the sexes differ in at least some neural and other morphological features that are in- volved in display of the dewlap (Crews and Green- berg, 1981; Wade, 1998; O’Bryant and Wade, 1999). The sexes also differ in the social context in which the dewlap is used. Males in agonistic interactions extend 1 To whom correspondence and reprint requests should be ad- dressed. Fax: (305) 284-3039. E-mail: rtokarz@miami.edu. Hormones and Behavior 41, 70–79 (2002) doi:10.1006/hbeh.2001.1739, available online at http://www.idealibrary.com on 0018-506X/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved. 70