An experimental test of the testosterone mediated oxidation handicap hypothesis in a wild bird Conor C. Taff a, ⁎, Corey R. Freeman-Gallant b a Department of Evolution & Ecology and Animal Behavior Graduate Group, University of California—Davis, USA b Department of Biology, Skidmore College, USA abstract article info Article history: Received 30 January 2014 Revised 16 April 2014 Accepted 23 May 2014 Available online 4 June 2014 Keywords: Androgens Oxidation handicap hypothesis Honest signaling Sexual selection The oxidation handicap hypothesis (OHH) proposed that honesty in sexual signals is maintained when testoster- one simultaneously promotes the development of elaborate signals and imposes an oxidative cost. Although there is evidence that testosterone enhances display traits in some cases, relatively few studies have tested the prediction that testosterone generates oxidative costs. We tested this prediction experimentally by administering testosterone (n = 14) and control (n = 14) implants to free-living common yellowthroat warblers (Geothlypis trichas) and quantifying testosterone and oxidative state before and 35 ± 15 days after implantation. We interpreted our experimental results in the context of a larger database of 83 unmanipulated males observed over five breeding seasons. In our observational data, testosterone was related to aspects of the carotenoid- based bib, but these relationships were age-dependent. Bib coloration was related to testosterone only for first time breeders, while bib size was positively and negatively associated with testosterone among experienced and inexperienced breeders, respectively. Two measures of oxidative metabolism—damage to DNA and total antioxidant capacity (TAC)—were unrelated to endogenous testosterone. Despite the correlation between endogenous testosterone and plumage, our experimental results failed to support the key prediction of the OHH. Testosterone treated males had higher levels of TAC upon recapture, but oxidative damage to DNA did not differ from controls. Because antioxidants can protect against the harmful effects of oxidative stress, one interpretation of our results is that males physiologically compensated for elevated testosterone, avoiding the honesty enforcing mechanism of the OHH. Taken together, our results suggest that testosterone is not a direct mediator of honest signaling in yellowthroats via its effects on oxidative stress. © 2014 Elsevier Inc. All rights reserved. 1. Introduction The immunocompetence handicap hypothesis (ICHH) posits that honesty in sexual ornamentation can be maintained by the dual action of testosterone, which both promotes the development of elaborate signals and suppresses the immune system (Folstad and Karter, 1992). Under this model, only high quality individuals are able to maintain the testosterone levels required to develop the most elaborate ornaments without succumbing to the immunological costs generated by testosterone. The ICHH has had a profound influence on the direction of research in sexual signaling over the past 20 years. Many studies demonstrate that testosterone contributes to signal development (Blas et al., 2006; Roberts et al., 2009) and that elevated testosterone generates immune costs (Duffy and Ball, 2002; Duffy et al., 2000; Fuxjager et al., 2011). However, other studies fail to find the predicted relationships, and the generality of the ICHH is still debated (reviewed in Roberts et al., 2004). One problem with evaluating support for the ICHH is that testoster- one has multiple physiological functions and potential costs beyond signal production and immune regulation (Wingfield et al., 2000). Therefore, negative or inconclusive results can either be interpreted as a lack of support for the ICHH or explained by invoking other mecha- nisms, such as physiological compensation (e.g. Alonso-Alvarez et al., 2007b). A fuller understanding of the role that testosterone plays in maintaining signal honesty will depend on incorporating more informa- tion on the complex costs and benefits of elevated testosterone with the existing literature on testosterone-dependent signaling. Recent work suggests that oxidative metabolism might mediate life history trade-offs (reviewed in Monaghan et al., 2009). Noting the connections between oxidative balance, signals, and testosterone, Alonso-Alvarez et al. (2007a, 2008) proposed the oxidation handicap hypothesis (OHH) as a complement to the ICHH. Like the ICHH, the OHH makes two key predictions: first, that testosterone drives the production of sexual signals, and second, that testosterone produces oxidative costs (e.g. increased production of reactive oxygen species). The first prediction is addressed by many previous studies of the ICHH. The second prediction, however, has so far been tested relatively rarely, especially when compared to the number of studies looking for Hormones and Behavior 66 (2014) 276–282 ⁎ Corresponding author at: Department of Wildlife, Fisheries & Conservation Biology, University of California—Davis, One Shields Avenue, Academic Surge, Davis, CA 95616, USA. E-mail address: cctaff@ucdavis.edu (C.C. Taff). http://dx.doi.org/10.1016/j.yhbeh.2014.05.006 0018-506X/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Hormones and Behavior journal homepage: www.elsevier.com/locate/yhbeh