Thermoregulation of two sympatric species of horned lizards in the Chihuahuan Desert and their local extinction risk Rafael A. Lara-Reséndiz a,b,n , Héctor Gadsden c , Philip C. Rosen d , Barry Sinervo b , Fausto R. Méndez-De la Cruz a a Laboratorio de Herpetología, Departamento de Zoología, Instituto de Biología, Universidad Nacional Autónoma de México, A. P. 70515, C.P. 04510 México, D.F., Mexico b Department of Ecology and Evolutionary Biology, Earth and Marine Sciences Building A316, University of California, Santa Cruz, CA 95064, USA c Instituto de Ecología, A. C., Miguel de Cervantes No.120, Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Chihuahua, Mexico d School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721, USA article info Article history: Received 2 October 2013 Received in revised form 21 November 2014 Accepted 24 November 2014 Available online 27 November 2014 Keywords: Climate change Ecophysiological model Habitat thermal quality Phrynosoma Thermal hours of restriction Thermoregulatory efficiency abstract Thermoregulatory studies of ectothermic organisms are an important tool for ecological physiology, evolutionary ecology and behavior, and recently have become central for evaluating and predicting global climate change impacts. Here, we present a novel combination of field, laboratory, and modeling approaches to examine body temperature regulation, habitat thermal quality, and hours of thermal re- striction on the activity of two sympatric, aridlands horned lizards (Phrynosoma cornutum and Phryno- soma modestum) at three contrasting Chihuahuan Desert sites in Mexico. Using these physiological data, we estimate local extinction risk under predicted climate change within their current geographical distribution. We followed the Hertz et al. (1993, Am. Nat., 142, 796–818) protocol for evaluating ther- moregulation and the Sinervo et al. (2010, Science, 328, 894–899) eco-physiological model of extinction under climatic warming. Thermoregulatory indices suggest that both species thermoregulate effectively despite living in habitats of low thermal quality, although high environmental temperatures restrict the activity period of both species. Based on our measurements, if air temperature rises as predicted by climate models, the extinction model projects that P. cornutum will become locally extinct at 6% of sites by 2050 and 18% by 2080 and P. modestum will become extinct at 32% of sites by 2050 and 60% by 2080. The method we apply, using widely available or readily acquired thermal data, along with the modeling, appeared to identify several unique ecological traits that seemingly exacerbate climate sensitivity of P. modestum. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Global climate change and habitat alterations due to human activities exert significant selective pressures and negative effects on natural populations of many taxa (Cahill et al., 2013; Chen et al., 2011; Huey et al., 2012; Kareiva et al., 1993; Pereira et al., 2010). Recent research shows 12% of 200 Mexican populations of Scelo- porus lizards have become locally extinct since 1975 as result of rising environmental temperatures (Sinervo et al., 2010). This oc- curs because when lizards are exposed to operative environmental temperatures (T e ) greater than their preferred body temperatures (T p ), activity time is restricted, and thus net energy gain becomes insufficient for females in their reproductive season (Dunham, 1993; Huey et al., 2010; Sinervo et al., 2010, 2011). Reduction in activity time is expressed as hours of thermal restriction (h r ), or hours of forced inactivity, which are the cumulative hours each day during the reproductive season when T e exceeds T p . Most lizards must thermoregulate to maintain their field body temperature (T b ) within an optimal range, but environments with high temperatures increase the risk of overheating, shortening activity time (Avery, 1982; Cowles and Bogert, 1944; Dunham, 1993; Huey, 1982), and thus elevating extinction risk. Therefore, thermoregulation studies of ectothermic organisms highly con- strained by their environment are essential to evaluate the beha- vioral capacity of different species to minimize high T b in ther- mally severe environments (Kearney et al., 2009; Vickers et al., 2011). Currently, evaluating temperature regulation in field-active reptiles can be approached with the protocol proposed by Hertz et al. (1993), which has been used widely for desert lizards and the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jtherbio Journal of Thermal Biology http://dx.doi.org/10.1016/j.jtherbio.2014.11.010 0306-4565/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author at: Department of Ecology and Evolutionary Biology, Earth and Marine Sciences Building A316, University of California, Santa Cruz, CA 95064, USA. E-mail addresses: rafas.lara@gmail.com (R.A. Lara-Reséndiz), hgadsden@gmail.com (H. Gadsden), pcrosen@email.arizona.edu (P.C. Rosen), lizardrps@gmail.com (B. Sinervo), faustor@ib.unam.mx (F.R. Méndez-De la Cruz). Journal of Thermal Biology 48 (2015) 1–10