Growth, Activity, and Survivorship from Three Sympatric Parthenogenic Whiptails (Family Teiidae)

—We surveyed whiptail lizard populations for seven summers (2000–2006) in riparian forests along the Rio Grande in central New Mexico. We captured 5,382 individuals from three parthenogenic species (Aspidoscelis exsanguis, Aspidoscelis neomexicana, and Aspidoscelis uniparens) including 129 hatchlings (young-of-the-year) that were later recaptured as adults. Growth data were fit to a logistic growth model and compared using a likelihood ratio test. Comparisons of growth rates showed that A. exsanguis grew faster than both A. neomexicana and A. uniparens and attained a larger snout–vent length (SVL). Comparisons of capture rates showed that species had similar activity patterns during the summer. Captures of adults peaked in mid-June and decreased in August. Hatchlings became active at the end of July and captures peaked in September. Some individuals were captured several seasons indicating that lizards lived for at least 3–4 yr. Our study shows both similarities and differences in life-history characteristics for three closely related and coexisting whiptail species. Whiptails (genus Aspidoscelis) are some of the most apparent lizards in the southwestern United States but are lesser known in terms of understanding their seasonal activity, growth rates, and longevity. Growth and activity are fundamental in life-history studies, and whiptail lizards provide a model system to study the demographics of coexisting populations. Studies on whiptails in this region have described their reproductive modes (Wright and Lowe, 1968; Dessauer and Cole, 1986; Reeder et al., 2002), life-history characteristics (Pianka, 1970; Congdon et al., 1978; Vitt and Breitenbach, 1993; Taylor and Caraveo, 2003), and habitat associations (Cuellar, 1979; Price et al., 1993; Bateman et al., 2008a). However, little has been published on body growth rates, longevity, and activity patterns of these lizards. Although mark– recapture methods can yield reliable data on growth from specific time intervals (Halliday and Verrell, 1988; Paulissen, 1999–2000), few attempts have modeled growth rates, seasonal activity patterns, and longevity for whiptails in a field setting (but see Carpenter, 1959). New Mexico is inhabited by 15 species of Aspidoscelis lizards (Stuart, 2005), eight of which are parthenogenic species. As part of a study designed to evaluate the effects of removing nonnative plants and fuels on wildlife, we monitored herpetofauna for seven years in central New Mexico (Bateman et al., 2008a,b). During our study, whiptail lizards composed the majority of captures. This provided an opportunity to follow individual lizards of parthenogenic Aspidoscelis exsanguis, Aspidoscelis neomexicana, and Aspidoscelis uniparens over time and record their rates of growth from hatchling to adult, longevity, and seasonal activity patterns in a field setting. Given that these lizards experienced generally the same climatic regimes and, thus, similar resource availability during the study, and that body-size differences exist among these species (Degenhardt et al., 1996), we tested the hypothesis that adult body size differences result from differences in growth rates among species rather than differences in longevity or survivorship. We can falsify our hypothesis if growth rates are similar among species or if smaller species have lower life expectancies or lower adult survivorship compared to larger species. In addition, we present information on seasonal activity patterns for adult and young-of-the-year (YOY). METHODS AND MATERIALS Study Site.—We conducted our study in the riparian forests along the Rio Grande in semiarid central New Mexico. The riparian forests contain a mixture of native Rio Grande cottonwood (Populus deltoides wislizenii), nonnative saltcedar (Tamarix chinensis and Tamarix ramosissima), and nonnative Russian olive (Elaeagnus angustifolia) trees. We captured lizards from June to September at 12 20-ha sites spanning 140 km of riparian forest from Albuquerque (35.0004uN– 106.4104uW) to Bosque del Apache National Wildlife Refuge (33u47959N–106u52959W). Field Measurements.—We captured lizards during the summers of 2000–2006 using trap arrays with pitfall and funnel traps set along drift fences. Trapping methods and array design are described elsewhere (Bateman et al., 2008a). Traps were open continuously from June through mid-September and checked three days per week. Animals that died during the study were deposited in the Museum of Southwestern Biology (Appendix 1). We identified lizards to species using field guides (Degenhardt et al., 1996) and followed current nomenclature (Crother, 2008). At each capture we measured snout–vent (SVL) and tail lengths with a 2 Corresponding Author. Present address: Applied Sciences and Mathematics, Arizona State University Polytechnic, Mesa, Arizona 85212 USA; E-mail: [email protected] 4 Present address: USDI National Park Service, Mojave Desert, Boulder City, Nevada 89005 USA. linear ruler (millimeters) and mass (grams) with a Pesola spring scale. At first capture, we assigned each lizard a unique toe clip. We identified YOY based on body size, presence of an umbilical scar, and tail coloration. Growth Comparisons.—We used SVL rather than mass to estimate growth to eliminate confusion potentially caused by changes in stomach contents, fat bodies, reproductive status, and hydration (Dunham, 1978). We calculated growth rate by organizing numbers of captures and SVL into monthly intervals during summer censuses. SVL was averaged monthly for each individual because we found that best represented the precision of our measuring techniques. We used growing seasons (April through November) as time intervals on a continuous basis. Because lizards are inactive during the winter and presumed not to grow, we excluded winter months (December through March; sensu Haenel and JohnAlder, 2002). Because data from individuals of known size, but not age, yield size-specific growth rates and not agespecific growth rates (Halliday and Verrell, 1988), we used data from individuals first marked as YOY and later recaptured. When an individual was not captured in successive seasons, we used unique characteristics (i.e., particular toes clipped, measurements, and tail regeneration status) to distinguish individuals for inclusion in analyses. We excluded questionable records (i.e., inconsistent unique characteristics) from analyses. To address potential treatment effects from the larger study design (described elsewhere, Bateman et al., 2008a), we evaluated growth relationships of each species in treated and untreated sites using a composite likelihood ratio test across species. To relate SVL to age (months of growth), we used a logistic growth model (equation 1). The growth equation, size~a=1⁄21z exp (b{c:age) , ð1Þ was defined as a 5 asymptote of maximum size (SVL), and b and c describe the shape of the logistic curve (Ratkowsky, 1989). No log transformation was necessary because parameters a, b, and c were estimated from empirical data using a nonlinear regression procedure (Ratkowsky, 1989). We used the NLMIXED nonlinear mixed model procedure (SAS vers. 9.1, SAS Institute Inc., Cary, North Carolina, 2004) for estimation with individual animals specified as analysis subjects to account for multiple remeasurements of some animals. To compare growth rates, we tested the null hypothesis that growth relationships did not vary among species. An initial growth model was constructed by pooling size and age of all three species and estimating a single a, b, and c (equation 1); then subsequent models were constructed by estimating a, b, and c for each species and for combinations of species. We selected the most parsimonious model to explain lizard growth based on likelihood ratio test and goodness-of-fit comparisons among candidate models (Mood et al., 1974). Seasonal Activity.—We plotted activity as weekly rate of captures from 2000 through 2006. We defined rate of captures as the number of lizards captured in each site per 100 trap days and trap days were averaged within sites. Individuals captured more than one time during the week were recounted at each encounter. We assigned the Julian number of each week to observations using WEEKNUM procedure (Excel, Microsoft Corporation, Redmond, Washington, 2003). We classified lizards as YOY or adult (juveniles captured in spring were classified as adults). Longevity.—We estimated individual lifespan by calculating the longest temporal interval over which lizards first marked as YOY were recaptured. We only used records from lizards continually present in summer censuses.