Hot rocks or no hot rocks: overnight retreat availability and selection by a diurnal lizard

I used radio telemetry to determine the effects of substrate size and composition on overnight retreat site selection by western fence lizards ( Sceloporus occidentalis). In watersheds of northern California (USA), these lizards occupy two habitat types differing in substrate characteristics: rocky cobble bars found in the dry, active channels of rivers and grassy upland meadows. Rocky substrates, found almost exclusively on cobble bars, provided warmer potential retreat sites than all available retreat sites on meadows during the first 5 h of inactivity. Only cobble and sand substrates provided retreats with temperatures within the preferred daily active range (32–36°C) during the inactive period for these lizards (1900–0900 hours). Females on a cobble bar used rocks as retreats on >90% of nights during the breeding season whereas females on a meadow used wood (>70% of nights) and burrows (>25% of nights). In contrast to females, cobble bar males used rocks significantly less frequently (<70%) and slept in the open air significantly more frequently (25% vs. <1%). Cobble bar females further, showed a significant preference for cobbles 15 cm thick, whereas the rocks used by males did not differ significantly in thickness from those measured in randomly placed transects. Rocks 15 cm thick were the warmest retreats commonly available on this habitat type. Thus, thermal microenvironments available to and chosen by gravid female lizards differ considerably between river and non-river habitats.     

Geographic variation in the life history of the sagebrush lizard: the role of thermal constraints on activity

Thermal constraints on the time available for activity have been proposed as a proximate mechanism to explain variation in suites of life history traits. The longer that an ectotherm can maintain activity, the more time it has to forage and the greater chance that it will encounter a predator and be eaten. Thus, the thermal environment may produce a trade off between growth and survival when variation in the environment favors increased activity. I used mark-recapture data from a demographic study of three natural populations of the sagebrush lizard (Sceloporus graciosus) and estimates of thermal opportunity for each population to evaluate whether variation in the thermal environment can explain patterns of growth and survival that occur over an elevational gradient. Lizards from the highest elevation population exhibited higher individual growth rates than those of lizards from lower elevation, while mortality rates increased with elevation for these populations. The covariation of fast growth and high mortality with increased thermal opportunity is the opposite trend expected if the thermal environment alone is to explain patterns of life history in these lizards. Additional factors including thermal heterogeneity in the distribution of microhabitats of lizards, adaptation to local environmental conditions, and a potential trade-off between resource acquisition and predation risk need to be addressed to obtain a satisfactory explanation of the causative mechanisms producing life history variation.     

Colonization of novel White Sands habitat is associated with changes in lizard anti‐predator behaviour

Colonization of novel habitats is often associated with differences in ecological community composition. For small diurnal animals, differences in predator diversity and abundance can lead to behavioural shifts in the novel habitat. The eastern fence lizard Sceloporus undulatus (Bosc and Daudin, 1801) recently colonized the gypsum dunes of White Sands, a predator‐poor community relative to the predator‐rich community of the surrounding Chihuahuan dark‐soil habitat. We used field experiments to assess S. undulatus anti‐predator behaviour in white‐sand versus dark‐soil habitats, and used laboratory assays to determine whether behavioural differences could be mediated by hormonal regulation. Overall, we found that white‐sand lizards were less vigilant but more wary than their dark‐soil counterparts; it took them longer to detect a simulated predator, but once detected they were more likely to retreat from their perches than dark‐soil lizards. At the proximate level, differences in anti‐predator behaviour could not be explained by differences in plasma hormone levels (corticosterone and testosterone); we detected elevated corticosterone for lizards in our stress treatment relative to control treatment, but found no differences between habitats in baseline or acute corticosterone levels. At the evolutionary level, we suggest that differences in anti‐predator behaviour may be explained by differences across habitats in predation environment, habituation, and/or the cost of retreating. Our study implicates changes in predator community composition in mediating ecological divergence in behaviour. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 657–667.     

Incubation temperature and phenotypic traits of Sceloporus undulatus: implications for the northern limits of distribution

Cold environmental temperature is detrimental to reproduction by oviparous squamate reptiles by prolonging incubation period, negatively affecting embryonic developmental processes, and by killing embryos in eggs directly. Because low soil temperature may prevent successful development of embryos in eggs in nests, the geographic distributions of oviparous species may be influenced by the thermal requirements of embryos. In the present study, we tested the hypothesis that low incubation temperature determines the northern distributional limit of the oviparous lizard Sceloporus undulatus. To compare the effects of incubation temperature on incubation length, egg and hatchling survival, and hatchling phenotypic traits, we incubated eggs of S. undulatus under temperature treatments that simulated the thermal environment that eggs would experience if located in nests within their geographic range at 37°N and north of the species’ present geographic range at latitudes of 44 and 42°N. After hatching, snout–vent length (SVL), mass, tail length, body condition (SVL relative to mass), locomotor performance, and growth rate were measured for each hatchling. Hatchlings were released at a field site to evaluate growth and survival under natural conditions. Incubation at temperatures simulating those of nests at 44°N prolonged incubation and resulted in hatchlings with shorter SVL relative to mass, shorter tails, shorter hind limb span, slower growth, and lower survival than hatchlings from eggs incubated at temperatures simulating those of nests at 37 and 42°N. We also evaluated the association between environmental temperature and the northern distribution of S. undulatus. We predicted that the northernmost distributional limit of S. undulatus would be associated with locations that provide the minimum heat sum (∼495 degree-days) required to complete embryonic development. Based on air and soil temperatures, the predicted northern latitudinal limit of S. undulatus would lie at ∼40.5–41.5°N. Our predicted value closely corresponds to the observed latitudinal limit in the eastern United States of ∼40°N. Our results suggest that soil temperatures at northern latitudes are not warm enough for a sufficient length of time to permit successful incubation of S. undulatus embryos. These results are consistent with the hypothesis that incubation temperature is an important factor limiting the geographic distributions of oviparous reptile species at high latitudes and elevations.     

墨西哥帕丘卡卵胎生条纹强棱蜥的繁殖周期

本文报道墨西哥伊达尔戈州帕丘卡地区干旱环境中卵胎生条纹强棱蜥的繁殖周期及特征.测定墨西哥两个科学收藏馆的浸制标本获得数据.雌雄个体达到性成熟时的体长相等,为40.0 mm.雄体睾丸重量在5月开始增加,至7、8月达到最大.睾丸重量变化与降雨量、温度和光周期均无关.雌体卵泡重量在7月开始增加,并一直持续至10月.从11月至次年7月,雌体怀有胚胎.卵黄发生及胚胎发育与光周期、温度和降雨量无关.窝仔数与雌体体长呈正相关.平均窝仔数为5.2±0.2个(2-10个).     

Anonymous nuclear markers for the eastern fence lizard,Sceloporus undulatus

We present results from a screen for de novo variable nuclear loci using a genomic library approach in Sceloporus undulatus, the eastern fence lizard. We tested amplification success for 77 primer pairs in S. undulatus, Sceloporus occidentalis and Sceloporus grammicus. Many loci amplified in all three species suggesting that our primers will be useful for developing sequencing or single nucleotide polymorphism (SNP) genotyping markers in other sceloporine lizards. We also sequenced 19 loci, containing 158 variable sites, for 91 S. undulatus individuals. We report high levels of nucleotide variation in this species with an average of 38 SNPs per kilobase.     

Sexual dimorphism in histological characteristics and contractility of the iliofibularis muscle in the lizard Sceloporus torquatus

The iliofibularis is a hindlimb muscle used in lizard locomotion that is composed of at least three types of fibres: fast‐twitch‐glycolytic (FG), fast‐twitch‐oxidative‐glycolytic fibre (FOG) and slow‐twitch‐oxidative (SO). The striated skeletal muscle is a highly plastic tissue undergoing phenotypic change in response to activity. The lizard Sceloporus torquatus has sexual differences associated with microhabitat use, which can be reflected in the physiology and anatomy of the muscle, and thus, in our study, we analysed the morphological and contractile characteristics of the iliofibularis muscle (IF) of S. torquatus males and females. We found a larger prevalence of FOG compared with FG and SO fibres in the muscle of both sexes. We also found that males show larger areas of the three types of fibres, develop greater strength but also faster fatigue than females, suggesting that strength is a key functional feature that enables males to perform faster movements (but for shorter periods), associated with the demands of territoriality.     

Seasonal effects on circulating leptin in the lizard Sceloporus undulatus from two populations

Characterizing leptin's structure and function in mammals has been the subject of thousands of studies since 1994. Recently, the study of leptin has expanded to include its distribution in non-mammalian taxa, and the role that leptin plays in the reproductive axis. We demonstrated in a previous study that Sceloporus undulatus, fence lizards (ectotherms), express a leptin-like protein. In the current study we quantified seasonal variation in this putative leptin among free-ranging fence lizards from two populations characterized by early and late reproductive maturation (after one or two years, respectively). Immunoblots were performed on whole blood samples to detect leptin and estimate its titer. Leptin titers were higher in the reproductive population of S. undulatus (early maturing: 2.5 ± 0.2 μg/mL; late-maturing 2.2 ± 0.3 μg/mL; mean ± 2 S.E.), but both populations showed the same seasonal pattern. Leptin titers were lowest in fall when fat stores are expected to be highest (spring: 2.6 ± 0.3 μg/mL; summer: 2.6 ± 0.3; μg/mL; fall: 1.8 ± 0.3 μg/mL), consistent with findings of seasonal variation in free-ranging mammals. Our data support previous work asserting that lizards express leptin and that it has a similar physiological function in endotherms and ectotherms. Our long-term goal is to use leptin to manipulate age at maturity and to test fundamental questions in the evolution of life-history strategies.