Lizard Sceloporus Occidentalis Research Articles

Population expansion, divergence, and persistence in Western Fence Lizards (Sceloporus occidentalis) at the northern extreme of their distributional range

Population dynamics within species at the edge of their distributional range, including the formation of genetic structure during range expansion, are difficult to study when they have had limited time to evolve. Western Fence Lizards (Sceloporus occidentalis) have a patchy distribution at the northern edge of their range around the Puget Sound, Washington, where they almost exclusively occur on imperiled coastal habitats. The entire region was covered by Pleistocene glaciation as recently as 16,000 years ago, suggesting that populations must have colonized these habitats relatively recently. We tested for population differentiation across this landscape using genome-wide SNPs and morphological data. A time-calibrated species tree supports the hypothesis of a post-glacial establishment and subsequent population expansion into the region. Despite a strong signal for fine-scale population genetic structure across the Puget Sound with as many as 8–10 distinct subpopulations supported by the SNP data, there is minimal evidence for morphological differentiation at this same spatiotemporal scale. Historical demographic analyses suggest that populations expanded and diverged across the region as the Cordilleran Ice Sheet receded. Population isolation, lack of dispersal corridors, and strict habitat requirements are the key drivers of population divergence in this system. These same factors may prove detrimental to the future persistence of populations as they cope with increasing shoreline development associated with urbanization.

RESPONSES OF A MONTANE LIZARD ASSEMBLAGE TO AN EXTENDED DROUGHT

Modern climate change will increase temperatures, as well as the frequency and severity of droughts and wildfires across the semiarid montane regions of the southwestern United States. Predictions of reduced habitat suitability and increases in extinctions precipitated by climate change derive from modeled responses to these conditions, especially increased temperatures. What is often lacking are empirical tests of those modeled predictions. Here I report findings from surveys of montane lizard assemblages during a period of increased temperatures, extended drought, and including a wildfire. I selected survey routes that included upper-elevation limits for western fence lizards (Sceloporus occidentalis), granite spiny lizards (Sceloporus orcuttii), and side-blotched lizards (Uta stansburiana), as well as the lower limits for southern sagebrush lizards (Sceloporus vandenburgianus), to measure the extent to which habitat occupancy shifted as drought conditions changed over 4 years. At the onset of drought, the abundance of all species declined. After initial declines, populations stabilized and I observed recruitment, positive population growth, and elevation shifts with only modest increases in rainfall. Once the drought abated, there were further increases in recruitment and population growth, but a colder winter and heavy snows in 2017 were coincident with declines and range contractions in both western fence and side-blotched lizards on the higher-elevation survey route. There was no indication that any of these species' populations were at risk of local extinctions; rather, shifts at range margins and use of microclimates revealed resilience and mechanisms of how these species might deal with future conditions, including occupying climate refugia.

Effect of host lizard anemia on host choice and feeding rate of larval western black-legged ticks (Ixodes pacificus)

Although ticks are known to exhibit preferences among host species, there is little evidence that ticks select hosts within a species based on physiological condition. It may be beneficial for ticks to choose hosts that are easier to feed upon if the ticks can perceive indicative chemical or other signals from the host. For example, if ticks can detect host hematocrit they may choose hosts with high hematocrit, facilitating a faster blood meal. It may similarly be adaptive for ticks to avoid anemic hosts because it may be difficult for them to obtain an adequate meal and feeding duration may be extended. We tested the hypothesis that larval western black-legged ticks (Ixodes pacificus) detect host hematocrit using external cues and choose healthy over anemic hosts, allowing them to feed more quickly. We presented groups of larval ticks with pairs of healthy and anemic male western fence lizards (Sceloporus occidentalis), allowed them to select a host, and measured the feeding duration of the ticks. We found that the ticks did not exhibit a statistically significant preference for healthy over anemic lizards, but that the ticks fed to repletion significantly faster on healthy hosts than on anemic hosts. Larval ticks may not be able to detect external cues indicating the health of the host, at least not in terms of their hematocrit. The extended feeding duration likely reflects the extra time needed for the ticks to concentrate the blood meal of their anemic hosts.

Impact of Malarial Infection on Metabolism and Thermoregulation in the Fence Lizard Sceloporus occidentalis from Oregon

We examined the influence of reduced hemoglobin concentrations resulting from malarial infection on metabolism and thermoregulation in the Fence Lizard, Sceloporus occidentalis. Malarial infections diminish the oxygen-carrying capacity of the blood, depressing hemoglobin concentrations by as much as 25%. Lizards collected from Oregon were screened for Plasmodium mexicanum. Malaria-infected and uninfected lizards were placed on a temperature-controlled treadmill at preferred body temperature (35°C) and induced to walk at constant speed for 5 min (0.7 m min−1) or sprint maximally to exhaustion (<2 min of activity). Excess oxygen consumption during recovery was over 30% higher in infected lizards when compared to uninfected lizards following similar levels of activity. Malarial infection significantly elevated blood lactate levels following sprint or endurance activity (20 and 36% elevation, respectively) when compared to uninfected lizards. Malarial infection significantly increased preferred body temperature tested in a thermal gradient following sustained activity (2–4°C increase) but had little impact on thermoregulation following exhaustive exercise. There was a strong negative correlation between hemoglobin concentration and postactivity body temperature. Our findings suggest that there are metabolic costs, measured as excess oxygen consumption and thermoregulation disruptions, associated with malaria-induced hemoglobin depression. These results may help explain limited size and reproductive success previously measured in malaria-infected lizards.

Clonal diversity within infections and the virulence of a malaria parasite,Plasmodium mexicanum

Both verbal and mathematical models of parasite virulence predict that genetic diversity of microparasite infections will influence the level of costs suffered by the host. We tested this idea by manipulating the number of co-existing clones of Plasmodium mexicanum in its natural vertebrate host, the fence lizard Sceloporus occidentalis. We established replicate infections of P. mexicanum made up of 1, 2, 3, or >3 clones (scored using 3 microsatellite loci) to observe the influence of clone number on several measures of parasite virulence. Clonal diversity did not affect body growth or production of immature erythrocytes. Blood haemoglobin concentration was highest for the most genetically complex infections (equal to that of non-infected lizards), and blood glucose levels and rate of blood clotting was highest for the most diverse infections (with greater glucose and more rapid clotting than non-infected animals). Neither specific clones nor parasitaemia were associated with virulence. In this first experiment that manipulated the clonal diversity of a natural Plasmodium-host system, the cost of infection with 1 or 2 clones of P. mexicanum was similar to that previously reported for infected lizards, but the most complex infections had either no cost or could be beneficial for the host.

Estimating maximum performance: effects of intraindividual variation

Researchers often estimate the performance capabilities of animals using a small number of trials per individual. This procedure inevitably underestimates maximum performance, but few studies have examined the magnitude of this effect. In this study we explored the effects of intraindividual variation and individual sample size on the estimation of locomotor performance parameters. We measured sprint speed of the lizard Sceloporus occidentalis at two temperatures (20 degrees C and 35 degrees C), obtaining 20 measurements per individual. Speed did not vary temporally, indicating no training or fatigue effects. About 50% of the overall variation in speed at each temperature was due to intraindividual variation. While speed was repeatable, repeatability decreased slightly with increasing separation between trials. Speeds at 20 degrees C and 35 degrees C were positively correlated, indicating repeatability across temperatures as well. We performed statistical sampling experiments in which we randomly drew a subset of each individual's full set of 20 trials. As expected, the sample's maximum speed increased with the number of trials per individual; for example, five trials yielded an estimate averaging 89% of the true maximum. The number of trials also influenced the sample correlation between mean speeds at 20 degrees C and 35 degrees C; for example, five trials yielded a correlation coefficient averaging 90% of the true correlation. Therefore, intraindividual variation caused underestimation of maximal speed and the correlation between speeds across temperatures. These biases declined as the number of trials per individual increased, and depended on the magnitude of intraindividual variation, as illustrated by running sampling experiments that used modified data sets.

Isolation and partial characterization of proteins involved in maternal transfer of selenium in the western fence lizard (Sceloporus occidentalis)

Selenium from dietary exposure is efficiently transferred from mother to offspring in oviparous vertebrates, where it can cause severe teratogenic effects. We isolated and partially characterized proteins involved in maternal transfer of selenium in the oviparous lizard Sceloporus occidentalis using size-exclusion chromatography, inductively coupled plasma-mass spectrometry, and polyacrylamide gel electrophoresis. Selenium from dietary selenomethionine exposure was incorporated into at least three egg proteins. One of these proteins was lipovitellin. The other two proteins may be part of a previously unknown mechanism of maternal transfer of Se that is independent of vitellogenesis or albumin secretion. Our results suggest at least three pathways for maternal transfer of Se in vertebrates that may vary in importance depending on the species.

Effects of Body Mass, Feeding, and Circadian Cycles on Metabolism in the Lizard Sceloporus occidentalis

Abstract We examined aspects of pre- and postprandial metabolism in the diurnally active Western Fence Lizard, Sceloporus occidentalis, by measuring rates of oxygen consumption (Vo2) and carbon dioxide production (Vco2) at 30°C. Sceloporus occidentalis exhibited strong circadian variation in metabolism that continued throughout digestion, with diurnal peaks in metabolism up to four times as high as nocturnal minimum values (standard metabolic rate, SMR). Metabolism increased with increasing body size (mass range = 3.65–19.02 g), with mass exponents ranging from 0.61–0.82. Metabolism of lizards fed meals equivalent to 1.4, 2.9, and 3.9% of their body mass was elevated above fasting metabolism, although significant differences in metabolism were not detected among the three meal sizes. Maximum metabolism during digestion was from 1.2–1.3 times that of maximum fasting metabolism, a value similar to that of other small, frequently feeding lizards. Specific dynamic action (SDA) ranged from 2.38–22.02 mL O2 and...

Measurement of Eggshell Thickness Between two Species of Sceloporus Lizards Using Scanning Electron Microscopy

Thermal environment exerts a strong influence on lizard life histories by constraining seasonal activity time, fecundity, growth, and embryonic development. in cold climates, extending the duration of egg retention within the female's body permits embryos to develop more rapidly than would be possible at ambient nest temperatures. The duration of egg retention time in the oviduct is largely constrained by the ability of the growing embryo to uptake oxygen and release carbon dioxide. Parchment shelled eggs of most squamates (lizards, snakes, and amphisbaenians) while permeable to water and gases, also acts as a diffusive barrier to gas exchange. As a result, most taxa with extended egg retention exhibit a concomitant reduction in eggshell thickness.To assess the influence of thermal environment on egg retention time, I sampled eggs from the lizards Sceloporus occidentalis and Sceloporus graciosus(Phrynosomatidae) from three populations at high, intermediate, and low elevation, in the San Gabriel Mountains of Southern California.

Life History of a Malaria Parasite (Plasmodium mexicanum) in Its Host, the Western Fence Lizard (Sceloporus occidentalis): Host Testosterone as a Source of Seasonal and Among-Host Variation?

Life History of a Malaria Parasite (Plasmodium mexicanum) in Its Host, the Western Fence Lizard (Sceloporus occidentalis): Host Testosterone as a Source of Seasonal and Among-Host Variation?

Variation in life-history traits of Plasmodium mexicanum, a malaria parasite infecting western fence lizards: a longitudinal study

The life history of malaria parasites (Plasmodium spp.) is directly related to their transmission, virulence, and population dynamics. I followed the life history of Plasmodium mexicanum in naturally infected western fence lizards (Sceloporus occidentalis) over a 4-year period, using a mark-recapture technique. The life-history traits measured included peak parasitemias and population growth rates of asexual forms, gametocytes, and total parasites. Among malaria infections, variation was high for all measured traits. Growth rates varied up to 11-fold, and among stable infections, average asexual parasitemias ranged from 0.2 to 13.2 and gametocytemias from 0.5 to 66.2 parasites per 1000 erythrocytes. This variation was not related to infection prevalences, which were similar among years and between male and female hosts. Host age and gender were not related to peak parasitemia or average growth rate of asexual forms. However, the growth rate of gametoctyes was higher in older lizards. Gametocytemia and parasitemia were significantly higher late in the warm season, when sand-fly vectors are active. These data reveal that life-history traits of P. mexicanum are highly variable within an infected host population, and that the variation is partially related to the age of the infected host or the time of year the host was examined.

Abundance of ticks (Acari: Ixodidae) infesting the western fence lizard, Sceloporus occidentalis, in relation to environmental factors.

We examined the impact of environmental characteristics, such as habitat type, topographic exposure and presence of leaf litter, on the abundance of Ixodes pacificus ticks infesting the western fence lizard (Sceloporus occidentalis) at the University of California Hopland Research and Extension Center (HREC), Mendocino County, California. A total of 383 adult lizards were slip-noosed and examined for tick infestation in April and May 1998. At least 94% of the lizards were infested by ticks and at least 20% of the females and 33% of the males carried > 15 ticks. This intensive utilization of western fence lizards (which do not serve as natural reservoirs for Lyme disease spirochetes) by subadult ticks, is probably the primary reason for the low prevalence of infection with Borrelia burgdorferi in I. pacificus nymphs and adults previously recorded at the HREC. Tick loads were higher on male than female lizards. Also, male lizards were generally more heavily infested in late April than in late May. The prevalence of tick infestation exceeded 88% in all habitat types but males collected in woodland and grass/woodland edges had higher tick loads than those collected in open grassland. Male lizards captured in open, exposed grassland tended to carry heavier tick loads in northern/eastern, as compared to southern/western, exposures, and when leaf litter was present.

Habitat Relationships of Amphibians and Reptiles in California Oak Woodlands

We used pitfall traps and time-constrained searches to sample amphibians and reptiles and to describe their habitats in oak woodlands at three areas in California. We captured 766 individuals rep- resenting 15 species during pitfall trapping and 333 animals representing 15 species during the time-con- strained searches. A total of 19 species were sampled. Across all study areas, several positive relationships were found between animal abundance and the cover of specific tree species. At Tejon Ranch, two salaman- ders-Batrachoseps nigriventris and Ensatina eschscholtzii croceater-were associated with canyon live oak (Quercus chrysolepis) and two lizards-Sceloporus occidentalis and Eumeces gilberti-were associated with California black oak (Q. kelloggii). At San Joaquin, B. nigriventris was associated with foothill pine (Pinus sabiniana), as was B. attenuatus at Sierra Foothill. Generally, salamanders were found in live oak woodlands on north-facing slopes at Tejon Ranch, or on north-facing woodlands dominated by foothill pine and interior live oak (Q. wislizenii) at Sierra Foothill and San Joaquin. In contrast, lizards used more xeric and open habitats dominated by California black oak, blue oak (Q. douglasii), and valley oak (Q. lobata). As would be expected for terrestrial and fossorial animals, litter depth, the development of grasses and forbs, and cover by downed woody debris and rocks were important in the habitat models. At Sierra Foothill and San Joaquin, these latter variables, in addition to slope, were of primary importance in the habitat models.

Thermal Ecology of a Malarial Parasite and its Insect Vector: Consequences for the Parasite's Transmission Success

We examined the transmission biology of Plasmodium mexicanum, a parasite of the fence lizard Sceloporus occidentalis, and its vector, the sandfly Lutzomyia vexator. Female L. vexator produced a clutch of eggs after each blood meal taken from a lizard. Mortality was high after oviposition, so few sandflies were likely to take two blood meals and almost none took three. Therefore, to maximize its transmission success, the parasite must complete development in its insect host before the vector lays its eggs and takes another blood meal. Between 16°C and 32°C, temperature did not affect the longevity of female sandflies, but did affect the rate of parasite development in the insect, the rate of maturation of sandflies' eggs, and the probability of sandflies becoming infected. The above relationships with temperature were non-linear and differed in shape among the variables such that an increase in temperature between 22°C and 32°C benefited the parasite by shortening its development while not reducing the time until the sandfly's next blood meal. We measured the temperatures available to the vectors in nature (burrows of ground squirrels). Within this range, there was a window that allowed successful transmission of the parasite (based on laboratory studies). In a thermal gradient, unfed female sandflies selected mean temperatures approximately 4°C below the minimum required for transmission. After a blood meal from a non-infected lizard, the insect's mean preferred temperature increased 1.6°C, presumably to aid digestion, and if a blood meal was taken from an infected lizard mean preferred body temperature increased by 3.6°C. Compared with 10 other Plasmodium species, P. mexicanum has a very rapid rate of development in its vector. The results suggest P. mexicanum enhances its transmission success through a combination of rapid development in the insect host and manipulation of the vector's thermoregulatory behaviour.

Prevalence of a Malarial Parasite Over Time and Space: Plasmodium mexicanum in its Vertebrate Host, the Western Fence Lizard Sceloporus occidentalis

1. We studied patterns of abundance of the malarial parasite Plasmodium mexicanum in its vertebrate host, the western fence lizard Sceloporus occidentalis, over a 13-year period at 51 study sites in northern California, USA. Abundance of the vectors, sandflies in the genus Lutzomyia, was also studied among sites during a single warm season, and among nights at one site during two seasons. 2. The parasite differed in prevalence (per cent of lizards infected) in males and females; males were more often infected. Prevalence increased with body size (= older lizards were more often infected). 3. Malaria prevalence varied among sites: 0-50% of lizards were infected. Topography explains part of this variation because malaria was rare or absent at sites. > 500 m elevation. However, sites at lower elevations, even when within short distances of one another, varied in malaria prevalence. 4. Abundance of vectors was not related to prevalence of malaria at a site; sandflies were found at some sites where malaria was rare or absent, including those at high elevations. Wind speed, relative humidity and sky brightness did not affect numbers of sandflies active, but vectors were active only when air temperature was > 16°C. 5. Parasite prevalence varied among years. Environmental conditions (temperature, rainfall and plant biomass produced at the study region) were not correlated with parasite prevalence. The changes in prevalence over time resembled a cycle of long duration (10 years). 6. Classical models developed for study of malaria in humans suggest explanations for variation in prevalence of P. mexicanum among sites, and a more recent theory suggests an explanation for the possible long-duration cycle observed in this study

The western fence lizard Sceloporus occidentalis: evidence of field exposure to Borrelia burgdorferi in relation to infestation by Ixodes pacificus (Acari: Ixodidae).

The role of the Western fence lizard Sceloporus occidentalis in the enzootiology of the Lyme disease spirochete Borrelia burgdorferi was evaluated in the Hopland and Ukiah areas of Mendocino County, California. In 1989, half of 74 lizards collected monthly from April to October at Hopland were infested by the immature western black-legged tick Ixodes pacificus at a mean intensity of 6.0 ticks per lizard. The prevalence of infestation of lizards by immature I. pacificus (36 of 73) at Ukiah was similar, but the mean intensity (12.9) was approximately twice as great. Overall, zero of 223 larvae and 2 (0.6%) of 330 nymphs from both sites were found to contain spirochetes by direct immunofluorescence. Larval and nymphal I. pacificus fit the negative binomial distribution in spring, and the prevalence and abundance of these stages were significantly greater in spring than in summer at both sites. Spirochetes were not visualized in thick blood films prepared from 133 lizards from both localities. Plasma antibodies against B. burgdorferi were detected in seven of 10 experimentally inoculated lizards, in five (8%) of 63 lizards from Hopland, and in 10 (14%) of 70 lizards from Ukiah. Adult lizards had a significantly greater tick burden and seropositivity rate than juvenile lizards only at Ukiah. In 1991, efforts to detect and culture spirochetes from the blood of 21 wild-caught lizards and from the tissues of 189 associated ticks that fed xenodiagnostically on them were unsuccessful.(ABSTRACT TRUNCATED AT 250 WORDS)

Perch Height Selection by Juvenile Sceloporus Lizards: Interspecific Differences and Relationship to Habitat Use

Perch height selection was studied in juvenile western fence lizards (Sceloporus occidentalis) and sagebrush lizards (Sceloporus graciosus) in the field and in the laboratory. In nature juvenile S. occidentalis were more arboreal than were juvenile S. graciosus. In laboratory experiments with lab-reared 3-4 wk old lizards, S. occidentalis perched off the ground more frequently. Differences between these two species in habitat preference are inferred to be at least partly innate, and emerge at an early age. Within each species, juveniles from different field populations differed in habitat use in the wild: lizards were more terrestrial at higher elevations. However, lab-reared juveniles from different populations of each species did not differ in laboratory tests of perch selection. In S. occidentalis there was evidence of individual variation in perch selection in the laboratory, but no among-family variation was detected.

Influence of Behavioral Thermoregulation on Microhabitat Use by Two Sceloporus Lizards

I investigated the relationship between microhabitat use and behavioral thermoregulation in the lizards Sceloporus occidentalis and S. graciosus. I studied two populations of each species along a 1300—m elevational gradient in the San Gabriel Mountains of southern California. These species occupy different, but overlapping, elevational ranges, with S. graciosus occurring at higher elevations. Because their ranges differ, the species experience different thermal environments; however, they are nearly identical in several measures of thermal physiology. Body temperatures of active lizards varied little with altitude, although air temperatures decrease substantially over this range. Both species compensated behaviorally for variation in thermal environments by varying basking frequency and microhabitat use. By thermoregulating, lizards maintained body temperatures that favor high levels of locomotor performance and other physiological functions. Microhabitat use varied markedly with elevation. Sceloporus occidentalis was almost completely arboreal at low elevation; lizards of both species were partly arboreal, partly terrestrial at intermediate elevation; and S. graciosus was mainly terrestrial at high elevations. Thus, the species converged in microhabitat use where sympatric. These shifts in microhabitat use paralleled altitudinal changes in (i) the spatial location of thermally suitable microhabitats and (ii) habitat structure. Behavioral thermoregulation, then, appears to be an important determinant of habitat use in these lizards. Results of this study may have important implications for lizard community ecology. Microhabitats that are otherwise appropriate for lizards can be thermally unsuitable depending on the local thermal environment. Hence, the thermal environment and lizard thermal biology constrain patterns of structural habitat use. As a result, physiological considerations could limit the behavioral and evolutionary options for species experiencing interspecific competition. For example, competitors in sympatry may be unable to evolve microhabitat resource partitioning because only a subset of microhabitats are thermally suitable.

Thermal sensitivity of growth rate in hatchling Sceloporus lizards: environmental, behavioral and genetic aspects.

To investigate the physiological, behavioral, and genetic contributions to growth rate, we studied the thermal sensitivity of growth rate in hatchlings of the iguanid lizards Sceloporus occidentalis and S. graciosus in the laboratory. We used a cycling thermal regime patterned after thermal environments found in nature. Growth rates increased with duration of access to radiant heat. Thus, variation in the thermal environment can cause phenotypic variation in growth rate and hence body size. The two species differed in both the magnitude and thermal sensitivity of growth rate, and these differences were associated with differences in behavioral thermoregulation. Thus, growth is determined interactively by both behavior and physiology. We found evidence of among-family variation in the growth rates of S. occidentalis, suggesting that growth rate has the genetic potential to evolve. In S. occidentalis, both growth rate and egg size affected body size of hatchlings at several weeks of age. In turn, hatchling size may affect fitness: for example, larger S. occidentalis hatchlings had higher sprint speeds and may therefore be more adept at capturing prey or evading predators. Our results demonstrate that growth rate has genetic, behavioral, and physiological components, and that the resulting effects on body size may have important consequences for ecological performance e.g., sprint speed.

Repeatability of Individual Differences in Locomotor Performance and Body Size During Early Ontogeny of the Lizard Sceloporus Occidentalis (Baird &amp; Girard)

Repeatability of Individual Differences in Locomotor Performance and Body Size During Early Ontogeny of the Lizard Sceloporus Occidentalis (Baird &amp; Girard)