Competing Behaviors of Thermoregulation and Ambush Foraging in the Timber Rattlesnake (Crotalus horridus horridus): A Mechanistic Assessment of Thermal Conduction

The interaction between the biophysical environment and ectotherm morphology elicits behaviors designed to maintain internal body temperature (Tb) within a range that promotes physiological functions. The short-term requirements of mass (energy requirements) and heat balance are subject to tradeoffs imposed by the organisms current physiological (heat and mass budgets) and environmental (biophysical, demographic, social, and predation) constraints and available resources. In temperate forests, extreme temperatures are common in summer even with intermittent sun exposure due to dense canopy cover. In Spring and Fall, temperatures can range from below freezing to 35 ℃ in 24 hrs. An ambush predator like the rattlesnake requires prolonged immobility (e.g., hours or days) to be an effective predator but thermoregulatory needs still take precedence. How then would a rattlesnake thermoregulate during periods of exposure to thermal extremes while remaining motionless for long periods? In this dissertation I explored the relationship between thermoregulation and ambush foraging using fixed videography, principles of heat transfer, and operative temperature models to address my general question. I found that in this study population C. horridus are rarely forced to make thermoregulatory movements. A combination of morphological features, thermal conduction, and positioning in the microhabitat are used to offset potentially extreme environmental temperatures throughout the active season. The results emphasize how adaptations for balancing tradeoffs between foraging and thermoregulation in a variable thermal environment help shape C. horridus life-history traits

Seasonal variation in movement and activity patterns of Crotalus willardi obscurus in the Animas Mountains, New Mexico

Movements by individuals of the New Mexico Ridgenose Rattlesnake, Crotalus willardi obscurus, at a site in the Animas Mountains were studied by telemetric methods over a multi-year period. Data are presented on a sample of 24 snakes in which transmitters had been implanted, all but nine of which were males. The results reveal that individual snakes were twice as active during the summer-fall monsoon (rainy season) than in the drier part of the warm season, a result consistent with non-telemetrically obtained observational data. Moreover, males showed greater distance movement per day than females in both seasons, although the sample size for females was too small to test for statistical significance. Such a gender difference has been suggested to occur in other snake species as the result of sexual selection, especially with respect to the energy conservation strategies associated with female reproduction. In the case of this rattlesnake, feeding efficiency and avoidance of predation may also be involved. Although the present telemetric study of movement in Crotalus willardi obscurus reveals no unexpected patterns of behavior for this snake, even this information contributes to the assembly a more complete picture of the biology of a poorly-known snake that will probably remain to be threatened at some level indefinitely

Nowhere to Go but Up: Impacts of Climate Change on Demographics of a Short-Range Endemic (Crotalus willardi obscurus) in the Sky-Islands of Southwestern North America.

Biodiversity elements with narrow niches and restricted distributions (i.e., 'short range endemics,' SREs) are particularly vulnerable to climate change. The New Mexico Ridge-nosed Rattlesnake (Crotalus willardi obscurus, CWO), an SRE listed under the U.S. Endangered Species Act within three sky islands of southwestern North America, is constrained at low elevation by drought and at high elevation by wildfire. We combined long-term recapture and molecular data with demographic and niche modeling to gauge its climate-driven status, distribution, and projected longevity. The largest population (Animas) is numerically constricted (N = 151), with few breeding adults (Nb = 24) and an elevated inbreeding coefficient (ΔF = 0.77; 100 years). Mean home range (0.07 km2) is significantly smaller compared to other North American rattlesnakes, and movements are within, not among sky islands. Demographic values, when gauged against those displayed by other endangered/Red-Listed reptiles [e.g., Loggerhead Sea Turtle (Caretta caretta)], are either comparable or markedly lower. Survival rate differs significantly between genders (female<male) and life history stages (juvenile<adult) while a steadily declining population trajectory (r = -0.20±0.03) underscores the shallow predicted-time-to-extinction (17.09±2.05 years). Core habitat is receding upwards in elevation and will shift 750 km NW under conservative climate estimates. While survival is significantly impacted by wildfire at upper elevations, the extinction vortex is driven by small population demographics, a situation comparable to that of the European Adder (Vipera berus), a conservation icon in southern Sweden. Genetic rescue, a management approach successfully employed in similar situations, is ill advised in this situation due to climate-driven habitat change in the sky islands. CWO is a rare organism in a unique environment, with a conserved niche and a predisposition towards extinction. It is a bellwether for the eventual climate-driven collapse of the Madrean pine-oak ecosystem, one of Earth's three recognized megadiversity centers

Population demographic parameters for <i>Crotalus willardi obscurus</i> (= CWO) in the Animas Mountain sky island compiled by life history stage and population.

<p>Comparison = Life history stages evaluated; df = degrees of freedom; Survival = F-value for comparison of survival by life history stage; Capture = F-value for comparison of capture probability by life history stage; Pop. Size = F-value for comparison of population size by life history stage.</p><p>** = Statistical significance at P < 0.05.</p><p>Population demographic parameters for <i>Crotalus willardi obscurus</i> (= CWO) in the Animas Mountain sky island compiled by life history stage and population.</p

Distribution of Crotalus willardi obscurus in North American (top picture), and magnified into a perspective of the three sky-islands in southwestern North America (bottom picture) that straddle southeastern Arizona (AZ), southwestern New Mexio (NM), and north-central México (MX).

<p>PEL = Peloncillos Mountains (AZ); ANM = Animas Mountains (NM); SSL = Sierra San Luis Mountains (México). Numbers indicate specific locations of study sites, two of which are within Animas Mountains: (1) = West Fork Canyon (ANM-W); (2) = Indian Creek (ANM-I); (3) = Peloncillos Mountain; (4) = Sierra San Luis Mountain.</p

Declines in population growth (= r) for <i>Crotalus willardi obscurus</i> in the Animas Mountain sky island under four levels of inbreeding depression (i.e., None; Low; Moderate; High), as gauged using both deterministic and stochastic population models.

<p>TtE = Time to extinction, in years.</p><p>Declines in population growth (= r) for <i>Crotalus willardi obscurus</i> in the Animas Mountain sky island under four levels of inbreeding depression (i.e., None; Low; Moderate; High), as gauged using both deterministic and stochastic population models.</p

Estimates for census size (= N), number of breeders (= Nb), and effective population size (= Ne) for the New Mexico Ridge-nosed Rattlesnake, <i>Crotalus willardi obscurus</i> (= CWO) in the Animas Mountain sky island.

<p>Metrics for Loggerhead Sea Turtle, <i>Caretta caretta</i> (= CC) from literature, and the Adder, <i>Vipera berus</i> (= VB) from literature or estimated in this study. Ratios of the various parameters with census size (N) are also provided.</p><p>Estimates for census size (= N), number of breeders (= Nb), and effective population size (= Ne) for the New Mexico Ridge-nosed Rattlesnake, <i>Crotalus willardi obscurus</i> (= CWO) in the Animas Mountain sky island.</p

Bioclimatic variables (WorldClim database) incorporated with193 GPS capture coordinates for <i>Crotalus willardi obscurus</i> in the sky Islands of southwestern North America provide climate envelopes and core habitat areas in ArcGIS 10.

<p>Color density = strong habitat preference, with black/dark grey being most positive. Red circles = sampling locations. 2014 = current climate envelope; 2080 = a conservative climate envelope projected 66 years in the future.</p

A Test of home range estimates for <i>Crotalus willardi obscurus</i> (= CWO) and six other rattlesnake species for which data are available, using analysis of covariance (ANCOVA) with gender and body size as covariates.

<p>Significant post-hoc pairwise differences are identified using Tukey’s Honest Significant Difference (HSD). Taxon = genus/species, where <i>C</i>. = <i>Crotalus</i> and <i>c</i>. = <i>catenatus</i>; N = number of individuals per study; Home Range = hectares with variance; P = Statistical probability of a larger value; Data Source = Literature citation.</p><p>A Test of home range estimates for <i>Crotalus willardi obscurus</i> (= CWO) and six other rattlesnake species for which data are available, using analysis of covariance (ANCOVA) with gender and body size as covariates.</p

Mean home range estimates for Crotalus willardi obscurus and six other rattlesnake species for which similar data were available.

<p>Horizontal red bar = mean value. Cva = <i>Crotalus viridis abyssus</i>; Ca = <i>Crotalus adamanteus</i>; Scc = <i>Sistrurus catenatus catenatus</i>; Cc = <i>Crotalus cerastes</i>; Sce = <i>Sistrurus catenatus edwardsii</i>; Ch = <i>Crotalus horridus</i>. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0131067#pone.0131067.t001" target="_blank">Table 1</a> for literature citations and test statistics.</p