Habitat Selection and Thermal Ecology of Great Basin Rattlesnakes (Crotalus lutosus) in a High-Desert Ecosystem in Eastern Nevada
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Authors
Irons, Colton Robert
Issue Date
2025
Type
Thesis
Language
en_US
Keywords
Habitat Selection , Habitat Suitability , Operative Temperature , Resource Selection Functions , Thermoregulation , Thermoregulatory Effectiveness
Alternative Title
Abstract
Rattlesnakes and other ectothermic species must select habitats that provide access to food, protection from predators, and opportunities for effective thermoregulation. Ectotherms in high-desert habitats face large seasonal and diurnal temperature fluctuations and may offset temperature extremes by alternating between warmer microhabitats (e.g., south-facing slopes, treeless rocky areas) and cooler microhabitats (e.g., dense vegetation, subterranean spaces). We evaluated habitat selection by the Great Basin rattlesnake (Crotalus lutosus) in a semi-arid montane region of eastern Nevada, using radiotelemetry data from 63 individuals tracked from 2010–2014 and 2023–2024.In chapter 1, we hypothesized that rattlesnakes would select rocky southeast-facing slopes and habitats with high shrub cover to thermoregulate while maintaining low visibility to predators. We also hypothesized that rattlesnakes would use open tree canopy areas near water sources (e.g., creeks, springs) for basking and higher prey availability. We integrated remote sensing data and topographic variables to assess habitat associations across the active season (April–October). Our results indicated that C. lutosus consistently selected low-elevation habitats (~2,000–2,200 m) characterized by moderately warm land surface temperatures (~30–35 °C), intermediate terrain roughness, and in proximity to water sources (<1 km). Snakes also favored habitats with moderate shrub cover in semi-open forests and low litter accumulation on the forest floor. These findings highlight the importance of thermal and structural habitat features in shaping seasonal movement and resource use in C. lutosus. For chapter 2, we evaluated the thermal ecology of C. lutosus in Great Basin National Park. During 2023–2024, we surgically implanted 29 snakes with temperature dataloggers to record field active body temperatures (Tb) and measured environmental (operative) temperatures (Te) in a wide range of available microhabitats. We recovered 23 temperature dataloggers in total and were able to successfully obtain data from 14 snakes (5 iButtons and 9 WeePits). These 14 dataloggers corresponded to 12 unique individuals (6 males, 4 nongravid females, and 2 gravid females). We deployed 60 operative temperature models (OTMs) during the mid-to-late active season in 2023 (July 9–October 20) and 2024 (June 22–September 21). We monitored daily body temperatures (minimum, mean, maximum) across the active season and identified the ingress period as 26 September–20 October 2023 and 8–21 September 2024 using temperature dataloggers. We derived an index of thermoregulatory effectiveness (E), interpreted as the degree to which snakes tended to achieve body temperatures nearer to their preferred temperature range than would be achieved by randomly sampling from their thermal environment. We predicted that thermoregulatory effectiveness would decline later in the active season as ambient and operative temperatures cool, reducing thermal opportunities and forcing snakes to spend more time in suboptimal microhabitats. As predicted, E was maximal in July and declined progressively toward mid-October. We hypothesized that gravid females would exhibit higher Tb and greater E than nongravid females and males. Our results indicated gravid females maintained higher average Tb (25.7 ± 4.7°C SD) than both males (22.3 ± 6.1°C SD) and nongravid females (23.1 ± 6.1°C SD). Gravid females showed higher thermoregulatory effectiveness throughout all parts of the diel cycle, with the highest E values during the day (~07:00–20:00: 0.83 ± 0.33 SD) and at dusk (~21:00–22:00: 0.84 ± 0.23 SD). Overall, gravid females exhibited the highest values of Tb and E, reflecting a strong capacity for maintaining warmer body temperatures that support optimal embryonic development during gestation. We expected thermoregulatory effectiveness to peak during the day, when snakes bask, and to remain moderate at night as they retreat into warmer subterranean refuges. E peaked during the day and persisted into the night; nocturnal values (0.32–0.49) indicated moderate thermoregulatory effectiveness, where E = 1 reflects active thermoregulation. Snakes maintained body temperature within the preferred range (Tset = 28–32°C; Cobb 1994) primarily during the day across open and shaded microhabitats; at night, Tb often exceeded Te and sometimes remained within Tset, indicating active thermoregulation. Our results contribute to the broader understanding of reptile habitat and thermal ecology, providing insights into conservation planning for montane snake populations across the Great Basin.