Great Basin Rattlesnake Study

This article was originally published in The Midden – Great Basin National Park: Vol. 16, No. 1, Summer 2016.

Figure 1. Great Basin Rattlesnakes are among the most variable of rattlesnakes in pattern and color.

NPS Photo

By Bryan Hamilton, Wildlife Biologist In order to effectively protect and conserve biodiversity, wildlife managers and decision makers require detailed information on species population biology and demography. Although experiencing declines worldwide, snakes receive far less attention for conservation than more charismatic species such as large mammals.

The recent discovery and spread of snake fungal disease in the United States has resulted in declines of both rare and common snake species. Snakes are economically valuable and provide a variety of ecological services, such as control of rodents and rodent-borne diseases (i.e., Lyme disease and hanta virus). Snakes often occupy positions in the center of the food chain and are thus vulnerable to both top-down and bottom-up conditions such as predation and prey availability. As secretive and highly cryptic predators, obtaining robust snake population estimates and demographic survival estimates is hampered by their low detectability.

Great Basin rattlesnakes (Crotalus lutosus) are endemic to North America’s largest desert, the Great Basin. As gape-limited ambush predators, Great Basin rattlesnakes feed primarily on small mammals, lizards, and the occasional bird. Great Basin rattlesnakes hibernate communally in ancestral winter dens and are highly philopatric, with strict fidelity to those sites. Rattlesnakes disperse from their dens to forage, mate, and give birth. Site fidelity is high at communal dens and nearly all individuals return to their hibernaculum in the fall. Like most rattlesnake species, Great Basin rattlesnakes have experienced local declines and extirpations, primarily due to human persecution. We used Capture-Mark-Recapture techniques (CMR) to describe the demographic parameters of Crotalus lutosus conducted between 2001 – 2015 in Great Basin National Park.

Snakes were restrained in clear plastic tubes during processing and marked using ventral scale clipping or Passive Integrated Transponder (PIT) tags. Snout to vent length (SVL) and tail length (TL) were measured in a squeeze box to the nearest millimeter. Mass was collected using Pesola spring scales (± 2-5g) or an electronic balance (±0.1g). Sex was determined by probing for the presence of hemipenes. Snakes were released at their exact site of capture and usually retreated into crevices or under rocks.

We captured 401 individuals 799 times (148 females and 253 males). Males were captured in higher numbers than females suggesting a male-biased sex ratio across survey locations (χ2 = 27.5; P < 0.0001). While the male-biased sex ratio held for adult snakes (χ2 = 32.3; P < 0.0001), juvenile sex ratios did not differ from 1:1 (χ2 = 0.3; P < 0.588). The peak capture date occurred on 27 April ± 11.2 days. Mean SVL was 59 ± 13 cm and mean mass was 186 ± 103 g. Males were significantly longer (5.3 cm; P < 0.0001) and heavier than females (61.8 g; P < 0.0001). Rattlesnake body condition was weakly correlated with the previous year’s total precipitation (Body Condition = 0.194(% precip) -0.206, df = F1,29= 7.51, P < 0.01040, R2 = 0.295; Figure 2). Of eleven snakes found dead, we were able to read the PIT tags or ventral clips on six. One snake had died of hyperthermia (possibly after disturbance by photographers), three had been decapitated and skinned, one found dead on a road, and one killed at a local residence. We were unable to identify the ventral clips or PIT tags of five mortalities: four decapitated and skinned snakes and a juvenile that died from an apparent rock fall.

Figure 2. Relationship between snake mass and snout to vent length (SVL) for Crotalus lutosus at four sites in eastern Nevada and western Utah. Data consist of 797 measurements of 394 individuals captured between 2001 and 2015.

Mean detection probability for all sites was 0.28 ± 0.06. Detection probability increased with SVL. Mean annual survival was estimated at 0.76 ± 0.10. Using the survival estimates, average life expectancy across sites was estimated at 3.6 years and median life expectancy at 2.5 years.

Rattlesnakes in many temperate environments are limited by prey availability. This limitation is often correlated with reproduction but can also show in demographic parameters such as survival. We found some evidence that resource limitation mediated by climate affected snake body condition. This relationship was likely a result of the interactions among precipitation, vegetation, and small mammal abundance, which are the primary prey for Great Basin rattlesnakes. Less precipitation reduced primary production and prey availability, which was reflected in the reduced body condition of rattlesnakes. In contrast to the relationship between precipitation and body condition, rattlesnake survival did not track precipitation. Rattlesnakes are ectotherms and can withstand resource limitation and periods of environmental difficulty without starving to death.

Empirically, we have data showing that some individuals lived much longer than the average age estimates. Two individuals were captured with 13-year intervals separating their initial and most recent capture dates. A female captured in 2001 was last observed in 2014. She was an adult when initially captured and grew 4 cm over those 13 years. She was found decapitated and skinned in 2014. A male snake, captured initially in 2002 and recaptured in 2015, had grown 20 cm over that interval. Two other individuals were captured with 12 years separating their captures. Another snake’s captures were separated by 12 years and she had grown 2 cm. Seven other individuals were captured with at least 10 years intervals, nine with nine years, and five with eight year intervals. These data indicate a skewed age distribution consistent with many wildlife species; mortality is high in neonates and juveniles, then declines with sexual maturity and larger size.

Declining abundance and smaller rattlesnake size could result in the general ecological collapse currently occurring in Great Basin ecosystems due to a simplification of trophic structure. Terry and Rowe (2015) found a dramatic decrease in small mammal diversity and biomass in the Great Basin. Similarly, Jenkins and Peterson (2008) found reduced small mammal abundance and changes in demographic patterns in Great Basin rattlesnakes at grazed or annual grass invaded sites. All told, these changes in vegetation due to conifer encroachment, land use, and annual grasses cascade through food webs. The trophic consequences of vegetation change may translate to reductions in rattlesnake abundance and density over time through reduced reproductive output and recruitment. Only long term data sets such as this can address such questions.

<sub>Jenkins, C. L., and C. R. Peterson 2008. A trophic-based approach to the conservation biology of rattlesnakes: Linking landscape disturbance to rattlesnake populations. Pages 265-274 in W. K. Hayes, K. R. Beaman, M. D. Cardwell and S. P. Bush, editors. The Biology of rattlesnakes. Loma Linda University Press.

Terry, R. C., and R. J. Rowe 2015. Energy flow and functional compensation in Great Basin small mammals under natural and anthropogenic environmental change. Proceedings of the National Academy of Sciences 112(31):9656-9661.</sub>