Rattlesnakes Emerging Earlier Due to Warmer Temperatures

This article was originally published in The Midden – Great Basin National Park: Vol. 17, No. 2 , Winter 2017.

A Great Basin Rattlesnake

NPS Photo

By Bryan Hamilton, Wildlife Biologist

We conducted a 17-year study of Great Basin rattlesnakes (Crotalus lutosus) at four communal hibernacula in eastern Nevada and western Utah. Surveys were conducted during spring emergence (March, April, and May) from 2000 to 2017. Hibernacula were visited approximately 10 times each year. We defined peak emergence as the date on which the greatest number of snakes were observed at a hibernaculum (sensu Brown 2008).

To determine peak emergence dates, we summed total captures per Julian day, by site and year. We then summed the total captures per Julian day across years and calculated the mean, median, and standard deviation of the dates of peak emergence. We also looked at the relationship between Julian day and mean April temperature using a simple correlation analysis.

Across all years, the mean date of peak captures occurred on 23 April. The mean peak capture date decreased by about one day per year (captures= - 1.1 (year) + 2250, df = F1,15= 7.57, P = 0.009, R2 = 0.3371; Figure 1). This decrease was related to mean April temperature (r = 0.40). Higher April temperatures correlated with earlier emergence dates.

Figure 1. Peak emergence date for rattlesnake emergence by year

One of the strongest signals of climate change in the Great Basin is an earlier onset of spring (Chambers 2008). Earlier spring onset brings a suite of ecological effects including a reduction in snowpack (Mote et al. 2005), earlier stream run-off (Stewart et al. 2005), and earlier plant phenology (Monahan et al. 2016). Our observations of earlier spring emergence of Great Basin rattlesnakes is consistent with predictions of a warmer and earlier spring. Stay tuned to The Midden and social media for more analyses of Great Basin rattlesnake data.

References
Brown, W. S. 2008. Sampling timber rattlesnakes (Crotalus horridus): Phenology, growth, intimidation, survival, and a syndrome of undetermined origin in a northern population. Pages 235-256 in W. K. Hayes, K. R. Beaman, M. D. Cardwell and S. P. Bush, editors. The Biology of Rattlesnakes. Loma Linda University Press.

Chambers, J. C. 2008. Climate Change and the Great Basin. Pages 29-32 in J. C. Chambers, N. Devoe and A. Evenden, editors. Collaborative management and research in the Great Basin — examining the issues and developing a framework for action.

Monahan, W. B., A. Rosemartin, K. L. Gerst, N. A. Fisichelli, T. Ault, M. D. Schwartz, J. E. Gross, and J. F. Weltzin 2016. Climate change is advancing spring onset across the U.S. National Park System. Ecosphere 7:e01465-n/a.

Mote, P. W., A. F. Hamlet, M. P. Clark, and D. P. Lettenmaier 2005. Declining mountain snowpack in Western North America. Bulletin of the American Meteorological Society 86:39-49.

Stewart, I. T., D. R. Cayan, and M. D. Dettinger 2005. Changes toward earlier streamflow timing across Western North America. Journal of Climate 18:1136-1155.