By Bryan Hamilton, Wildlife Biologist, Great Basin National Park. First published in The Midden*, Vol. 16, No. 1, Summer 2016.*
Figure 1: Great Basin rattlesnakes are among the most variable rattlesnakes in color and pattern. NPS photo.
Why study snakes
To protect wildlife, managers need to know how populations grow, survive and die. Snakes are declining worldwide but get far less conservation attention than charismatic animals like large mammals. Snake fungal disease, recently found and spreading in the U.S., is hitting rare and common species alike.
- Useful: snakes are economically valuable and control rodents — and rodent-borne diseases such as Lyme disease and hantavirus.
- Exposed: sitting mid food chain, they feel both predators from above and prey shortages from below.
- Hard to count: they are secretive and well camouflaged, so reliable population and survival estimates are rare.
The Great Basin rattlesnake
Crotalus lutosus lives only in the Great Basin, North America's largest desert.
- Diet: an ambush predator limited by the size of its mouth — mostly small mammals and lizards, sometimes a bird.
- Winter: it hibernates in groups in ancestral dens, and is extremely faithful to them: nearly every snake returns to its den in the fall.
- Summer: it spreads out to feed, mate and give birth.
- Threats: like most rattlesnakes, it has declined or vanished locally, mainly from human persecution.
The method
From 2001 to 2015, the park used capture–mark–recapture:
- snakes were held in clear plastic tubes while being processed;
- marked by clipping belly scales or with PIT tags (passive integrated transponders);
- measured for snout–vent length (SVL) and tail length in a squeeze box, to the nearest millimeter;
- weighed on spring scales or an electronic balance;
- sexed by probing for hemipenes;
- released at the exact spot they were caught — most slipped into crevices or under rocks.
What they found
| Measure | Result |
|---|---|
| Snakes caught | 401, captured 799 times |
| Sex | 253 males, 148 females — a male-biased ratio among adults; juveniles about 1:1 |
| Peak capture | 27 April, ± 11.2 days |
| Size | mean SVL 59 ± 13 cm; mean mass 186 ± 103 g; males 5.3 cm longer and 61.8 g heavier |
| Detection | a 0.28 ± 0.06 chance of catching a snake that's there — higher for bigger snakes |
| Survival | 0.76 ± 0.10 a year |
| Life expectancy | 3.6 years on average; 2.5 median |
Figure 2: mass against snout–vent length for snakes at four sites in eastern Nevada and western Utah — 797 measurements of 394 snakes, 2001–2015. NPS.
Rain, rodents and body condition
A snake's body condition was weakly linked to the previous year's rainfall (R² = 0.295). Less rain means less plant growth, fewer small mammals, and thinner rattlesnakes. Survival, though, did not follow rainfall: as ectotherms, rattlesnakes can ride out lean times without starving.
Some live much longer
- 13 years apart: a female first caught as an adult in 2001 was last seen in 2014, having grown 4 cm — found decapitated and skinned. A male caught in 2002 and again in 2015 had grown 20 cm.
- 12 years: two more, one a female that grew 2 cm.
- At least 10 years: seven others; nine at nine years; five at eight.
This skewed age structure is common in wildlife: newborns and juveniles die at high rates, and survival improves with maturity and size.
How they died
Of eleven snakes found dead, six could be identified:
- one from overheating, possibly after being disturbed by photographers;
- three decapitated and skinned;
- one on a road;
- one killed at a home.
The five that couldn't be identified: four more decapitated and skinned, and a juvenile apparently killed by a rockfall.
A warning for the Great Basin
Fewer and smaller rattlesnakes would simplify the food web, part of a broader ecological collapse under way in the Great Basin. Small mammal diversity and biomass have dropped sharply (Terry and Rowe, 2015), and grazed or cheatgrass-invaded sites show fewer small mammals and changed rattlesnake demographics (Jenkins and Peterson, 2008). Conifer encroachment, land use and annual grasses ripple through the food web, and over time may mean fewer rattlesnakes through fewer young. Only long-term records like this one can answer that.
Sources
Based on "Great Basin Rattlesnake Study," by Bryan Hamilton, The Midden — Great Basin National Park, Vol. 16, No. 1, Summer 2016, National Park Service; a work of the United States government in the public domain. It cites C. L. Jenkins and C. R. Peterson, 2008, "A trophic-based approach to the conservation biology of rattlesnakes," in The Biology of Rattlesnakes (Loma Linda University Press), 265–274; and R. C. Terry and R. J. Rowe, 2015, "Energy flow and functional compensation in Great Basin small mammals under natural and anthropogenic environmental change," PNAS 112(31): 9656–9661.
In these publicationsGreat Basin National Park
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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