At the conclusion of my last post (29 October), I mentioned that it appeared that all of the rattlesnakes had settled into their winter shelters and, indeed, two days later, they had done so. Of the six telemetered rattlesnakes for which I have refuge locations from last winter, five of them returned to the same refuge this winter. The exception is Male 38, who spent last winter under a log in the flat floodplain but this winter is high on the hillside near the northern preserve boundary. On 31 October, I climbed the bluff and found Male 38’s location just a few meters away from Male 37, who had returned to his 2014/2015 location. Both animals’ radio signals were originating from under thick vegetation covered by wild grape vines high on the steep hillside. Both are undoubtedly underground, likely in ground squirrel burrows.
Of the eight telemetered rattlesnakes this winter, three are hibernating together in one refuge, two are together in another, and three are hibernating alone in different locations (including Males 37 and 38). Of course, there may be non-telemetered rattlesnakes with any or all of them but that’s hard to determine at this stage. I will do some burrow camera work soon but that is often inconclusive, as their shelters are often too deep or have too many sharp turns for the camera. Even if I reach them, I cannot visually identify the snakes without looking for paint in their rattles, which are usually covered by their coils. In warm weather I often poke them with the camera to get them to expose their rattles but I don’t want to disturb them when they are cold.
So how cold are they? The table below shows the ground surface temperature in sun and shade, as well as the coldest, warmest, and average body temperatures of the telemetered rattlesnakes over the last five weeks of 2015. I will continue to occasionally collect temperature data until spring emergence. That will be foretold by the snakes beginning to bask in the morning sun, probably sometime in March. Even if I don’t catch them basking, they won’t be able to hide their elevated body temperatures; then we’ll know they’re about to begin their 2016 season.
Brumation vs. hibernation
Several people have asked me about the term “brumation” and the difference between brumation and hibernation. According to Harvey Lillywhite’s Dictionary of Herpetology (2008, Krieger Publishing, Malabar, FL), brumation is:
“A condition of torpor during extended periods of low temperature (winter dormancy), intended to distinguish such states of inactivity of amphibians and reptiles from the term ‘hibernation’ that is used commonly in reference to birds and mammals. Term coined by W. Mayhew (Comp. Biochem. Physiol. 16:103–119, 1965)”
When used regarding endotherms (animals that regulate their body temperature internally, like birds and mammals), “hibernation” generally describes a state of deep sleep in which an animal lowers its body temperature and metabolism to conserve stored nutrients during harsh winters, such as many bears and bats. In reality, ectotherms like rattlesnakes do essentially the same thing with two exceptions: they remain responsive and do not “sleep” although they become very sluggish as they get cold and, after all, it is not a big physiological change for ectotherms because their body temperature varies with the environment year-round. As a result, many herpetologists ignore brumation and use hibernation to describe winter inactivity in reptiles and amphibians, too (e.g., F. Harvey Pough, et al. 2016. Herpetology, 4th ed., Sinauer Associates, Sunderland, MA).
Best wishes for a safe and Happy New Year!
Mike