Well, it’s March and many recent days have been sunny and quite warm. Even though daytime highs have been listed in the 50s, that’s measured in shade and the temperature in the sun near the ground is always considerably warmer. So why were rattlesnakes basking in good numbers by late February last year but not yet this year?
There are two parts to the answer. First, most of the snakes spend the winter deep enough underground that they are insulated from daily temperature fluctuations. In other words, they do not feel daytime highs or nightime lows. Deep in their winter shelters, the temperature creeps up or down gradually in response to the mean (average) outside temperature. The second part of the answer is that natural selection tends to remove animals from the gene pool that don’t spend the winter deep enough and/or that venture out too early and get caught in freezing weather. The animals that select optimal winter shelters tend to live longer, and thus produce more offspring carrying the genes for those successful behaviors. (For those of you interested in the evidence for genetic influence on behavior, Time Love and Memory (2000) by Pulitzer Prize winning author Jonathan Weiner is an entertaining and astonishing summary, written in easy-to-understand language but with references to the scholarly research.)
So how do we know the temperature in the underground shelters?
Besides having recorded hourly temperatures for nearly four years with a data logger in a burrow 1 meter (39″) below ground in the Mohave Desert during a previous study, we can routinely calculate body temperatures of the hibernating snakes from the pulse interval of their transmitters during the winter.
The transmitters come from the manufacturer (Holohil Systems Ltd.; www.holohil.com) with a calibration graph showing the correlation between pulse rate and temperature for each transmitter (they vary slightly).
I routinely check the pulse rate in a water bath at room temperature before implanting a transmitter and adjust the mathematical conversion, if it differs slightly from the factory data. Then it is a simple matter in the field to time the pulse interval with a stopwatch and convert that measurement to temperature later. And since the transmitter is implanted inside the snake’s abdominal cavity, transmitter temperature equals core body temperature.
I also record hourly shade air temperatures with a data logger in the nature preserve at Effie Yeaw Nature Center. This gives me a standard local reference against which to compare rattlesnake behavior and look for correlations with weather conditions.
I can then compare rattlesnake behavior, body temperatures and air temperatures. This is especially interesting when one year is compared to another.
You can see about six weeks’ data from this year in the chart above. The fluctuations in mean (average) air temperature (open circles) reflect the daily changes based on cloudy verses sunny days and cold air masses moving through verses warmer southern air (the same factors discussed every day by meteorologists). The five filled circles are the mean body temperatures of the twelve telemetered rattlesnakes on the days that I recorded their data. There are more air temperature records because the data logger records automatically around the clock while rattlesnake body temperatures are only recorded when I visit the study site (which I do less frequently when the animals are inactive).
Now take a look at the corresponding data from last year:
Checking my behavioral observations from last year, I find a few rattlesnakes were starting to bask occasionally by the end of January and most were basking regularly on sunny days by the end of February. Regular basking continued through mid-March. Then, suddenly, between 17 and 19 March, most of the rattlesnakes left their hibernation shelters, as if someone had given them the “all clear” signal. This was multiple rattlesnakes leaving multiple shelters at the same time – pretty amazing! By the way, you may also notice that I visited and recorded data more frequently in February last year. That’s because there were rattlesnakes to see and behavior to record (besides the science; it’s just more fun when there are rattlesnakes to see!).
If I combine the raw data from 2016 and 2017 for comparison, the chart gets pretty messy and trends are hard to make out. But we can create mathematical models for those same data that present a much clearer picture.
You can see that mean February air temperatures started out much lower this year compared to 2016. And, in 2016, with mean air temperatures exceeding 50F by early February and staying there, the rattlesnakes began to bask and their body temps took off as a result. This year, air temps started nearly ten degrees lower, rose briefly, but then took a nosedive in late February, keeping the rattlesnakes cool and underground so far.
Of course, some individuals do not behave in the same manner as most others. I have heard some reports of a few rattlesnakes being sighted in the past few weeks and one dog reportedly bitten already. While none of the telemetered rattlesnakes at Effie Yeaw have emerged from hibernation yet, one rattlesnake without a radio has been basking early – just as he has done each of the past two years. My old skinny Male 40 has been frequently laying in the sun at the entrance to his winter shelter for more than a week.
As you may recall, Male 40 was implanted with a transmitter in 2014 but I removed his second radio last spring and released him without one because he was not producing useful behavioral data. It was not that he was just behaving differently than the others (after all, unusual behavior is interesting and valuable to record), he has been chronically and significantly underweight, appears to be quite old, moves very little during the summer, and has never been found courting a female. Given his poor body condition, I was afraid that he would die during hibernation and his transmitter would not be retrievable. But, even without a radio, he continues to be sighted regularly and has now made it through another winter.
But most others seem to be waiting for some sustained warmer weather before venturing out.