I have been conducting research out in the Mojave Desert to understand how sidewinder rattlesnakes (Crotalus cerastes) behaviorally adjust their body temperature. Thermoregulation is the process of maintaining an individual’s internal body temperature within a specific range; all organisms have a range of body temperatures that they must stay within or else they risk dying. In humans, we thermoregulate via automatic responses to temperature change, such as shivering and sweating, which our bodies do naturally (ie. we have no control over these actions). Behavioral thermoregulation, on the other hand, encompasses active behaviors that are performed in order to regulate internal body temperature. Some examples of behavioral thermoregulation are huddling and moving from cool areas to warm areas (or vice versa).
 |
| A horned lizard behaviorally thermoregulating by basking in the sun. |
Reptiles are ectotherms, meaning their body temperature is dependent on the temperature of the environment around them. Thus, they must behaviorally thermoregulate if they want to change their body temperature. For desert reptiles, thermoregulation is most often accomplished via movement across thermal gradients, such as moving from a cool burrow to a warm basking rock. Rattlesnakes, however, present an interesting scenario regarding behavioral thermoregulation: they are sit-and-wait predators. They will remain in ambush for hours at a time, during which they hardly move at all. Not only that, but they maintain their ability to strike both quickly and accurately throughout the duration of the night.
 |
Sidewinder (Crotalus cerastes), photo courtesy of
Tim Garvey
|
One solution
for sidewinders, which has not been well-documented, is through cratering. In
1992, Timothy Brown & Harvey Lillywhite coined the term “cratering” to
describe the behavior that sidewinders often exhibit whereby they bury their
outer coils in the sand when ambushing. The theory behind this behavior is that
it keeps the snakes’ body temperatures warmer during the night and cooler in
the morning because ground temperatures do not fluctuate as much as air
temperatures do. One goal of my research was to answer the question “Does
cratering provide thermal insulation for sidewinders?”
 |
A cratered, ambushed
sidewinder, photo courtesy of Tim Garvey
|
 |
Two “craters” left behind in
the sand after a snake left its ambush positions,
photo courtesy of Malachi
Whitford
|
To
answer my question, I surgically implanted sidewinders with temperature-sensitive
transmitters, which send a signal to a hand-held receiver. The signal is just a
continuous series of beeps that get louder as the person holding the
receiver moves closer to the snake. Additionally, the time between
each beep is dependent on the body temperature of the snake: if the snake is
warmer, the time between the beeps is shorter. This conveniently functions as
both a way to locate the snakes in the field, and as a way for me to determine
internal body temperatures of the snakes.
I recorded the signal from these transmitters by placing a receiver in a cooler along with a voice recorder near an ambushed snake. I would record the snake’s body temperature for the whole night, providing me with a complete temperature profile for that snake on that night. I coupled these recordings with video footage so that I could link body temperature with behavior. I quantified the crater intensity of the snake (none/light, moderate, or heavy) using the video footage, then I examined how the snake’s body temperature related to both the air temperature and the ground temperature.
 |
A temperature-sensitive radio transmitter
(length-wise it’s about the size of a quarter)
|
 |
My temperature-recording set
up. Inside the box is a receiver and a voice
recorder, with the antenna on the
outside.
|
If
cratering is providing thermal insulation for the snakes, we expect that snakes
with a heavier crater will have body temperatures more similar to the ground,
and that snakes with a light crater will have body temperatures more similar to
the air. However, my data does not support this. I found no relationship
between the level of crater intensity and difference of body temperature to
ambient temperatures.
 |
Speckled rattlesnake (Crotalus mitchelli), photo courtesy of Steve
Hein
|
So
how do sidewinders do it? Previous research has demonstrated that this species
of rattlesnake has a wide range of preferred body temperatures, which means
that they can be fully functional at many different temperatures. One researcher
found their preferred range to be 13.6 - 40.8°C, which is relatively large when compared to the
sympatric speckled rattlesnake (Crotalus
mitchelli), whose range was determined by the same researcher to be 18.8 –
39.3°C (Moore 1978). This
means that as long as their body temperatures are within that range,
sidewinders can allow their body temperatures to conform to ambient
temperatures and do not need to behaviorally thermoregulate.
One
important thing to consider is that my research was conducted during the summer
months, when even night temperatures are relatively warm, which could mask the
effect of cratering during the night. During the summer, cratering would be
most beneficial for snakes in the morning, when ambient temperatures begin to
rise rapidly beginning at 5:30 AM. I am therefore continuing to explore
cratering by determining whether snakes with a heavier crater stay out longer
in the morning when compared to those with a light crater. Further research
will try to determine why sidewinders crater if it is not a thermoregulatory
behavior. One possibility is that it aids in camouflage, but additional
research will need to be conducted to determine this.
References:
