Tuesday, October 6, 2015

Behavioral Thermoregulation in Sidewinder Rattlesnakes: Guest Post by Grace Freymiller

Grace is a new Master's student in the Clark Lab. She has been conducting research on desert rattlesnakes for the past three years, and she describes the work for her undergraduate thesis below:

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.