Ground squirrels use an infrared signal to deter rattlesnake predation

Rundus et al. 10.1073/pnas.0702599104.

Supporting Information

Files in this Data Supplement:

SI Movie 1
SI Movie 2
SI Text
SI Figure 4
SI Figure 5




Fig. 4. Temperature at dawn, dusk, and nighttime in Winters, CA. Mean (p) and peak (Ü) tail temperatures of squirrels in this study are contrasted with the average ambient temperatures at this location. Regional temperatures are shown during April-August, when rattlesnakes are most likely to prey upon ground squirrel pups. ', average temperature during April-May when rattlesnakes exhibit crepuscular activity patterns; □, average temperature from June-August when rattlesnakes are more nocturnally active. Note that both the mean and peak tail temperatures often well exceed the background temperatures at these times. Points are connected for ease of reading and do not imply continuous measurement.

1. Clark RW (2005) Behav Ecol Sociobiol 59:258-261.





Fig. 5. Overhead schematic diagrams of the testing chambers used in experiments 1 and 2. (A) In Experiment 1 the squirrel's home cage (1) was placed adjacent to an opening into the main testing chamber (2) within which the cage (3) housing the stimulus animal was located. (B) Experiment 2 used a two chamber apparatus consisting of a starting chamber (1) linked by an enclosed runway to the testing chamber (2). The testing chamber housed a small, simulated squirrel burrow containing a euthanized rat pup (3) and a biorobotic ground squirrel model (4) during control and infrared trials. The dashed line indicates the gap cut in the testing chamber floor, preventing vibration from the biorobotic model servo to the rest of the chamber.





SI Movie 1

Movie 1. A California ground squirrel interacting with a gopher snake, filmed by using an infrared imaging video camera. This clip from a gopher snake trial illustrates typical snake directed antipredator behavior such as tail flagging and elongate posturing. Note the tail region of the squirrel, which remains approximately the same temperature as background.





SI Movie 2

Movie 2. A California ground squirrel interacting with a rattlesnake, filmed by using an infrared imaging video camera. This clip from a rattlesnake trial illustrates typical snake-directed antipredator behavior such as tail flagging and elongate posturing. Note the tail region of the squirrel, which is considerably warmer than the background temperature.





SI Text

Defensive behavior of rattlesnakes

The antipredator and defensive behaviors of rattlesnakes, in particular those of the northern Pacific rattlesnake, Crotalus oreganus, have been well documented (1-5). Historically they have been described as falling loosely along a continuum from less defensive (including immobility/procrypsis) to more defensive (coiling and cocking-to-strike) and typically begin less defensively and progress to greater defensiveness. The additional behaviors of rattling and slow/exaggerated tongue flicking are typically deployed when the snake is most defensive, often coinciding with the coiled and cocked postures. More recently, a new system for classifying the antipredator has been proposed that attempts to address the complexity inherent in this suite of behaviors (6). Rather than using a one-dimensional characterization of antipredator behavior, this new formulation examines antipredator behavior across three dimensions: movement with respect to the predator, amount of movement, and apparent function.

However, the ground squirrel-rattlesnake system described here suggests even more complexity: a triadic interaction between adult squirrels, rattlesnakes, and juvenile squirrels. One key feature of this system is that adult squirrels are not the prey items of rattlesnakes due in large part to their behavioral capabilities at avoiding strikes and resistance to rattlesnake venom. It is the ground squirrel pups, with limited venom resistance, that are the prey items of these snakes. Thus, interactions between adult squirrels and rattlesnakes fall outside of formulations designed to characterize more typical predatory encounters. In this paper we therefore categorize generalized defensive behavior by the rattlesnakes along a defensive continuum while recognizing that, in other antipredator contexts, these snakes behavior might be better classified on a multidimensional scale.

The results from this experiment show that our rattlesnakes were progressing farther along this defensive continuum during the infrared trials than in the baseline or control trials. Snakes adopted the highly defensive coiled and cocked-to-strike postures and rattled almost exclusively during the infrared trials. During control and baseline trials these snakes exhibited a more predatory pattern of behavior, moving more (usually toward the squirrel burrow) and remaining in an elongated posture. The induced shift from predatory to defensive behavior by the rattlesnakes in our infrared trials resembles natural encounters with adult squirrels that are quite capable of inflicting serious wounds to these snakes.

1. Duvall D, King MB, Gutzwiller KJ (1985) Nat Geogr Res 1:80-111.

2. Graves BM (1989) Copeia 1989(3):791-794.

3. Greene H (1997) Snakes: The Evolution of Mystery in Nature (Univ of Calif Press, Berkeley, CA)

4. Klauber LM (1982) Rattlesnakes: Their Habits, Life Histories, and Influence on Mankind (Univ of Calif Press, Berkeley, CA)

5. Mattison C (1996) Rattler! A Natural History of Rattlesnakes (Sterling, New York)

6. Mori A, Burghardt GM (2004) Herpetolol J 14:79-87.

This Article

  1. Published online before print August 17, 2007, doi: 10.1073/pnas.0702599104 PNAS September 4, 2007 vol. 104 no. 36 14372-14376
  1. OA Abstract
  2. Figures Only
  3. OA Full Text
  4. Full Text (PDF)
  5. » Supporting Information
About Our New Site Design >>
Link: Advanced Search

This Week's Issue

  1. July 22, 2008, 105 (29)
  1. Current Issue
  1. Alert me to new issues of PNAS

Most