Probing,Assessment and Management during Interactions between Ground Squirrels (Rodentia: Sciuridae) and Rattlesnakes (Squamata: Viperidae). 2: Cues Afforded by Rattlesnake Rattling

The predator-prey relationship between California ground squirrels (Spermophilus beecheyi) and northern Pacific rattlesnakes (Crotalus riridis oreganus) is a useful system in which to explore risk assessment and management. Rattlesnakes are major predators of ground-squirrel pups, but pose only a sublethal threat to adult squirrels. Adults approach, harass, and even attack rattlesnakes when confronted with them. A rattlesnake's response to such harassment can include rattling and striking. Not all rattlesnakes pose the same risk to an adult squirrel. Larger, warmer rattlesnakes strike in ways that may be more effective at overwhelming the defensive leaps of squirrels, and larger snakes can inject more venom if they are successful in landing a bite. It would therefore benefit squirrels to assess and respond appropriately to rattlesnakes of different body size and temperature. We looked for cues in rattling upon which such assessments might be based. We recorded and digitally analyzed the rattling sounds of snakes of different sizes, each tested at four different body temperatures — 10, 18, 27, and 35°C. Results indicate that warmer snakes rattle with faster click rates, higher amplitudes, and shorter latencies. Similarly, larger snakes produce rattling sounds of higher amplitude and lower dominant frequencies. Thus, rattling provides reliable cues about rattlesnake dangerousness. Nevertheless, this highly ‘informative’ characteristic of rattling has its origins in physical and physiological constraints, not in specialization for communication. Ground squirrels appear to probe for the information extractable from rattling, for example by pushing loose substrate at the snake and thus inducing it to rattle. Future reports will discuss the degree to which ground squirrels actually exploit these cues.