Question

3. California ground squirrels have a characteristic behavior "When encountering rattlesnakes, both pup (recently weaned) and adult ground squirrels approach, often within striking distance, and invariably tail-flag (wave tail side-to-side)." (Barbour and Clark 2012) Further, snakes tend to give up the chase when they observe this signal, even leaving their ambush site. Recent work has revealed an infrared display component to the squirrel behavior. From Rundus et al 2007: ... ground squirrels engaged both rattlesnakes and gopher snakes, but not the control (conspecific) stimuli, with behavioral repertories similar to those seen in natural encounters. ... Although anti-snake behavior was qualitatively similar for the two snake species, video records of the encounters taken with an infrared imaging camera told a different story. Infrared emission from these squirrels varied with stimulus condition, and these differences were confined to the tail regions. ... Squirrels were increasing infrared emission from their tails during encounters with infrared-sensitive rattlesnakes compared with (non infrared- sensitive gopher snakes). Suggest an ultimate hypothesis for why the squirrels would behave this way. Be sure to explain what costs and benefits they might have for approaching a rattlesnake. (5 pts) Other species of squirrel do not have tail-flagging or infrared tail displays, and this suggests that the ancestors of California ground squirrels did not either. In the ancestral squirrel, observing a nearby snake likely caused the squirrel to turn away and raise its tail while running. Would you describe the tail raising behavior in the ancestral squirrel as a signal? Why or why not? (5 pts) Write out a series of evolutionary transitions that could have let to the current behavior, starting with an ancestral squirrel that did not tail-flag or have a heated tail patch and a snake that continued to chase squirrels with raised tails. Be sure to address how both species have evolved. (6 pts)

Answer 1

This phenomenon is an example of communicative signals that have evolved as part of Natural Selection. Here the California ground squirrel produces infrared radiation as a form of "radiant heat" to deter rattlesnakes that have infrared sensitivity causing a shift in rattlesnake behavior from predatory to defensive. This capacity to defend themselves comes with infrared signaling mechanisms and also the ability to neutralize snake venom. But this signalling system works on snakes that has infrared sensing ability such as rattle snakes or gopher snakes. Squirrels showing this behavior would not be safe from predation if the snake doesnt have this ability.