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This clip shows the basic signaling interaction between a California ground squirrel and a Northern Pacific rattlesnake. The coiled snake is difficult to see, but it is in the grass in front of the squirrel. Notice that the squirrel gets quite close during the interaction, inspecting the snake frequently while vigorously giving the tail-flag display. The squirrel has been marked by our research team with an "11" in black pelage dye on the side. The snake has been implanted with a radio transmitter to allow us to relocate him in the field. One of our main projects focuses on determining how rattlesnakes might alter their foraging behavior in response to these displays. Interestingly, interactions that involve unrestrained, free-ranging snakes like this almost never escalate beyond signaling. The squirrel inspects and tail-flags for a while, and then moves on. For the most part, snakes never exhibit any overt offensive or defensive responses during the display period.

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Squirrels definitely expose themselves to risk by coming close to snakes. Snakes eat a significant number of ground squirrels each year--mainly pups (squirrels that are just a few months old), as adult California ground squirrels are both resistant to snake venom, and fairly adept at avoiding snake strikes.  This clip shows a predatory strike by a rattlesnake, which bites a ground squirrel pup, drags it in to the burrow, releases it, and then later emerges to begin strike-induced chemosensory searching (the process used by venomous snakes to relocate envenomated prey). Note that an adult squirrel runs over to investigate, and tail flags and inspects the snake during this process.

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This is another clip of a tail-flagging squirrel approaching and investigating a coiled snake, contrasted with a  non-tail-flagging squirrel that approaches the snake unaware . In the first clip, the Northern Pacific Rattlesnake is coiled at a burrow entrance, with just its head and upper body on the surface in a ready-to-strike position. The squirrel comes quite close to investigate while tail flagging. In the second clip, a squirrel that isn’t tail flagging wanders close to a coiled snake and is struck. We think one of the functions of the tail flagging behavior is that it allows the squirrel to signal that it is aware of the danger from the snake, and is ready to dodge out of the way of any potential strike (Matt Barbour explores this topic extensively in his soon-to-be-published Masters thesis). Rattlesnakes may be less likely to try to ambush a squirrel that is signaling its readiness to dodge. Thus, perhaps this “vigilance signal” effectively protects the squirrel from attack while it is investigating and gathering information about the snake.

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In order to truly understand how a signal functions, researchers need to be able to conduct playback experiments that recreate elements of the display in a controlled fashion. To create a playback of a complex visual signal we, like many other animal behaviorists, have turned to the burgeoning field of biorobotics. That is, with collaborators at UC Davis, we have created a mechanical model of a squirrel that we think is a reasonable representation of the squirrel display. Details of the robosquirrel model and our efforts to date are summarized in Joshi et al 2011. This video shows some of our preliminary tests of the robosquirrel. It is potentially a very useful tool for more experimental work in this system, but we still need to iron out some wrinkles in terms of field deployment.  Check back soon! 

This clip shows two of our preliminary field trials with robosquirrel. In the first clip, you can see the snake hiding in the tall grass in ambush position. The robosquirrel track is put in place, and the squirrel mount is slid down the track close to the snake and made to tail flag via remote control. The snake exhibits no overt reaction. In the second clip, the snake is again hiding in ambush position at a different location. The squirrel model again is slid into position down the track, but this time with no tail flagging. The snake launches a predatory strike at the squirrel, biting it in the head region. We collected several large deposits of expelled venom from the pelage of the squirrel model after this trial.