Ambushing the Predator: Squirrels and Rattlesnakes

As a young boy growing up in the Adirondack Mountains of Upstate New York, old-timers would often say that if you encounter a black bear in the woods you should shout and wave your arms in an effort to make yourself appear larger and more threatening. Thus, I have anecdotal evidence that this strategy works. However, I didn’t realize at the time that these predator-deterrent signals are fairly common in the animal world.

Animal behaviorists note that both predator and prey benefit from predator-deterrent signals because both parties avoid the costs of conflict. That is, predators are unlikely to be successful once prey detect them, and prey avoid the costs of a potential injury during an attack, even if it is unsuccessful. Researchers also note that many species give predator-deterrent signals when no predator is present.
Barbour and Clark (2012) state three possible reasons why these behaviors may occur when predators are not visible, “including (i) dishonest predator detection; (ii) deflecting attacks of undetected predators to non-vital body parts, such as a tail; or (iii) deterring attacks from undetected predators by honestly advertising vigilance.”

Barbour and Clark (2012) tested the third explanation by filming the tail-flag display of the California ground squirrel (Spermophilus beecheyi), during ambushes by free-ranging northern Pacific rattlesnakes (Crotalus oreganus). When ground squirrels encounter rattlesnakes, the squirrel approaches, often within striking distance, and begins waving its tail side-to-side (aka tail-flagging) (Video 1). Previous studies have shown that ground squirrels also increase the temperature of their tails when tail-flagging towards rattlesnakes. This suggests that tail-flagging disrupts the infrared-sensors of rattlesnakes, making them miss if they do strike at the squirrel. However, squirrels also frequently tail-flag when no snake predator is present.


Video 1. A sample video showing a rattlesnake successfully ambushing a juvenile squirrel followed by an adult tail-flagging at the burrow entrance. (https://www.youtube.com/watch?v=eyQ1lLmj_Tc&feature=player_detailpage)

By video-recording and quantifying the ambush behavior of rattlesnakes to tail-flagging ground squirrels, the researchers found tail-flagging deterred snakes from striking squirrels (Figure 2). The likely reason is that squirrels advertise their “readiness to dodge a snake strike,” making it less likely the snake will waste the energy. Snakes were also more likely to abandon their ambush site if a squirrel tail-flagged in front of them presumably because nearby squirrels had been altered to the snake’s hideout.

tail-flag
Figure 2. The probability of rattlesnake strikes (strike, 1; no strike, 0) to ground squirrels that either tail-flagged (black triangle) or not (grey
diamond) at various distances within their observed strike
range (31 cm). (From
Barbour and Clark, 2012)

So why would a ground squirrel tail-flag when no snakes were visible? Perhaps the squirrels do so to signal their vigilance to snakes that remain hidden and undetected – to hedge their bets.

References

Matthew A. Barbour and Rulon W. Clark. (2012) Ground squirrel tail-flag displays alter both predatory strike and ambush site selection behaviours of rattlesnakes. Proc R Soc B 2012 : rspb.2012.1112v1-rspb20121112.