Tuesday, April 10, 2012

Squirrels Have a Weapon: Fluffy Tails

Given that it's almost impossible to walk around campus without encountering a furry squirrel, have you ever been curious about the evolutionary purpose of the fluffy tails? Besides the fact that fluffy tails make the squirrels look super adorable (in my opinion), the tail actually serves as a "weapon" to alarm predators, specifically to ward off rattlesnakes.

Researchers have noted particular defensive behaviors that California ground squirrels (Spermophilus beecheyii) display when they come face-to-face with venomous rattlesnakes. Initially, the squirrel kicks pebbles and dirt directly at its slithery foe. The following behavior is what has baffled researchers: the squirrel erects its tail, fluffs out the hairs, and wave it vigorously side-to-side. It probably seems like an intimidation tactic to display aggressiveness, but the squirrel exhibits this "tail flagging" even at night in complete darkness, when snakes cannot see. Even more interesting is the observation that tail flagging is specifically used against rattlesnakes, not other predators that are birds or mammals.

Over the past million years, squirrels and snakes have coexisted in a predator-prey relationship. Squirrels have actually evolved immunity to poisonous rattlesnake venom. Although it may seem like a cute, innocent squirrel is no match against a hungry rattlesnake, a threatened squirrel will actually bite an approaching snake to defend itself.

Rattlesnakes have sensory organs called pit organs that allow them to sense infrared thermal radiation. The pit organ is lined with thousands of thermoreceptors that can detect very slight changes in temperature. In fact, a rattlesnake can "see" radiant heat at certain wavelengths with great accuracy such that a blind snake would still be able to strike and attack its prey.

So the question is: why do squirrels exhibit tail flagging as a defense mechanism, even in the dark when the snakes cannot see their behavior?

Researchers have found that adult squirrels use tail flagging to protect their offspring, who are vulnerable to snake venom since they have not yet developed immunity to it. Rattlesnakes often target squirrel pups, which make up 2/3 of the rattlesnake diet in the spring and summer seasons. To investigate the phenomenon, scientists used infrared cameras to record squirrels' body temperature changes when they engage in defensive tail flagging behavior against rattle snakes.

The researchers recorded an increase in temperature - by as much as 12 degrees - in the tail region during tail flagging. By waving their "hot tail" around, the squirrels essentially enlarge their infrared image and appear much bigger than they are in reality to rattlesnakes.

To investigate the rattlesnakes' reaction to the harassment, scientists designed a "robo squirrel" that could wave its tail threateningly with or without extra heat. They found that as the robo squirrel's tail generated more heat, the snake's body posture changed from slithering to a coiled, defensive position. The hypothesis is that rattlesnakes must have learned to back off based on previous interactions with aggressive adult squirrels. Upon sensing the tail flagging behavior, the rattlesnake ditches its predatory behavior and goes on the defensive, reversing its role in the confrontation.

However, the heated tail does not pose a threat to the rattlesnake. Researchers believe that the hot tail signals how prepared the adult squirrel is to defend its young -- by waving its bushy tail back and forth, it is warning the hungry snake that it is not defenseless and will resort to violent acts (e.g., biting) if necessary to protect its dear pups.

So the next time you see a Rice squirrel, be sure to note any tail flagging behavior!

1. Blumstein DT. Feeling the heat: Ground squirrels heat their tails to discourage rattlesnake attack. Proc Natl Acad Sci USA. 2007; 104(36):14177-8. 
2. Norris, Scott. "Squirrels Heat Their Tails to Fend Off Rattlesnakes." National Geographic News. 13 August 2007. National Geographic Society. <http://news.nationalgeographic.com/news/2007/08/070813-squirrel-snake.html> 
3. Minkel, JR. "Squirrel Has Hot Tail to Tell Snakes." Scientific American. 14 August 2007. Scientific American, Inc. <http://www.scientificamerican.com/article.cfm?id=squirrel-hot-tail-tell-snakes>


  1. I like squirrels.

    So the squirrels with less-fluffy tails on campus (due to infection) probably have a bigger consequence than I expected.

    1. Good thing there are no rattlesnakes in Rice! ...Or are there?

  2. The purpose of the tail is also to maintain balance in the treetops, therefore, treetop squirrels tend to have longer and fluffier tails than those belonging to squirrels on the ground.

  3. i wonder if there is a correlation or pattern of this evolutionary significance to the length of tails?

    1. Good point. I would think that a longer tail would make it appear larger, and would therefore be selected for.

  4. A major component of the diet of snakes is rodents, How come squirrels are one of the few rodents that have large tails (i.e. mice, rats, chipmunks, etc. do not have large tails)? Usually when something works, natural selection favors it, so why not favor it in other species?

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