I’ve noticed while driving around the Bloomington-Normal area that there are lots of squirrels running into the street when cars are fast approaching. I’ve also seen it where a squirrel is waiting on the side of the street and instead of running across when the coast is clear, they tend to run across when a car is right in front of them. Do squirrels lack peripheral vision or the ability to turn their head to look both ways? Just curious.
—Narry Kim ’93, M.S. ’95, ’99; events coordinator, Illinois State University
The behavior you are observing is a result of their evolved anatomy and predator avoidance behaviors. Our eyes are on the front of our head, so the fields of view from each eye overlap in the center, allowing us to have the depth perception necessary to judge how quickly a speeding object is approaching. Squirrels, on the other hand, have eyes more on the sides of the head, so they have more side, or peripheral, vision than us.
They can actually see more around the back of the head than us, which helps in detecting predators approaching from behind. But there is a trade-off that comes with this advantage, which is that each eye’s field of view does not overlap to the degree it does in humans, so their depth perception is not as acute as that of most primates.
There is another piece to this puzzle. An effective predator avoidance behavior is to freeze as a predator is approaching and then dart out of the way at the last moment. The predator is unable to change directions as quickly and thus often misses its squirrely prey. This behavior is an adaptation, or a trait that has genetic origins and increases fitness, but this is a perfect example of how adaptations result in a species being better fit to survive in their current environment.
When the environment changes, such as a vehicle traveling faster than any predator, what was once an adaptation may end up reducing fitness. The squirrel’s predator avoidance behaviors probably have a net positive effect on fitness, meaning they do a better job of reducing their predation than increasing being hit by a car. And since they have such a high reproductive rate, typical of rodents, we probably will not see them going extinct anytime soon due to vehicular traffic.
Rebekka Gougis, assistant professor, School of Biological Sciences
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