By Dr. Robert Thorson

It was a dark and foggy night. The frogs were migrating. Collisions with my car were unavoidable. Hop — Thud — Squish. Frog pizza.

Road kill can be pretty gross; especially when a whitetail deer is macerated and smeared over the highway by a tractor-trailer. But there are plenty of silver linings in this gruesome red cloud.

First, the carnage we witness on roads helps remind us where our meat comes from: dead animals. This is good thing in a world where human carnivorousness is so sanitized, one in which millions of burgers are ordered by intercom, picked up at the drive-through window and paid for by plastic. Second, roadkill helps remind us that organic evolution is not just about fossils and arguments about religion. It’s about our cars.

With more then 20 trillion road miles being driven in the United States each year, humans have become an important agent of natural selection. In fact, a subdiscipline called road ecology is now part of transportation planning. Third, with so few large toothy predators on the prowl, roadkill helps keep the animal population down.

Alfred Lord Tennyson described the co-evolution between predator and prey as “Nature, red in tooth and claw.” Were he alive today, he might write of the cars and critters of southern New England: “Automobiles, random but effective.”

Southern New England is a wonderful place to be a crow. They can soar above rural roads until they see a spot of red pavement or the fuzz from a fur-ball knocked down for the final count. Then they swoop down, peck, pull and gulp with one eye constantly on the road.

But it’s a hazardous place to be a whitetail deer because they must forage widely, especially at dawn and dusk when we commute, and are easily immobilized or panicked by the lights of a car.

There are several reasons automobiles exert such a disproportionate evolutionary pressure in southern New England per unit mile. First is habitat fractionation. The high population and intricate road network is laid out over a mosaic of habitats that have been diced into pixels of their former selves. With territories to inspect, mates to find and food to acquire, many animals have no choice but to run, slither, crawl or waddle across gauntlets of asphalt, often several times per night.

Second, our commuting schedule coincides with the dawn and dusk activity schedules of many animals, especially mammals. Truly nocturnal creatures such as possums, raccoons and skunks have an advantage over those who move under the low-light conditions of our daily commutes, though they still get killed, in part because they are prone to scavenge roadkill.

A third factor is the speed with which we surprise creatures on our hilly, windy town roads. For slowmoving ones like turtles, porcupines and most amphibians, getting killed is a matter of random timing. For fast-moving ones, fatal confusion is usually the cause of death, whether by mesmerizing headlights or by vehicles simultaneously coming from opposite directions.

Squirrels, I’ve been told, prefer to return back across to the road to where they know it’s safe, rather than take the shorter distance into the unknown. This fits with my experience: All of the squirrels I’ve killed seemed to have leaped right at my car. Armadillos reportedly do the same, though we don’t have to worry about them here. When will they ever learn?

Finally, the roads most characteristic of southern New England are precisely those that kill the most critters. These are the two- lane secondary roads on which there is neither too much traffic, nor too little.

Given all the fuss about wildlife habit and environmental protection, I recommend we pay more attention to the enormous influence our cars have on the world we live in.