Coyotes, Carbon, and Corn

I was surprised when the neighbors across the road told me they had killed a dozen coyotes in our woods. I was working on my mother's farm in Indiana last spring. (I handle the technical jobs, like pulling weeds in the woods and picking up trash.)

A dozen coyotes? Really? I couldn’t image what they found to eat in the woods. Their sharp noses and antennae ears help them catch small critters that I never see, but it’s not a very big woods (unless you’re trying to pull all the garlic mustard in it). Our woods couldn’t support many coyotes.

I wondered about the coyotes while I attended the dinner and bluegrass concert at the library in town that evening. When I got to my aunt’s house, where I was staying, she said that the coyotes had been going over to our neighbor’s house. Oh, right. I remembered hearing dogs barking across the road. Then I knew what the coyotes had been eating: they weren't finding it in our woods.

"What’s the carbon isotope ratio of coyotes on the east side of Tucson?"

That was the most interesting question I got during my Ph.D. oral exam at the University of Arizona. The question meant, "Are the coyotes eating plants--and animals that eat plants--that grow in cool seasons or in warm seasons"?

All plants use carbon dioxide (CO₂) from the air to build their cells and make the food their cells need. Plants get CO₂ through tiny openings on their leaves, called stomates. But warm season plants are more efficient than cool season plants at converting CO2 into structures and food. Because they’re more efficient, warm season plants don’t have to keep their stomates open as long.

Whenever their stomates are open, plants are also losing water, through evaporation. This means that warm season plants lose less water through these openings. Although Tucson summers are hot, winters are cool, so both types of plants thrive; they just grow at different times. But which ones do coyotes and their prey eat most? Another difference between the two types of plants, and between different atoms of carbon, holds the answer.

Because cool season plants open their stomates more often, they bring in a fresh batch of CO₂ more often than warm season plants do. When plants use carbon, they prefer typical carbon atoms to the 1% of carbon atoms that are slightly heavier. These two types of atoms are called isotopes.

Since warm season plants open their stomates less often, they have to use more of the less-preferred carbon isotope that cool season plants do. Scientists can measure the amounts of these two isotopes in plants and know whether they grew in a warm, or a cool, season. This isotopic information is passed on to animals that eat the plants, and to predators that eat other animals.

There’s also a tiny amount of a third isotope of carbon in the atmosphere. Carbon 14 lets us determine the age of plant and animal remains up to 80,000 years old.

During my exam, I guessed that the carbon isotope ratio of the coyotes was in between that of warm and cool season plants, as I supposed that the coyotes and their prey were eating both types of plants, depending on the season.

The professor who asked the question said that the carbon isotope ratio of coyotes on the edge of Tucson is similar to that of warm season plants. Warm season plants like corn, to be exact. The coyotes come into town at night and clean up dog food left in back yard bowls: the coyotes are living on corn kibble.

Our Indiana neighbor's dog food bowl is a more reliable, and better-tasting, source of food than the few critters in our woods. And dog food is a lot easier to catch.

Urban coyotes in Edina, Minnesota

I emailed my mother about coyotes and corn the next day. That reminded her that Edina, near where she lives, had been "overrun by coyotes" a few of years earlier. "People saw them in the parks, and they attacked small pets," she emailed back.

The city thought about trapping the coyotes, but Wile E. Coyote wasn’t just a figment of some cartoonist’s imagination: coyotes are wary and difficult to catch. Killing coyotes inside the city limits didn’t seem like a good idea, either. The City of Edina decided that getting people to stop feeding coyotes, either on purpose or by leaving dog food out, was the best way to reduce the number of coyotes in parks and people’s yards.

As more people move into suburban and exurban areas, there are more conflicts between wildlife, people, and pets. We need to understand the wildlife that we share these areas with and recognize how we affect the wild animals we enjoy. When we stop leaving dog food out, coyotes will have less reason to come around. Then we’ll be able to enjoy their nighttime concerts without worrying about the band partying in our yard.

-------- Update

Edina now encourages residents to haze coyotes.