See a Magnum Photos gallery of animals eating.

You’re always going on about the environmental impact of the food we eat. What about the food our pets eat? Cats and dogs consume a lot of meat, after all.

So far, there hasn’t been much rigorous, firsthand research on the cradle-to-muzzle impacts of pet food. But one thing seems clear: How we answer this particular question is going to depend on how we define the word meat.

You’re right that the carnivorous diets of cats and dogs are likely to be worse for the environment than those of, say, birds and guinea pigs. But the meat we feed to our pets isn’t quite the same as the stuff we eat ourselves. Most commercial dog and cat food is made from the parts we humans don’t eat, like organs, scraps, and rendered bones and tissues.

Looked at one way, then, pet food is a kind of recycling operation: It takes waste products and finds a use for them. From an economic perspective, these less-than-palatable parts aren’t that big of a deal. Clark Williams-Derry, blogging for the Seattle-based think tank Sightline Institute, notes that byproducts account for at most 15 percent of a livestock animal’s value. Thus, he argues, the pet food industry contributes relatively little to the total environmental impact of a meat-producing cow, chicken, or pig. We grow and slaughter those animals to feed our yen for meat—not to make the scraps that go into pet food. So 100 calories of byproduct meat should be credited with a lower impact than 100 calories of human-grade meat.

The other way of looking at the question is that it takes the same resources—feed, land, fertilizer—to make organ meat as it does to make flank steak. Nathan Pelletier, an ecological economist who specializes in food production systems, points out that we do life-cycle analyses precisely because the market isn’t very good at telling us about the true environmental costs of our choices. (Case in point: cheap plastic.) It’s irrelevant that we value some types of meat over others—100 calories of meat is 100 calories of meat, he argues.

Last fall, a pair of sustainable architecture experts from New Zealand, Robert and Brenda Vale, raised pet-lover hackles when an article based on their book Time To Eat the Dog? was published in New Scientist magazine. According to the Vales’ back-of-the-envelope calculations—which assumed that all the meat in pet food came from chickens and that pet-grade meat is no different than people-grade meat—a medium-size dog has roughly twice the ecological footprint of a Toyota Land Cruiser. (An ecological footprint is the average amount of land and sea required to create a product and then absorb its waste.)

Several bloggers cried foul, claiming that the Vales overestimated how many calories a dog requires and underestimated the impacts of the Land Cruiser. The Lantern has been doing her own due diligence on the Vales’ assumptions and has enough questions that she’s not ready to accept or reject their conclusions just yet. But it does seem clear to her that a carnivorous pet—especially a large one with a big appetite—requires a significant amount of resources. Does that mean you should ditch your Alsatian for the sake of the planet? No. The Lantern understands that pets are important members of many families and that suggesting that readers get rid of them—no matter how big of a paw print they may have—is a bit like suggesting you kill yourself to spare the Earth (a recommendation that appears with some frequency in the Lantern’s inbox). But the emotional attachment you feel to your dog or cat isn’t a free pass to ignore its contribution to your family’s overall consumption patterns. Maybe it means you make some personal trade-offs to balance out your choice of animal companion—like riding your bike instead of driving, for example.

In the meantime, what’s an eco-conscious pet owner to do? The way the Lantern sees it, there are two options worth exploring. The first is a variation on one of our cardinal rules for humans: Eat less meat. Some vegans and vegetarians put their cats and dogs on equally abstemious diets. The Lantern doesn’t believe humans should be required to give up all meat, so she’s not going to suggest that your pet should, either. But according to Marion Nestle—a public-health and nutrition expert who’s recently been focusing her attention on pet food—the research clearly shows that dogs and cats can get all the nutrients they need from complete-and-balanced, all-veggie commercial foods. (No one has done any long-term clinical trials comparing various diet options, however.) Even if you don’t want to take meat out of the equation entirely, you might be able to cut back, by replacing some of your pet’s fleshy fare with grain-and-vegetable-based meals.

The other option is to enlist your pet in your own food recycling efforts. As we’ve noted before, one of the most straightforward ways to make your diet more eco-friendly is to consume everything you buy, whatever it is. We Americans waste a lot of food—around 30 percent to 40 percent, according to recent estimates—which means we also waste all the resources that went into producing that food. Now, the Lantern isn’t advocating you treat your pet like a dumpster, filling its bowl with ossifying Oreos and rotting Chinese takeout. But if you have more meat or vegetables in your fridge than you know what to do with, consider turning that excess food into some pet chow. There are plenty of books on the market to show you how. Just make sure that any changes to your pet’s diet are made gradually, and keep your vet apprised of your experimentation.

Slate V: What’s Greener, Your Pet or Your SUV?

Is there an environmental quandary that’s been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

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