We have all met dogs that, with their small heads and large bodies, bear a striking resemblance to the people with shrunken heads from the movie Beetlejuice. Some dogs naturally come with this particular head-to-body ratio, but for others, it is man-made.

In the 2014 book Dilemmas in Animal Welfare, Peter Sandøe, professor of bioethics at the University of Copenhagen, and his co-authors estimate that about one third of companion dogs in developed countries are overweight. On top of that, they suggest, “more than one in 20 is obese.”

And yet people often do not realize that their dogs are hauling too many pounds. In a study published in 2011, University of Nottingham researchers asked owners to categorize their dogs as underweight, normal, overweight or very overweight. Owner assessments were then compared with the judgments of veterinarians, a population specifically trained in this critical assessment of animal well-being.

How did owners do? They did know numbers! Almost 70 percent offered an estimate, suggesting they were at least somewhat familiar with their pet’s weight. But they did not always understand what that figure meant for the animal’s health. In fact, owners tended to underestimate whether dogs were too heavy. For example, almost 40 percent of owners with a dog deemed overweight by a veterinarian described the poundage as normal (and one owner even thought their overweight dog should gain more!). Although some owners did acknowledge that their dogs were overweight, the veterinarians often labeled these dogs as very overweight, whereas the owners tended to classify them as simply overweight.

Canine obesity is not a joking matter, and it often impairs welfare and quality of life. Obese dogs face a higher risk of serious health problems such as diabetes, cardiovascular issues, and joint and bone disorders, as well as shortened life span (typically by about two years). In the U.K., the Royal Society for the Prevention of Cruelty to Animals (RSPCA) adds that overfeeding can be “just as cruel as underfeeding.”

Is it really possible that some owners do not realize their pets are overweight? When researchers investigated the disconnect between owner and veterinarian weight assessments, they learned that the former sometimes had misguided ideas about what made up weight. For example, an owner might not consider a dog fat but rather “all muscle” (something you might imagine us saying about ourselves). Others reasoned that because Rover had lost weight, he was now good to go—although the veterinarians would still consider the animal to be overweight.

How to Spot Excess Weight

Another possibility is that owners simply do not know how to assess their dog’s body condition. After all, what constitutes overweight versus very overweight for a particular dog?

The good news is that anyone can easily learn to evaluate a dog’s body condition. Veterinarians often rely on 5- or 9-point body condition scales where dogs are examined and ranked from underweight to markedly overweight and everything in between. These tools help determine whether a dog is a healthy size or not. The American Society for the Prevention of Cruelty to Animals (ASPCA) recommends the Body Condition Tools from Purina, which offer overhead and side views of each weight category, for both dogs and cats. Looking down at your dog from above, do you see an “hour-glass” indentation near the waist? Can you feel the ribs, easily? If this is not what you feel and observe, you could have a dog that stands to lose a few.

Fat dogs do not happen overnight. In fact, they happen over many nights, as was highlighted in the first episode of a wonderful series called Dogs: Their Secret Lives that aired in 2014 in the U.K. That episode, “Fat Dogs,” traced the way three dogs put on their extra pounds and their roads to taking them off. A hidden camera revealed, for instance, that one of them, a Cavalier King Charles spaniel named Chancer, perked up whenever Grandma ate. Grandma, it turned out, was generous to both her two- and four-legged grandchildren!

Decreasing a dog’s weight after a lifetime of reinforced overeating is not easy. On eating habits in dogs, illustrator Mark Ulriksen notes—in his book Dogs Rule, Nonchalantly (Goff Books, 2014)—“Each of my dogs had their own favorite meal .... Though I’ve yet to meet a dog that’s a fussy eater.” Dogs like to eat. People like to feed them.

That said, being overweight is not necessarily a permanent condition. After identifying the many factors (and people) that contribute to a dog’s high poundage, it is possible to reverse course. For example, in “Fat Dogs,” Chancer’s family learned that feeding is not just about what goes down, it’s also about how it goes down. Enter puzzle toys and slow feeders; these devices can force dogs to work at getting their food, which stretches out meals and can prevent scarfing and overeating. Slow feeders can be useful for normal-sized dogs as well as those who are overweight. Of course, consulting a veterinarian or specialist is a good first step if you have questions about your dog’s weight.

I can pretty much guarantee that dogs are unlikely to wake up one morning, reflect on their expanding midsection and proclaim, “Ya know, there’s really no need to keep eating like this. I’m going to cut back.” Instead it takes diligent pet parents to figure out what is contributing to a dog’s weight gain and commit to a diet and exercise plan for improved health. This might just be an area where dogs and humans are in a similar boat.

The Dog Killer in Your Pocket

Here’s another danger that might surprise you

Just as dog owners often don’t realize their canine friends are too heavy, they may have a blind spot about another threat. Surprisingly, the lowly penny can become a lethal weapon against dogs—specifically pennies minted after 1982.

Although all pennies are equal in value—one cent, no matter what year it is—their compositions are not. Pennies that were produced between 1962 and 1982 are predominantly copper (95 percent), whereas pennies churned out in 1982 and after are mostly zinc (97.5 percent).

Zinc is an essential mineral but is undesirable in excessive amounts. When pennies meet the acid in a dog’s stomach, the zinc gets released rapidly, which can destroy red blood cells and, in turn, lead to a number of debilitating conditions, including kidney or liver damage.

Certainly no one was thinking about dogs when the U.S. Mint approved the transition from a copper to a zinc penny. But if Abraham Lincoln were alive today, I’m sure he would say, “One score and sixteen years ago, our fathers brought forth on this continent a new penny, conceived in zinc. So, please, keep them out of reach of your dogs.” —J.H.