Matters of the Heart

If you take an elephant and put it on a scale and record its weight, and then you remove its heart and weigh it, and finally divide the weight of the heart by the weight of the elephant, you will get a dividend of approximately 0.6 percent. 

Then, if you take a mouse and put it on a scale and record its weight, then remove its heart and weigh it, and finally divide the weight of the heart by the weight of the mouse, you also will get a dividend of approximately 0.6 percent. 

Now, if you take a human or a cow and weigh either one, then weigh its heart and put it on a scale and record its weight, and finally divide the weight of the heart by the weight of the person or the bovine, you still will get a dividend of approximately 0.6 percent.

In fact, if you compare the size of the heart of any mammal to its total size you will always get a ratio of roughly six-tenths of one percent.  And, you can even plot these comparisons on a graph, with animal weights along the bottom and heart weights along the side.  You will get a straight line with a slope of 0.6%.

All mammals’ heart weight ratios will lie near this line.  Except for the dog.

If you compare the weight of a dog’s heart to its total body mass you will get a ratio of 0.8%.  That’s a third again as large as other mammals!

Now, a human marathon runner’s heart will increase in size under intensive training, perhaps up to 0.8%, which is an average dog’s size.  But, a marathon racing husky’s heart also will increase with lots of exercise, from 0.8% to 1.0%.  All the training in the world will never give a human super-athlete a heart as proportionately large as a sled dog’s.  Marathon huskies are the aerobic super-athletes of the world.

Even more interesting is how the sled dog got that way.  We would like to assume that humans “engineered” the sled dog from a wolf into an aerobic marathon runner through selective breeding–the way we have turned the water buffalo into a milk cow, or the way we morphed the wolf into the poodle, the chihuahua, the great Dane, and the shih-tsu.  But, we’d be wrong.

It turns out the wolf also has a heart size ratio far higher than other mammals.

Wolves are born to run long distances and they have been for as long as there have been wolves and the wide-ranging prey upon which they feed.  And, sled dogs, with whom wolves share a common ancestor, have carried aerobic wolf genes in their chromosomes for millennia.  We humans have done little to make the sled dog a marathon runner.  They evolved for it.

We may have selectively molded the wolf into the Irish setter or the Dalmatian or the Mexican hairless on the outside, but on the inside, the dog’s physiology has changed very little for thousands of years.  Like its lupine cousins, who will routinely trot up to 50 miles per day as a pack in search of game, the sled dog will trot along with its team mates for hours on end, reveling in the arctic scenery, and knowing that there’s a hot meal waiting up ahead.

If humankind had stood on the shores of the Great Flood as it receded, and surveyed all of the pairs of critters disembarking from Noah’s ark, to choose the one best animal with which to run a thousand-mile sled dog race, the wise person would have chosen the dog.

While it is doubtful that this is how it happened, there is no question that the aboriginal inhabitants of the Arctic made the right decision when they let the friendlier of the wild dogs of the North creep toward their fires to scavenge the crumbs of their feasts.  For that first courageous overture, the primitive wolf’s descendants have been amply rewarded with the best food, shelter, and humane care possible in such a harsh environment.  In return, the aboriginal peoples, and the white immigrants with whom they shared their knowledge of the wilds, were given the greatest of all athletes in the world to carry their loads and share their journeys under the northern lights.

So on Valentine’s Day, when our thoughts turn to open hearts, thankful for all of the loves in our lives, we mustn’t forget to put our arms around that super-athlete at our feet.  He’s got the biggest heart on the planet.

Dr. Jerry Vanek has been a musher or sled dog race veterinarian for the past 30 years, including five Yukon Quests.  He is a former officer of theISDVMA and he continues to write and speak widely on the subject of sled dog medicine.Author: Jerry Vanek, DVM