Dog Discoveries

Evolution Explains Why Dogs Have Four Toes

 

dog four toesHave you ever looked at your hands and feet and compared them to your dogs’? If so, perhaps one of the most prominent contrasting features you may notice is that your hands and feet have five fingers and five toes (that makes you pentadactyl, by the way), whereas, your average dog has only four functional toes.

Also, the fingers and toes of your hands and feet are quite long, while your dog’s toes instead are quite short, why is that? For sure, “Mother Nature” must have known what she was doing as she was weighing out the pros and cons of having toes. She then made her final verdict that less was better than more.

“Five is a common number for digital division among vertebrates; and our own hands and feet present these digits in a high degree of perfection.” ~Felter, 1912

A Matter of Speeddog lung

If we look at the toes of many animals, we may notice that the general standard is to have five toes at the end of a limb. Humans have 5 toes and so do bears, primates, weasels, bats, and several types of reptiles and birds. Some animals such as dogs, foxes, wolves and coyotes though have though four fully functional toes instead of five. What do these four-toed animals have in common?

Fossil evidence shows that animals that show a loss of digits are for the most part animals who were required to maintain high speeds for long distances, explains John Buckwalter, Emeritus of Biology at Alfred State College.

Such animals are known as “cursorial animals” and they are distinguished for their long limbs, shortened digits and reduced number of toes. Dogs for example have four toes instead of five, and horses (cursorial grazers) have only one (the hoof is simply the distal phalanx of the 3rd digit).

The Recipe for Running

In order to become swift runners, a change in basic physiology was necessary so  several speed adaptations were necessary. While us humans and bears are plantigrades, meaning that we place our whole foot on the ground with a rolling heel-to-toe action, dogs are digitigrades, meaning that they only walk on their digits, a quality that makes them faster and quieter runners as their heels are not required to touch the ground, explains Stanley Coren, in the book “Do Dogs Dream?: Nearly Everything Your Dog Wants You to Know.”

This means that the bones of the dog’s wrist (carpals), palm (metacarpals), sole (metatarsals) and ankle (tarsals) are kept off the ground. We therefore make a mistake when we compare our dog’s paws with our hands and feet.

In reality, we are mainly looking at their toes as seen in these illustrations.  Basically, the bones that correspond to our wrists and ankles are set much higher than we would think which explains why many people often confuse the dog’s ankles (tarsal, hock) for the dog’s knee (stifle). However, this doesn’t answer our question:”why do dogs have only four toes?”

” What evolution did to dogs was to rock their legs forward so that their heel would no longer touch the ground. In so doing they became a digitigrade species, meaning that they walk on their digits.” Stanley Coren

dog cursorialLoss of Weight

In us humans, our extra digit, the thumb, played a very important role in our evolution allowing us to manipulate things, but why would a cursorial animal do better with less toes? Dogs didn’t rely much on their digits to manipulate things, but there’s likely an additional reason for having four toes: it’s likely an issue of weight.

An extra toe may weigh just a few ounces, but its extra weight on the foot  may ultimately have an impact when it’s carried along for the ride as an animal runs to catch his dinner or runs for his life.

So evolution decided that it was in the dog’s best interest to not have a fully developed 1st digit, which is the equivalent of our thumb. Reduced weight is also a plausible explanation also as to why a dog’s legs has concentrated muscle mass at the top, while at the bottom the legs get more slender with a lighter foot. “An increase in limb length adds to stride length, whereas reduction in limb weight especially distally, enhances stride rate.”says  S. M. Kisia in the book “Vertebrates: Structures and Functions.”

“Members of the dog family (canids) have small feet, with usually four digits in contact with the ground. The small size and weight of their limbs requires less energy to move, allowing them to run more efficiently.”~ John Buckwalter,

dog dewclawA Vestigial Remnant….

It would be wrong though to say that the dog has totally lost his fifth toe. In reality, that extra digit is still there but it’s just reduced. You may stumble on it if you take a look at your dog’s foreleg and notice a claw that’s located higher up which doesn’t normally touch the ground as the other toes.

That claw is known as the “dewclaw“and it’s a remnant of that original fifth digit dogs used to have in the past. In technical terms, it’s a vestigial structure, meaning that it has lost its original function and therefore has atrophied.

Note: If you don’t see this fifth digit on your dog’s front feet it likely was removed shortly after birth to give the dog a cleaner look or to prevent it from getting caught on things.

“Dewclaws are vestigial remnants of a fifth digit that was reduced in the process of evolution.”~John Buckwalter

With a Purposedog agility role of tail

It would be also wrong to say that a dog’s dewclaws have completely lost their functionality. Turns out, contrary to what we may have heard, those dewclaws at times actually touch the ground, but because it happens when dogs run at high speeds we hardly notice it.  

Christine Zink, a veterinarian working with canine athletes, has found that those dewclaws that many people chop off and think of them as useless appendages, actually help prevent torque on the leg especially when the dog is galloping and making tight turns, as often seen in canine sports.

Without the dewclaw, there are risks that leg may twist which over time can lead to problems such as carpal arthritis and other injuries to the dog’s  elbow, shoulder and toes!

Did you know? Some dog breeds (like the Great Pyrenees or Briard for example) have one or even two dewclaws on their rear feet as well!

References:

  • MadSci Network, Why do dogs have dewclaws? and why are they only in the front?John Buckwalter, retrieved from the web on April 10th, 2o16.
  • Do Dogs Dream?: Nearly Everything Your Dog Wants You to Know” by Stanley Coren, W. W. Norton & Company; Reprint edition (July 16, 2012)
  • Evolutionary Biology 8/e by Dr. B. S. Tomar, Dr. S. P. Singh. Edition. 9th Revised Edition
  • Chapter 15, Limbs in Mammalian Evolution, by P. David Polly, retrieved from the web on April 10th, 2015
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2016. The Animal Diversity Web (online).
  •  Vertebrates: Structures and Functions, By S. M. Kisia, CRC Press (April 12, 2010)
  • Course Hero, Mammalian Specializations, Chapter 21,  retrieved from the web on April 10th, 2015
  • Do the Dew(claws)?  by M. Christine Zink DVM, PhD, DACVSMR, retrieved from the web on Aprile 10th, 2016

 

Photo Credits:

Paw and pads in dog by Amos T Fairchild,  CC BY-SA 3.0, edited to add text and adjust size

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