Pet Column for the week of June 1, 1998
Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
By Theresa A. Fuess, Ph.D.
University of Illinois
College of Veterinary Medicine
"The class of animals called mammals includes bats that fly, dolphins that live in water,
moles that live underground, armadillos that wear a coat of armor, rodents as small as your
thumb, elephants that weigh 14,000 pounds, and humans that write books about all this,"
says Dr. Gerald J. Pijanowski, professor of morphology at the University of Illinois College
of Veterinary Medicine at Urbana. "Most of us know that only mammals have hair and
mammary glands that secrete milk to suckle the young. However, also unique to mammals
are some basic features of the musculoskeletal system and brain. The refinements of these
basics by different species give rise to the enormous variety we observe."
For an example, let's compare the limbs of familiar mammals that appear very dissimilar.
The accompanying figure highlights the equivalent bones in the hind limb of a cat and a
horse and in the human leg. The basic bone structure of the limbs of these animals is very
similar, but bones that make up the equivalent of human feet are highly specialized.
People, bears, and other animals that walk on feet are called plantigrades. Cats, dogs, and
other animals that walk on the equivalent of human toes are called digitigrades. Notice in the
figure that the the metatarsal bones that make up part of the human foot is part of the cat's
leg. Horses, cows, and other animals that walk on tiptoe are called ungulagrades. The
horse's leg includes not only the metatarsals but also the "toes," which makes the leg and the
stride relatively longer than that of digitigrades and planitgrades.
Plantigrades use toes at the end of the foot to help them balance on two hind limbs.
Digitigrades are able to move more quietly and, in general, they can attain faster speeds
than plantigrades or ungulagrades. Ungulagrades have relatively long, slender limbs, and
they can sustain a high speed for longer periods.
The ends of the digits or toes are also modified in each of these species. People have
toenails, cats have claws, and horses have hooves. Each of these is a modification of
basically the same material. The combination of a cat's stealth and claws make it a very
good hunter. A horse's hooves are well suited for supporting its body weight while running
long distances on rugged, abrasive terrain. These hooves and long, slender legs enable a
horse to outdistance a cougar, given enough head start.
The extremities of front and hind limbs in four-legged mammals are very similar. Primates
are unique in having an opposable thumb, which enables them to grasp an object in one
hand between their fingers and thumb. Other mammals with flexible fingers must always use
two hands to grasp an object.
"So, if you have any questions about comparative anatomy," says Dr. Pijanowski, "you can
use your opposable thumb to pick up the phone and call your local veterinarian."
CAPTION: Skeletal bones of the hind limb of a cat and a horse and a human leg. The cat
stands on the equivalent of human toes. The horse stands on "tip toe."
Office of Public Engagement /
2001 S Lincoln Ave /
Urbana, IL 61802 /
Last modified Thu Dec 23, 2004 by DSL
Your comments about the site are welcome, but we cannot dispense medical advice via the Internet.