by John Wible
Armadillos are placental mammals that first appeared in the fossil record in South America 60 million years ago. Today, there are 21 species, only one of which is found in North America—the nine-banded armadillo, Dasypus novemcinctus.
All armadillos have a protective shell, or carapace, made of bony plates in the skin. These osteoderms not only cover the trunk, but the head, limbs, and, except for the naked-tail armadillo, even the tail. Armadillo in Spanish means “little armored one,” but the Aztec name is more descriptive, translating to “turtle-rabbit.” The carapace of the nine-banded armadillo has two major shields, one at the shoulders and the other at the pelvis, connected by a series of overlapping bands, which provide some degree
of flexibility. As you can guess from its name, there are usually nine such bands in Dasypus novemcinctus (as in the mother pictured here), but this varies from seven to 10. The underbelly is not similarly protected with osteoderms, but the skin is tough and leathery.
The evolution and biology of the nine-banded armadillo, the state small mammal of Texas, have fascinated me for years. However, because we just celebrated Mother’s Day, I want to comment on the amazing armadillo mother. Dasypus
novemcinctus is the only vertebrate that gives birth to identical quadruplets every time! A female produces a single egg that, once fertilized, splits into four genetically identical embryos that share one placenta. How and why this unique pattern evolved and continues to be maintained is a mystery. We usually consider genetic diversity a plus for organisms with multiple births as
it increases the chances that some offspring will survive in an ever-changing environment, but the nine-banded armadillo bucks this by producing clones.
Other than making milk for her young, the nine-banded armadillo mother is not particularly attentive, and the father is even less so. Yet, perhaps her major role is providing sufficient nutrition for her young to grow and prosper. Building a bony carapace requires extra nutrients, primarily calcium, and phosphate. Although ossification of the osteoderms begins in utero, you can imagine that birth (of four babies no less!) is facilitated by them having a thin and flexible carapace, that is, one that
is not fully formed. Indeed, thankfully, most of the carapace’s development occurs after birth.
The armadillo mother makes this all happen largely on an insectivorous diet, a generally poor source of calcium. Add to that, armadillos have a lower metabolic rate than most mammals. Somehow, despite these perceived handicaps, the young born at 100 grams (less than a quarter pound) grow rapidly and are able to forage for themselves at two months. How the armadillo mother is able to do all this for her four identical babies is as mysterious as how she had four identical babies in the first place.
John Wible, PhD, is curator of mammals at Carnegie Museum of Natural History. He studies the evolutionary history of mammals and lives in a house full of them, some human (wife and two sons) and some non-human (cats, rabbits, and guinea pigs).