Mammals
Nine-banded Armadillo
Dasypus novemcinctus · also called nine-banded armadillo, common long-nosed armadillo
The nine-banded armadillo is a small, armour-plated insect-eater that looks like a relic of the age of giant ground sloths — which, in a sense, it is. It has a set of biological quirks unusual enough that it has become a standard research animal, and it is one of the great range-expanders of modern North America.
Overview
Nine-banded armadillos live in forest, scrub, grassland, and wetland edges wherever the soil is soft enough to dig and does not freeze hard for long. They are solitary, mostly nocturnal in hot weather, and spend their active hours snuffling through leaf litter and topsoil for beetles, grubs, ants, and termites, which they locate by smell and dig out with strong claws. Each animal maintains several burrows within its home range.
Armour and senses
The “shell” is a carapace of bony plates overlaid with horn, with a rigid shield over the shoulders and hips and a set of flexible bands (usually eight or nine) across the middle that let the animal bend. Unlike the three-banded armadillos of South America, the nine-banded species cannot roll into a ball; its defence is to run for a burrow, wedge itself in, or dig rapidly out of sight. Its eyesight is poor and it relies on smell and hearing.
A strange reproductive system
Two features stand out. First, the female can delay implantation of a fertilised egg for months or even years, timing birth to good conditions. Second, and uniquely among mammals as a rule, that single egg then splits into four identical embryos, so every litter is identical quadruplets of the same sex. This makes the armadillo valuable for studies that need genetically matched subjects.
Leprosy
The nine-banded armadillo is the only known wild reservoir of the bacterium that causes leprosy. Its cool body temperature suits the microbe, which is otherwise very difficult to culture, so armadillos have been central to leprosy research and vaccine work. A handful of human cases in the southern US each year are linked to handling or eating armadillos; the practical advice is to do neither.
An expanding range
Since the mid-1800s the species has spread from Texas across the southern and central United States, reaching Illinois, Nebraska, and the mid-Atlantic, and it is still moving north and east, limited mainly by cold winters and its need to eat every day.
Status
The nine-banded armadillo is Least Concern and increasing — one of the few medium-sized mammals actively gaining ground in North America.
Nine-banded Armadillo: common questions
Why does the nine-banded armadillo always have identical quadruplets?
The female produces a single fertilised egg, which implants and then splits into four genetically identical embryos, so every litter is four clones of the same sex sharing one placenta. This is called obligate polyembryony, and this armadillo and its close relatives are the only mammals known to do it as a rule. Why it evolved is unclear — one idea is that the shape of the armadillo's uterus allows only one implantation site, and splitting the embryo is the only way to produce more than one young per pregnancy.
Do armadillos really carry leprosy?
Yes. The nine-banded armadillo is the only wild animal known to be a natural reservoir of Mycobacterium leprae, the bacterium that causes leprosy (Hansen's disease). Its low body temperature suits the bacterium, which is hard to grow any other way, so armadillos have been essential to leprosy research and vaccine development. In the southern United States, a small number of human leprosy cases each year are linked to handling or eating armadillos, though the overall risk is very low and the disease is curable with antibiotics. The sensible precaution is simply not to handle wild armadillos or eat their meat.
How does an armadillo cross a river?
It has two methods. For a narrow stream it can hold its breath — for as long as about six minutes — and simply walk across the bottom, since it is dense enough to sink. For a wider river it gulps air to inflate its stomach and intestines, roughly doubling its buoyancy, and paddles across on the surface before belching the air out on the other side. The choice seems to depend on how far it has to go.
Why are so many armadillos seen dead on roads?
Partly because there are simply a lot of them, and partly because of a specific reflex. When startled at close range, a nine-banded armadillo jumps almost a metre straight up into the air. This may startle a predator, but with a car it is often fatal — an animal that would have passed safely beneath the bumper instead leaps up into it. Their poor eyesight and habit of foraging at dusk and along roadside ditches add to the toll.