Do Armadillos Always Have Quadruplets? Identical Births Explained

How do armadillos have identical quadruplets?

In most mammals, identical twins are a rare accident. In the nine-banded armadillo they are the rule. The process is called polyembryony, which means “many embryos” from one egg.

It happens in a few steps:

1. The female releases a single egg, which is fertilized after mating.
2. The egg develops into a tiny ball of cells (a blastocyst), which then pauses without attaching to the womb.
3. Months later, it implants, and early in its development the embryo divides into four.
4. Each of the four develops into a pup, and they share one placenta, though each has its own umbilical cord.

Because all four come from the same egg and the same sperm, they are genetically identical clones of each other. That also means a litter is either all male or all female.

Delayed implantation: a pregnancy with a pause

Nine-banded armadillos in the United States mate mainly in summer, in July and August. But the young are not born until the following spring, usually in March or April.

The reason is delayed implantation. After fertilization, the early embryo floats in the womb for about three to four months before it attaches. Only then does it split into four embryos and develop for roughly four more months. Pausing the pregnancy this way means the pups are born in spring, when insects are plentiful and the weather is mild. The full story of the species is in our nine-banded armadillo profile.

Why do armadillos have quadruplets?

Scientists are still debating why polyembryony evolved. The leading idea is linked to the shape of the armadillo’s womb. In the nine-banded armadillo, there is only one site in the uterus where an embryo can implant. That limits the female to a single egg per pregnancy. Splitting that one embryo into four is a way to raise a larger litter despite this limit.

There is a cost. Since all the young are genetically identical, a litter has no genetic variety, which may make it less able to cope with disease or changing conditions. Still, the system clearly works: a female can breed every year from about one year old and may produce dozens of young in her lifetime.

Are there ever exceptions?

The pattern is extremely reliable, but it is not perfect. Litters of fewer than four young are sometimes recorded, for example when one embryo fails to develop. Very rarely, genetic studies have found litters in which not all the young were identical, suggesting that two eggs were fertilized. These cases are unusual, so the rule of “four identical pups” holds for the vast majority of births.

Which armadillos have identical young?

Polyembryony is found only in the long-nosed armadillos of the genus Dasypus, not in all armadillos as is often believed.

The southern long-nosed armadillo of southern Brazil, Uruguay, Paraguay and Argentina can produce some of the largest litters of identical young of any mammal. Three-banded armadillos, the famous ball-rolling species described in can armadillos roll into a ball?, have a single pup.

Life as a baby armadillo

Newborn armadillo pups weigh about 85 g (3 oz). Their eyes are open, and they can walk within hours, but their shells are soft and leathery, rather like a fingernail before it hardens. The shell gradually stiffens over the following weeks.

The pups stay in the burrow drinking their mother’s milk for about two to three months and then start following her as she forages for insects. They become independent within their first year, and many females have their own first litter at about one year old. Young armadillos with soft shells are much more vulnerable than adults to coyotes, bobcats, birds of prey and cars.

Why armadillo quadruplets matter to science

Natural sets of four identical siblings are rare in nature, so armadillos have been useful to scientists. They have been used to study how genes and environment shape development, since any difference between the four young must come from their surroundings rather than their genes.

Armadillos are also one of the very few animals other than humans that can carry the bacterium that causes leprosy, and their low body temperature suits it. Since the 1970s they have been important in leprosy research. Health agencies advise people not to handle wild armadillos, though the risk to the public is considered low.