Which animal almost always gives birth to same-sex quadruplets?

The term “armadillo” means “little armored one” in Spanish, and refers to the bony, armor-like plates covering their bodies. The name is fitting as an armadillo’s primary defense mechanism is its shell, or carapace.

There are about 20 known species of armadillos worldwide, but the nine-banded armadillo (which can have 7 to 11 bands) is the only one found in the US. The females of the species almost always give birth to same-sex litters of four identical quadruplets, a trait that's unique to this mammal.

Armadillos are nocturnal and spend most of their waking time burrowing and feeding; their diet consists primarily of insects, Mating takes place during a two-to-three month long mating season, which occurs from July–August in the Northern Hemisphere and November–January in the Southern Hemisphere. A single egg is fertilized, but implantation is delayed for three to four months to ensure the young will not be born during an unfavorable time. Once the zygote does implant in the uterus, a gestation period of four months occurs, during which the zygote splits into four identical embryos, who share a common placenta. The quadruplets are born in March and weigh about 3 oz (85 g). The babies look very much like adults, but are smaller and softer than their armored parents. After birth, the young remain in the burrow, living off the mother’s milk, for about three months. Eventually they begin to forage with the mother, and strike out on their own after six months to a year.