Nine-banded armadillos practise obligate polyembryony. One fertilised egg implants, then divides, and the resulting embryos develop as separate individuals sharing a single genome. The typical result is four pups, all clones of one another and all the same sex. Occasional litters fall outside four, but the four-way split is the rule rather than an anomaly.
The species also delays implantation. A fertilised egg can remain dormant in the uterus for months before development resumes, which lets births fall in a favourable season regardless of when mating occurred. Combined with a long life for an animal of its size, this reproductive system has helped armadillos colonise new ground quickly.
Two corrections are worth making. Polyembryony of this kind belongs to the genus Dasypus rather than to armadillos in general, so other genera do not share it. And the armadillos that spread across the southern United States from the late nineteenth century are now usually treated as a separate species, the Mexican long-nosed armadillo. Why the trait evolved at all remains debated, since four copies of one genotype forfeit the genetic variety that usually makes a litter worth having.