Which mammal always bears four identical young of the same sex?

The mammal that always bears four identical young of the same sex is the armadillo, specifically the nine-banded armadillo. Its four babies are genetically identical because they develop from one fertilized egg that splits into four embryos.

This reproductive process is called polyembryony. Unlike ordinary fraternal twins, the young share the same genetic material, so a litter is either all male or all female. The nine-banded armadillo is the best-known vertebrate example of this unusual pattern.

The babies are born with soft, flexible shells rather than the hard armor seen in adults. They remain dependent on their mother for several weeks before becoming more mobile and developing tougher protective plates. A common mix-up is assuming every armadillo produces four identical babies; the rule is most famously associated with the nine-banded species, while related armadillo species can differ.