Armadillos are placental mammals (Order Xenarthra) that occupy the southern United States, Central America, and the northeastern half of South America. They differ from other placental mammals in numerous ways. The uterus is simplex, just like that of humans, but there is no vagina and instead, a urogenital sinus serves as vagina and urethra. Males have internal testes and no scrotum, and have among the longer penes of mammals, reaching one third the length of the body in nine-banded armadillos (Dasypus novemcinctus). The oddities are not limited to morphology, but include the physiology as well.
Nine-banded armadillos have unusual delayed implantation. Armadillos breed in June, July or August, and the only fertilized egg becomes a blastocyst after 5-7 days at which point it enters the uterus. Development then ceases and the blastocyst remains free-floating in the uterus until November or December when it implants, and then the zygote divides twice to form four identical embryos. After 5 months of gestation, four identical quadruplets are born usually in May. Although identical twins are known to occur in humans, nine-banded armadillos regularly have identical offspring, most often four of them. But the mystery does not end there: a captive female held in solitary confinement gave birth to a litter of four females 24 months after her capture, or 32 months after she could have mated in the wild! Although the mechanism is not clearly understood, either her implantation delay lasted 23-24 months, or this particular female had produced two eggs, one of which would have remained dormant for at least 15 months.
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