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Adactylidium is a type of mite which probably has the fastest (and possibly the most disturbing) mating cycle of any animal: the male mates with his sisters in the mother's womb . The baby mites then eat their way out of the mother and the process is repeated in four days . In some species, the male emerges from the mother alongside his sisters and then dies a few hours later, while in other species, he doesn't emerge at all, because his only purpose was to impregnate his unborn sisters

While not quite as extreme as Adactylidium , the common stoat has a very bizarre mating strategy for a mammal. While males sexually mature at about a year old, females are sexually mature at three weeks old , when they're still blind, deaf, and hairless, and the mother will let adult males mate with her female kits while they're still infants. Female stoats are able to retain sperm from mating for up to ten months later before fertilization and spend almost all their life pregnant.