Unlike sea otters, river otters burrow into hillsides near the water to make cozy dens where they sleep and where females have their litters of pups. Females give birth to 1-4 pups after 63 days of embryo development. But that could be more than a year after mating! Otters have the unusual ability to delay embryo implantation in the womb for 8-10 months, perhaps to wait for the easiest pup-rearing conditions.