There is much discrepancy in the scientific literature regarding the length of river otter gestation periods, with reports ranging from 288 - 375 days (Chapman 1974; Wren 1991). This extreme difference has been attributed to a process known as delayed implantation, in which young are born following a period of arrested development. In this process, fertilized eggs develop to the blastocyst stage, and then remain floating in the uterus for a variable period of time before implanting into the uterine wall. Litters are born nearly a year following conception, generally from November to May; though in the Pacific northwest, pups are delivered from March through May. The actual gestation period following implantation is estimated to be 60-62 days (Chapman and Feldhamer 1982; Dronkert-Egnew 1991; Nebraska Game and Parks Commission 2001).Litter size ranges from 1- 6 pups per litter, with 2-4 being most common (Chapman 1982). Pups open their eyes at 21-35 days. At 25-42 days, they begin to play. Parents introduce pups to water at 48 days, and they begin to venture outside the den alone at 59-70 days. Pups are fully weaned by 91 days, and will leave parents at approximately 1 year old (Chapman 1982).
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