Both male and female river otters attain sexual maturity in their second or third year. Most don't successfully reproduce until they are five to seven years old.
Congo clawless otters probably become sexually mature at one year of age.
Female sea otters become mature at about four years. Males are capable of mating at age five or six, but usually don't become active breeders for several more years.
Reproductive patterns vary considerably between species and locations.
Eurasian otters are polyestrous, having a four- to six-week cycle with two weeks of estrus.
Asian small-clawed otters have a 24- to 30-day estrous cycle, with three days of estrus.
The reproductive cycle in California sea otters is about 12 months. If a female's pup does not survive, she may experience postpartum estrus.
In England, Eurasian otters mate throughout the year. In the more severe climates of Sweden and Siberia they mate in late winter and early spring.
North American, spot-necked, smooth, and marine otters in North American zoological facilities mate during winter and spring.
Sea otters breed throughout the year. Males may breed with more than one female during the year.
A male river otter appears to approach females indiscriminately until it finds one that is receptive. A male approaches a female and attempts to embrace her with his forelegs or rub and sniff her body. If she is receptive, the two roll and frolic; if not, she will push him away. Copulation is preceded by vigorous play - chasing, swimming, diving, twisting, cork-screwing, and lunging. In some species, the male bites the female's upper jaw or nose, in other species the male grasps the female by the scruff of the neck. Copulation takes place in the water, and lasts 10 to 30 minutes.
When a male sea otter finds a receptive female, the two engage in playful and sometimes aggressive behavior. The male often grasps the female's nose in his teeth, causing it to become swollen and bloody. The wounds form pink scars that researchers can use to identify individual females.