Sea otters have been characterized as having sexually segregated social organizations, with "male areas" and "female areas".^1,2,3-5 Adult males enter female areas and establish a territory.^4-8 Males may copulate in serial fashion with females that enter their territories. The copulation rates of territorial males correlate closely with the shelter and food availability of that particular territory.

Evidence, based on morphological histological examinations of reproductive organs, field observations and captive animals observations, indicate the sea otter breeds throughout the year with breeding activity reaching a peak in the fall. Parturition occurs in all months of the year with a maximum frequency in spring. Some features of the reproductive cycle such as the time of year during which peaks in mating and pupping occur, the length of pup dependency periods, weights at birth, and the length of time between successive reproductive attempts in females may vary depending on environmental conditions and carrying capacity of the population.⁹ In general, reproduction is more seasonal in Alaska than in California.

Age at sexual maturity can be estimated by physical characteristics. Sea otter testicles increase in size until 5 to 6 years of age, after which their size does not change, suggesting that male sea otters reach sexual maturity at this time.³ In Alaska, males less than 6 years of age are not able to successfully maintain territories and breed. However, in captivity a 2 year old has exhibited mating behavior and copulated. A captive male at 19 years of age fathered young at the Vancouver Aquarium.¹¹ The female sea otter reaches puberty at 3 to 4 years of age.

When mating, a pair bonds for 1 to 4 days. During this period, the pair conducts all activities in close proximity to each other and several copulations may take place in the water. During copulation, the male grabs the female by the nose and mounts her from the back. Coitus is preceded and followed by mutual nuzzling, pawing, and tumbling together.¹² Occasionally, females are seriously injured and stressed in breeding attempts. The pair-bond is terminated when the female leaves the male's territory. At this time the male may attempt to prevent the female from leaving. It appears that a female's sexual receptivity ends before pair-bond dissolution and is likely to be the reason the pair-bond ends. It is suggested that estrus is 3 to 4 days long based on the duration of the pair-bond.^2,6

Parturition has been observed on land, but usually occurs in water.^2,23-15 The normal pupping interval is about 12 months¹⁶ and the gestation period approximately four to six months. Pups weigh 1.8 to 1.9 kg at birth. Twin fetuses have been observed at necropsy and in one case, the dystocia was the apparent cause of death of the mother.¹⁷

Recently, twinning was observed in the wild but the mother abandoned one of the pups on the first day.¹⁸ The abandoned pup was retrieved and raised at the Monterey Bay Aquarium. Successful rearing of twins has not been reported in sea otters.

Parental care, provided exclusively by the female, continues for five to eight months. Such intensive maternal investment helps prepare the young for survival in the harsh aquatic environment after weaning. The sea otter pup's nourishment comes exclusively from mothers's milk during the first month of age; by four months of age the pup subsists mainly on solid food obtained by the mother. At fourteen weeks of age, most pups are able to swim independently, dive proficiently, and groom themselves without the assistance of their mothers. Pups are able to capture and break open hard-shelled prey, using rock tools, by 20 to 24 weeks of age.¹⁹ Pups become independent at body weights as low as 10 kg or as high as 20 kg. Mothers in poor health may abandon their pups before they're old enough to survive on their own. Severe storms periods may also promote premature separation of female/pup pairs.²⁰

References

1.  Lensink, C.J. The history and status of sea otters in Alaska. Ph.D. Thesis, Purdue Univ., Lafayette, Indiana. 186pp. 1962.

2.  Kenyon, K.W. The sea otter in the eastern Pacific Ocean. N. Am. Fauna 68:1, 352 pp., 1969.

3.  Schneider K.B. Sex and age segregation of sea otters. Alaska Dept. Fish and Game, Juneau, Final Rep. Fed. Aid Wildl. Restor. Proj. W-17-4 through W-17-8. 45 pp. 1978.

4.  Garshelis, D.L., and J.A. Garshelis. Movements and management of sea otters in Alaska. J. Wildl. Manage. 48(3):665-678. 1984.

5.  Garshelis, D.L., A.M. Johnson, and J.A. Garshelis. Social organization of sea otters in Prince William Sound, Alaska. Can. J. Zool. 62:2648-2658. 1984.

6.  Vandevere, J.E. Reproduction in the southern sea otter.Pages 221-227 in Proc. Seventh Annual Conference on Biological Sonar and Diving Mammals. Stanford Res. Inst., Menlo Park, Calif. 1970.

7.  Calkins, D.G., and P.C. Lent. Territoriality and mating behavior in Prince William Sound sea otters. J. Mammal. 56(2):528-529. 1975.

8.  Loughlin, T.R. Activity pattern, habitat partitioning, and grooming behavior of the sea otter (Enhydra lutris) in California. Ph.D. Thesis, Univ. California, Los Angeles 110 pp. 1977.

9.  Rotterman, L.M., T. Simon-Jackson. Sea otter Enhydra lutris in Selected Marine Mammals of Alaska, Pub. no. PB88-178462, Lentfer, J.W., Ed., National Technical Information Service, Springfield, VA. pp. 237-275. 1988.

10. Garshelis, D.L. Ecology of sea otters in Prince William Sound, Alaska. Ph.D. Thesis, Univ. Minnesota, Minneapolis. 321 pp. 1983.

11. Hewlett, Gil., Vancouver Aquarium, Vancouver British, Columbia, Canada.

12. Shelley, Mark., Montery Bay Aquarium, Monterey, California, Video Tape. 1988.

13. Barabash-Nikiforov,I.I., S.V. Marakov, and A.M. Nikolaev. The Kalan or sea otter. Izdatel'stvo "Nauka," Leningrad. 184 pp. (Transl. from Russian by A.L. Peabody, U.S. Dep. Commer., NOAA, Natl. Mar. Fish. Serv., Off. Int. Fish., Language Services Div., Washington, DC 20235). 1968.

14. Antrim, J.E., and L.H. Cornell. Reproduction of the sea otter, Enhydra lutris, captivity. Int. Zoo Yearb. 20:76-80. 1981.

15. Jameson, R.J. Evidence of birth of a sea otter on land in central California. Calif. Fish and Game 69(2):122-123. 1983.

16. Jameson, R.J., and A.M. Johnson. Reproductive characteristics of female sea otters (Enhydra lutris). U.S. Fish and Wildl. Serv., Natl. Ecology. Res. Center, San Simeon Field Sta., Calif. In prep.

17. Williams, T.D., Mattison, J.A., and Ames, J.A. Twinning in a California sea otter. J.Mammal. 61, 1980.

18. 18.Jameson, R.J., and J.L. Bodkin. An incidence of twinning in the sea otter (Enhydra lutris). Mar. Mammal Sci. 2(4),:305-309. 1986.

19. Payne, S.F. and R.J. Jameson. Early behavioral development of the sea otter, (Enhydra lutris). J. Mamm. 65(3):527-531. 1984.

20. Morejohn, G.V., J.A. Ames, and D.B. Lewis. Post Mortem Studies of Sea Otters, (Enhydra lutris) in California. marine Resources Tech. Rept. No. 30, Calif. Dept. of Fish and Game, 86pp., 1975.