REPRODUCTIVE TIMING IN EURASIAN OTTERS ON THE COAST OF NORWAY

The seasonal distribution of reproductive phases in female otters and the recruitment of cubs were studied by using information from carcasses collected along the central and northern coast of Norway. Relationships between available prey and female body condition were investigated. Conception, parturition and rearing of cubs, at least up to the age of 5-6 months, occurred successfully at all times of the year. However, the main birth peak occurred in summer and autumn. The frequency of all phases of the breeding cycle, from follicle enlargement to early stages of cub rearing, varied consistently between seasons. Food availability when the cubs were c. 2 months old, and energetic demands on the female assumed to be greatest, was probably the main ultimate cause of the seasonal variation in the recruitment of cubs. Intrauterine mortality, possibly due to the seasonal variation in body condition of females, and loss of litters, may have been important mechanisms in creating the seasonal recruitment patterns. Although the prey biomass on average showed seasonal variation, the peak season shifted among years and locations. It is argued that the availability of suitable prey for otters tend to be unrelated to the seasonal productivity in aquatic environments, due to species-specific growth and behavior of prey and restrictions on the hunting capability of otters. Continuous reproduction with seasonal variation in birth rates may have been maintained by selection for reproductive riskreduction in a nutritionally unpredictable, although generally seasonal, environment.