Little is known about mating in cetaceans, especially in species which occur primarily offshore. Mating systems in Dall's porpoises are unknown.
Little is known about the reproductive biology of Dall's porpoises. Two calving periods have been reported for portions of the eastern North Pacific, one in winter, from February through March, and the other in summer, from July through August. Some segregation of animals seems to occur with juveniles found closer to shore and larger adults well offshore. In offshore areas, females in late pregnancy or lactation seem to be distributed in northern areas, and southern areas are mainly occupied by males and females not accompanied with calves. This seems to indicate that not all females become pregnant every year. Females usually reach sexual maturity between the age of 3 to 6 years, whereas males reach sexual maturity between the ages of 5 to 8 years. Gestation is believed to last about 11 months, and lactation periods are usually about 2 years.
Phocoenoides dalli dalli appear to have three major breeding grounds. Two occur in the North Pacific north of 45 degree latitude, and another breeding site occurs in the central Bering Sea. Phocoenoides dalli truei may breed off the northern coast of Japan.
Breeding interval: Individual females probably do not breed every year. Breeding intervals may be as long as 3 to 4 years because of the length of dependence of calves.
Breeding season: Mating is likely to occur after the calving seasons each year which occur in winter, from February to March, and in summer, from July to August.
Average gestation period: 11 months.
Average weaning age: 24 months.
Range age at sexual or reproductive maturity (female): 3 to 6 years.
Range age at sexual or reproductive maturity (male): 5 to 8 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous
Average gestation period: 347 days.
Average number of offspring: 1.
Females feed and care for their offspring for extended periods of time. It is likely that some form of extended learning occurs during this period as well. Male parents do not contribute parental care.
Parental Investment: no parental involvement; precocial ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); extended period of juvenile learning