Although mating systems have not been described for north Pacific right whales, it is likely that mating is similar in other right whale species such as southern right whales and north Atlantic right whales. Southern right whales and north Atlantic right whales form non-aggressive mating aggregations in which individuals engage in gentle physical contact and nuzzling. Females are likely to mate with multiple males and males do not compete aggressively for females. Behaviors associated with mating in right whales include fin and tail slapping at the surface, "headstanding," and breaching. In headstanding, right whales float at the water surface in a vertical position with the flukes extended into the air. They may also rock back and forth while in this position and hold it for several minutes. This is thought to be a mating display. Breaching and tail/fin slapping are also more common during mating aggregations and may be a kind of courtship display.
The sex organs of male right whales are positioned on their abdomen. Right whales have no scrotum or baculum, however they have exceptionally large testes. One north Pacific right whale was reported to have a testis 201 cm in length, 78 cm in diameter, and 525 kg in mass. Males have a darkly pigmented, long, slender, retractile penis that can reach 215 to 270 cm in length. The ovaries of female right whales are also positioned on the abdomen. The largest measured ovary weighed 6.3 kg. Right whales have a bicornuate uterus, and it is thought a fetus can develop in either horn of the uterus, as with most baleen whales.
Mating System: polygynandrous (promiscuous)
Like other right whales, northern Pacific right whales have exceptionally low reproductive rates. They give birth in winter to a single young every 3 to 4 years after a gestation period of just over a year. Breeding generally occurs in winter with births occurring the next spring. The length of lactation is unknown but has been estimated to last 6 or 7 months. Although the duration is unknown for right whales, weaning is usually gradual and prolonged in ceteceans. A 1-year-old north Pacific right whale was found with a large amount of milk in its stomach, and another individual 11.3 m in length appeared fully weaned. Males reach sexual maturity when they reach 15.2 m in length and females at 15.8 m, which is about 10 years in age.
Breeding interval: Female north Pacific right whales give birth to a single young every 3 to 4 years.
Breeding season: North Pacific right whales breed during the winter.
Average number of offspring: 1.
Range gestation period: 12 (low) months.
Average age at sexual or reproductive maturity (female): 10 years.
Average age at sexual or reproductive maturity (male): 10 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous
North Pacific right whales measure up to 6 m in length when they are born, and they grow quickly for their first few years, reaching lengths up to 12 m by 18 months of age. The milk of mother north Pacific right whales is high in protein and is lipid-rich. Mothers nurse, protect, and care for their young, investing significant energy into each offspring. Little is known about the duration of lactation and care, but it is likely to be long, given the 3 to 4 year interval between breeding attempts in females. There may be a long period of association with the mother and an extended period of learning. Female right whales have strong inclinations to protect their young. Females position themselves between their claves and sources of danger including other whales, boats, aircrafts, or divers.
Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female)