Although little is known of gray whale mating behavior, group mating events of three or more individual have been documented. Gray whales have a high reproductive rate, relative to other baleen whales.
Mating System: polygynandrous (promiscuous)
Gray whales mate throughout the year, however, most conceptions occur during the fall migration. After 13 to 14 months of gestation, females give birth to a single calf (one occurrence of twin fetuses was reported in 1987), which nurses until it is 6 to 7 months old. Eastern gray whale calves are born in late January in the warm coastal waters of Baja California, Mexico; however, early calving during the fall migration has been documented. Although less information is available for western gray whales, their winter calving grounds are thought to be along the coast of the South China Sea and likely have characteristics that are similar to the calving grounds of their eastern counterparts. Calving grounds are typically in shallow lagoons that are less than 4 m in depth and are hyper-saline. Preference for shallow water during calving may have contributed to the extirpation of the north Atlantic population in the mid 1700's.
Sexual maturation in gray whales occurs around 8 years of age, but has been documented in individuals as young as 5 and as old as 11. Nevertheless, studies suggest that size may be a better indicator of sexual maturity than age. Males average 11.1 m in length at time of sexual maturation and females average 11.7 meters. Sixty percent of the population consists of sexually mature adults. The average generation length (number of years between an individual's birth and the age at which they give birth) for gray whales is 22 years.
Breeding interval: Every other year
Breeding season: Year round mating with most conceptions occuring in late November to early December
Range number of offspring: 1 (high) .
Range gestation period: 13 to 14 months.
Range birth mass: 500 to 600 kg.
Range weaning age: 6 to 7 months.
Average time to independence: unknown years.
Range age at sexual or reproductive maturity (female): 5 to 11 years.
Range age at sexual or reproductive maturity (male): 5 to 11 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous
Gray whales replenish fat reserves during the summer. Pregnant females are especially dependent on these reserves. From the time they leave the summer feeding grounds in the fall, to when they return in early summer, females rely on fat reserves for energy and milk production. During times of limited food availability, interval between individual calving events may be extended.
Gray whale cows often hold newborn calves to the surface to help them breathe and are fiercely defensive of their young, especially against potential predators such as orcas (Orcinus orca) and human whalers. Gray whales inherit their mother's feeding grounds and are often seen, 1 year after they become independent, in their mother's feeding grounds.
Parental Investment: precocial ; female parental care ; pre-hatching/birth (Provisioning: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); inherits maternal/paternal territory