Fin whales are seen in pairs during the breeding season and are believed to be monogamous. There have been sightings of courtship behavior during the breeding season. A male will chase a female while emitting a series of repetitive, low-frequency vocalizations, similar to humpback whale songs. However, these songs are not as complex as those observed in humpback whales or gray whales. One study has shown that only males produce these low-frequency sounds. Low frequencies are used because they travel well in water, attracting females from far away. This is important because fin whales do not have specific mating grounds and must communicate to find each other.

Mating System: monogamous

Both mating and calving occur in the late fall or winter when fin whales inhabit warmer waters. Each female gives birth every 2 to 3 years, birthing one calf per pregnancy. Although there have been reports of fin whales giving birth to multiple offspring, it is rare and those offspring rarely survive. The gestation period is 11 to 11.5 months. The mother then undergoes a resting period of 5 or 6 months before mating again. This resting period may extend to a year if the female fails to conceive during the mating period.

Fin whale calves are born at an average length of 6 meters and weighing 3,500 to 3,600 kilograms. Calves are precocial at birth, able to swim immediately after. The age of sexual maturity ranges in from 4 to 8 years. Male fin whales become sexually mature at a body length of about 18.6 meters while females mature at a body length of 19.9 meters. Physical maturity does not occur until the whales have reached their full length, after 22 to 25 years of age. The average length for a physically mature male is 18.9 m and 20.1 m for females.

Breeding interval: Fin whales breed every 2 to 3 years.

Breeding season: Breeding occurs from November to January in the northern hemisphere and June to September in the southern hemisphere.

Range number of offspring: 1 to 1.

Range gestation period: 11 to 12 months.

Range birth mass: 3,500 to 3,600 kg.

Range weaning age: 6 to 7 months.

Range time to independence: 6 to 8 months.

Range age at sexual or reproductive maturity (female): 4 to 8 years.

Average age at sexual or reproductive maturity (female): 7.7 years.

Range age at sexual or reproductive maturity (male): 4 to 8 years.

Average age at sexual or reproductive maturity (male): 6.7 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous

Average number of offspring: 1.

The mother nurses the infant for 6 to 7 months after it is born. Since the calf does not have the ability to suckle, like land mammals, the mother must spray the milk into the mouth of the baby by contracting the circular muscles at the base of the nipple sinus. Feeding takes place at 8 to 10 minute intervals throughout the day. At weaning the calf is usually 14 meters long, it then travels with its mother to a polar feeding area where it learns to feed itself independent of its mother.

Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)