Females typically give birth to a single calf measuring 450 cm in length. There are reports of rare multiple fetuses. The calf nurses for six or seven months. Young reach sexual maturity at 10 years of age, but do not reach full adult size until they are about 25 years old. Sei whales may live as long as 74 years.
Females typically give birth every other year, but a recent increase in pregnancies has been noted.
Researcher think this may be a response to the predation rate. Behavior Little is known about the actual social system of these animals. Groups of two to five individuals are typically observed, but sometimes thousands may gather if food is abundant. However, these large aggregations may not be dependent on food supply alone, as they often occur during times of migration. During mating season.