Sperm whales are the largest toothed and one of the most sexually dimorphic cetaceans, as the males are much larger than females. Males can grow to lenghts of 18 meters and females can grow to be 12 meters.
In the Odontoceti group, males are generally larger than females, however this difference is usually very small and very hard to notice in the wild. Sperm whales are very special in that aspect! 🙂
Female Sperm whales reach sexual maturity around 9 years of age when they are roughly 9 m long. At this point, growth slows and they produce a calf approximately once every five years. After a 14-16 month gestation period, a single calf about 4 m long is born. Although calves will eat solid food before one year of age, they continue to suckle for several years.
Females are physically mature around 30 years and 10.6 m long, at which time they stop growing. For about the first 10 years of life, males are only slightly larger than females, but males continue to exhibit substantial growth until they are well into their 30s. Males reach physical maturity around 50 years and when they are 18 m long. Unlike females, puberty in males is prolonged, and may last between ages 10 to 20 years old. Even though males are sexually mature at this time, they often do not actively participate in breeding until their late twenties.
Most females will form lasting bonds with other females of their family, and on average 12 females and their young will form a family unit. While females generally stay with the same unit all their lives in and around tropical waters, young males will leave when they are between 4 and 21 years old and can be found in “bachelor schools”, comprising of other males that are about the same age and size. As males get older and larger, they begin to migrate to higher latitudes (toward the poles) and slowly bachelor schools become smaller, until the largest males end up alone. Large, sexually mature males that are in their late 20s or older, will occasionally return to the tropical breeding areas to mate. Here, in the Azores, we mostly see the groups of females and youngsters but we also see adult males regularly.
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