Cetaceans are anything but monogamous, but a long-term paring of a male and a female has never been reported for these animals, no more than among other marine mammals. Consequently, the mother is left with the upbringing of her young, a role she tend to with a complete devotion.
Baleen whales, such as minke, fin, blue and humpback whales, lead a relatively solo lifestyle and usually do not rely on the assistance of others. Therefore, the baleen whale mothers are left completely alone with caring and protecting their young calves. On the other hand, toothed whales have learned that the social life is the most secure way. Together, they increase their chances of hunting successfully and of rearing and caring for their young. The solo lifestyle has, however, demanded larger body sizes. As a result, the largest animal ever have roamed the earth is a female blue whale, but baleen whale females usually grow to be larger and heavier than the males. That gives them an advantage in nursing and protecting a young calf on their own.
The toothed whale mothers, such as dolphins and sperm whales, are no less devoted in the upbringing of their young. However, they usually do not take on that job alone. They are assisted by other females sharing the same group. Sometimes these are unrelated animals that have formed a lifelong alliance or family members such as sisters, aunts, grandmothers and cousins.
Edda Elísabet Magnúsdóttir
Marine biologist and a whale specialist at the University of Iceland