Menopause… in whales?

A recent study reveals that humans aren’t the only ones with a post-reproductive life stage. Reasons behind menopause, a mid-life event in which females cease menstruating, have been interpreted in a variety of ways.

The most liable explanation seems to be the ‘grandmother’ hypothesis. When a woman is no longer able to reproduce, she can redirect her efforts in rearing her children and grandchildren. From an evolutionary standpoint, raising and caring for your own grandchildren increases their chances of survival, thereby increasing the survival of one’s own genes through generations. One might also argue that being unable to produce more offspring reduces competition for resources for the existing offspring, further increasing their survival chances.

Aside from the biological explanation, menopause and a post-reproductive life is a large component of our complex social system. Imagine not having those memories of sitting on grandma’s lap on the eve of Christmas, or confiding her your biggest secrets hours after your bedtime. This social system forms a crucial part of our lives.

As is turns out, the same phenomenon can be seen in other species. The study of 2 killer whale populations revealed that killer whale females have the longest non-human menopause. They can live up to 90 years old, and reach menopause in their 30s or 40s. That’s over half of their lifespan spent without the ability to reproduce.

A family of killer whales, retrieved from The Whale And Dolphin People Project

As in humans, this phenomenon forms a part of a very complex social dynamic. Killer whale mothers place an enormous amount of effort in caring for their offspring following birth. They spend their lifetime swimming side by side. Research shows that this is particularly important for male young.

If a male is over 30 years old and loses his mother, the chance of him dying in the following year increases by 14 times, whereas it has little effect on female offspring.

The theory behind this is that a mother increases the survival rate and reproductive success of her son. Her son’s offspring will then carry some of her genetic makeup, but be reared by the population of its mother in a different group of whales. However, if the daughter reproduces, her offspring must be cared for by that same group, thereby competing for that group’s resources.

Therefore, the extra effort placed in taking care of her son is the most efficient way for a mother of ensuring reproductive success of her offspring, without compromising the resource availability and functionality of her own group of whales. Have a read of Science Daily for more details on the study and the associated theory.

It is always interesting to study these animals. Whales are particularly interesting because of the complexity of their social system, which, let’s face it, is not unlike our own. New biological findings such as this uncover further pieces of the puzzle, extending our understanding of the group dynamics of these wonderful creatures.

Aurelien Delaval