The animal kingdom is full of strange reproductive strategies, but when it comes to menopause, humans are among the weirdest. In just three species on the planet — humans, killer whales and pilot whales — do females routinely stop breeding years before the end of their lives. Human women spend about a third of their life span after menopause.
Seems counterproductive to stop having children so early in life when evolution is supposed to favor those with the most offspring, right?
Many species become less fertile as they age. For instance, female chimpanzees experience declining fertility rates from their early 30s onward until their reproductive chances reach zero around age 45. What makes humans and some whales different is that they carry on living and surviving with good odds for so long after menopause, whereas chimpanzees and other animals rarely survive much beyond the point where their eggs run out, even in captivity, said Virpi Lummaa, an evolutionary biologist at the University of Sheffield in England.
Female killer whales reach menopause before 50, but can live for up to 90 years and female pilot whales, which stop breeding by age 36, can live to be 65.
The jury is still out on why few species experience menopause while most do not. Some researchers believe it has to do with the family structures in which both humans and certain whales live. [Can Animals Commit Suicide?]
Humans, killer whales and pilot whales become more genetically related to those they live with as they age. In humans, females have traditionally left their family group to join a husband’s family. At first, they are genetically unrelated to the group, but as they age and their children start to reproduce, they become more closely linked, genetically, to those around them.
Such a situation may have made it favorable to cease reproduction in order to help younger generations of kin reproduce — a model of late-life helping that researchers call the grandmother hypothesis.
Yet other experts believe that mothers experience menopause as their daughters reach reproductive age to cope with scarce resources, such as food and for modern humans, money. “Several generations living under the same household cannot all reproduce successfully when resources are limited,” said Lummaa.
Dubbed the reproductive conflict hypothesis, some researchers refer to this as the “Father of the Bride 2” hypothesis, because it bears resemblance to the predicament in which actor Steve Martin finds himself in the 1995 comedy where both his wife and daughter are pregnant at the same time.
The exact origins of menopause in humans and whales may always remain somewhat shrouded in mystery, according to Lummaa. There’s scant archaeological evidence for how it evolved. It leaves no traces in fossils, and few records of menopausal age were kept prior to pre-industrial times.
Animals almost always continue to reproduce until they die. There are just three exceptions that we know of: humans, short-finned pilot whales, and killer whales. In all three species, females lose the ability to have children, but continue living for decades after. That’s menopause. Female killer whales go through in their 30s or 40s. Why? Why sacrifice so many future chances to pass on your genes to the next generation?
One of the most compelling explanations is called the grandmother hypothesis. Proposed in 1966, it suggests that older females forgo the option to bear more children so they can support their existing ones. By helping their children and grandchildren to survive and thrive, they still ensure that their genes cascade down the generations.