No offence to all you grannies out there, but why do they exist? Evolutionarily it doesn’t seem to make much sense for individuals to live for many years after they stop reproducing. In the brutal world of natural selection, what’s the point in living if it’s not to pass on your genes? But luckily for us grandchildren, grannies do exist. So why is this the case?
To answer this, the world of comparative psychology can come in very useful. We can look at the species where females experience the menopause and continue to live for a number of years and compare them to see how their environmental pressures may be similar. The list of animals that we know experience menopause is a short one:
- Killer whales
- Pilot whales
The Grandmother Hypothesis in humans argues that older females ensure the survival of their genes through helping their grandchildren to survive. The more females there are to take care of a particular child, the more likely it is to survive. In addition, older females can contribute knowledge and skills to enhance group fitness. If the other members of the group are kin then this increases the fitness of the grandmother as her genes are more likely to be passed on down more generations.
Although very little data exists on the post-reproductive life of pilot whales, a recent study by Emma Foster and colleagues has proposed a different theory for the existence of this phenomenon in killer whales. Female killer whales live for up to 90 years (males live to around 30 or 40) but stop reproducing in their 30s or 40s. Foster et al found that rather than increasing the survival of their grandchildren, as in humans, these older females increased the survival of their own offspring, specifically their adult sons. They found that in the year following an older female’s death, the sons showed a 14 fold increase in the likelihood of dying. This effect was not seen in daughters. The researchers suggest that because the sons mate outside of the family group, they focus their attention on their survival, rather than the survival of their grandchildren.
The three species above have a number of things in common including long lives, stable social structures, strong family units and whales show matriarchal societies. Elephants also exhibit strong family units, matriarchal societies and are long lived. It is unclear as yet whether they actually experience the menopause but many females live for at least 10-15 years after their last calf is born. The evidence so far is more in line with the grandmother hypothesis in humans than the findings in killer whales. McComb and colleagues found that elephants derive significant benefits from the influence of older leaders in the family group; particularly in response to predatory threats but also very likely in the search for food and appropriate migration routes. Therefore longevity should also be selected for in elephants so that the whole family can benefit from this wealth of knowledge because older females can make better decisions in times of ecological uncertainty.
|A female looks out for the safety of her calf.|