Why do animals, notably women, outlive their reproductive years? Nature would seem to have little or no use for us once we reach middle age, let alone our dotage; after all, the only thing evolution cares about—by which I mean, acts on—is how many offspring we leave. Why, then, should we live beyond the time when we can reproduce?
That mystery has given rise to the grandmother hypothesis: the idea that primates, elephants and a few other long-lived species survive long enough for their offspring to have offspring because older generations retain a store of knowledge that helps their descendants survive and multiply.
In the case of the elephants, scientists describe in a new study in the journal Biology Letters how they followed three family groups during the 1993 drought in Tanzania’s Tarangire National Park, the most severe one in that region in the past 35 years. Sixteen of 81 elephant calves in the three groups died; normally, only two or three would have. But the mortality was not spread evenly among the three groups. The two groups that left the park suffered lower mortality rates than the one that stayed; the wanderers apparently found enough food and water outside the park to provide for themselves and their young.
Which raises the question: why did the two groups leave the park? They both had elderly matriarchs—45 and 38 years old, respectively. The group that stayed put had a 33-year-old matriarch. These older females may have been able to draw upon memories of an earlier drought and how they survived it: the 45-year-old, for instance, born in 1948, would have been 10 when the great drought of 1958-61 began. The 38-year-old (born in 1955) would have been only three then, but was 6 when it ended, old enough to remember. The 35-year old, born the first year of the previous drought and only 3 when it ended, would have been too young to remember it.
That suggests that experienced matriarchs give their families an edge in periods of drought through their memories of distant, life-sustaining sources of food and water, says Charles Foley of the Wildlife Conservation Society, who led the study: “Older females with knowledge of distant resources become crucial to the survival of herds during periods of extreme climatic events. It’s enticing to think that these old females and their memories of previous periods of trauma and survival would have meant all the difference.”