GRANDMOTHER"S
FOOTSTEPS
by Mirkka Lahdenpera
on 11 March 2004 in the Economist
( originally ublished in Nature.)
Nature is unsentimental. In Herbert Spencer's famous phrase, it is
a question of the survival of the fittest. You are born. If you are
lucky, you reproduce. Then you die. Indeed, once you can no longer
reproduce you are, evolutionarily speaking, dead anyway. That, at
least, is the conventional wisdom. So it has been a bit of a mystery
why women, who normally become infertile in their mid-40s with the
arrival of the menopause, often live on and on. Indeed, the average
lifespan of women in almost all societies exceeds that of men—and
most men remain fertile well into old age.
The presumed explanation depends on a more sophisticated understanding
of what “fittest” means. It is not enough to survive and
have children. Your line has to continue through your grandchildren.
In this context, the survival of women beyond menopause makes sense
if it translates into extra grandchildren. Proving that hypothesis,
though, is difficult. But Mirkka Lahdenpera, of the University of
Turku, in Finland, and her colleagues, think they have managed to
do so. Their work, just published in Nature , draws on records made
in the 18th and 19th centuries in Finland and Canada. The results
are striking. In both countries, a woman gained two extra grandchildren
for every decade she survived beyond the age of 50.
The post-reproductive
survival of humans—women in particular—is truly unusual.
Non-reproductive “helpers” of individuals who are breeding
are found in many species. But they are usually young animals that
have yet to establish themselves, rather than relics from previous
generations. The post-reproductive elderly just die. Chimpanzees,
for example, have a similar pattern of fertility to people. A female
chimp's fertility peaks in her late 20s, and is more or less extinguished
by her mid-40s. But in chimpanzees, mortality rises as fertility declines.
Nor, despite the increase in average lifespan in the rich world over
the past few centuries, is post-reproductive survival a modern phenomenon.
Until very recently, most of the increase has been due to better survival
in childhood and youth, rather than the increased prolongation of
middle age into old age. Even in pre-industrial populations, around
a third of women were over 45.
All this suggests that the prolonged old age of people—women
in particular—is indeed an evolved phenomenon, rather than being
a consequence of better living conditions. And the quality of the
records Dr Lahdenpera was working with allowed her to dig into the
details of how this came about.
First, it was nothing to do with a long-lived woman having had more
offspring in the first place, and the number of her grandchildren
thus merely being a function of the number of her children. Nor was
there any detectable effect due to the socio-economic status of the
women involved. Rich or poor, high- or low-born, all showed the same
trend. And, contrary to the beliefs of some workers in the field,
both sons and daughters benefited from the survival of a matriarch,
not merely daughters alone.
Nor was the increase due to a single effect. Instead, Dr Lahdenpera
was able to disentangle a range of beneficial phenomena which, when
added together, resulted in the increased reproductive success that
she observed. People whose mothers were still alive both gave birth
to more offspring and raised a higher proportion of those offspring
to adulthood. They also gave birth to their first children at a younger
age than those whose mothers had died, and the births of their children
were more closely spaced. All of these things contribute to reproductive
output.
Moreover, the physical presence of the matriarch was vital. Children
who lived more than 20km from their mothers produced significantly
fewer offspring than those who lived in the same village. That suggests
the increase in the number of grandchildren was due not to some subtle
genetic effect, but rather to help—whether physical or in the
form of advice—that matriarchs were contributing.
Perhaps the most evolutionarily significant finding, though, is the
age at which the matriarchs in the study died. The average lifespan
of post-menopausal Finns was 68. Of Canadians, it was 74. These ages
correspond to the points where the matriarch's children themselves
had stopped reproducing. At that point, a woman's fitness plummets.
And so, sadly, does her life expectancy.
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