Giving talks about ageing is always pleasant. Audiences are usually as old as I am and are often attentive because they hope for some unexpected good news or need to air strong opinions on how ageing progresses and affects our minds. Their questions follow two main themes. One is that, in contradiction of any statistical probability they have a relative given to flagrant, excessive and persistent smoking, alcohol consumption and unhealthy living who triumphantly reached a great age with no mental impairment whatever. Another is why women, on average, live longer than men.
It is not wise to pick up the first theme by talking about variation about means, extreme statistical outliers or even to plagiarise David Spiegelhalter’s witty descriptions of how to use Bayesian statistics to interpret risk factors 2 My co-evals want reassurance that their futures are not necessarily nailed by bleak numbers and that we can hope for luck that we have done nothing to deserve – some outrageously unfair free pass to prolong life’s journey. I only offer reciprocal anecdotes of a formidable survivor, my German aunt-in– law, Maus, who claimed that a lifetime as an industrial chemist exposing herself to noxious substances had inoculated her against Cognac, whose apartment was crusted brown by cigarette smoke, whose main survival risk was driving a BMW very badly and too fast and who lived merrily past 90. The issue why women are more durable than men is much trickier because there is too much data to puzzle through. I offer a little of it here and I should be very grateful indeed to any readers who have more information or ideas than I have found and care to share it with me.
For both sexes average lifespan projected at birth depends mainly on access to good food, water, medical care, education and a hefty helping of affluence in societies in which violence and accidents are rare. In 1998 1 at the top of this league were the millionaires’ sanctuary of San Marino (84; Women 86; Men 82 ) with Japan (83; Women 86, Men 79) and Singapore (82; Women 84 Men 80) close behind. In the poorest countries these long life-spans were almost halved: Botswana (47; W 46; M 47); Central African Republic ( 49; W 51; M 47) and Chad (50 W. 51 M 49). In the United States the overall average of 79 (W 81; M 76) disguised huge regional and racial differences as it did in the UK (82; W 84; M 80) where even within a square mile of a single city, such as London or Birmingham, life expectancy can vary between 60 and 85.
Differences in affluence and safety strongly determine survival but, because they seem to affect men and women by similar amounts, we must suspect that sex differences in survival are not mainly due to life-circumstances but to basic differences in biological robustness. Support for this idea is that the sex difference is present throughout the entire life-span; male fetuses and infants are more at risk and this mortality gap widens at the other end of life so that we have very many more female than male nonogenarians and centenarians. This continuing female advantage throughout the lifespan weakens the idea that a main reason why, in middle-age, older women outnumber men is that men have been culled while young by accidents and violence during dangerous “testosterone storms” in adolescence and early adult life (see pictures).
Another straw of evidence against the t-storm hypothesis is that the gender gap is not just found in humans and other primates but in most other mammals, many types of birds and even in insects. This suggests that it must be of evolutionary benefit, and prompts us to ask why. Obvious arithmetic is that males can impregnate many females but, at least among most mammals and many other species they then do little or nothing to rear their young (devoted male Sea Horses are tender exceptions). After their brief acts males become disposable because they are redundant to the future of their species.
For me an uncomfortable extension of this idea is that old men are obvious liabilities and nuisances who lounge about eating and drinking more than they can provide while pretending to tolerate each-others’ lies about their former value to their families and societies. Meanwhile older women still bustle about, bringing up kids, doing more than their share of gathering, gardening, laundry and supermarket shopping and power-nag to promote communal hygiene and a generally more civilised lifestyle. In this scenario menopause is an adaptive liberation from childbearing that allows many useful engagements, most especially rearing children of stressed, multiple-mothers who might otherwise die from neglect.
Like most other ideas in evolutionary biology this is fun to consider, intuitively attractive and almost impossible to test directly. On the other hand, though their science is difficult, Biological Anthropologists do have more fun than laboratory gerontologists. Such speculations as they can make need field data that can often be collected in places much nicer than rainy Manchester or windy Newcastle. For laboratory gerontologists life’s consolations are not travel, adventure and interesting strangers but only regular coffee and the Jaffa cakes that our volunteers do not finish.
Rebecca Sear and Ruth Mace 3 asked where most of the calories that fuel simple tribes come from – from the strenuous (and often dangerous) hunting of their men or the gardening and gathering activities of women ? They found that, in some South American tribes, and for the Hadze in Tanzania, agriculture and gathering, which were mainly carried on by women who contributed most of the calories a village consumed. This seems excellent evidence for the continued value of middle-aged and elderly women in these particular societies but it does not settle the question because women remain more durable even in societies such as the Inuit, where male hunting is almost the sole source of food. Sear and Mace further asked how far longer survival of individual relatives benefited their extended families. In line with the male-redundancy hypothesis they found that death of a mother always greatly increases risk of death for her children but, in contrast, death of a father usually has little, and often no effect. In most of the tribes that they studied children with older relatives were better nourished but the idea that grand-parenting is especially beneficial did not stand up across the board. Differences between children who had and did not have living grandparents were small, only slightly greater than those between children with and without fathers. Maternal grandparents seemed to help more than paternal grandparents. Presence or absence of uncles and aunts seemed not to be of much consequence, though having an outstandingly prosperous uncle was often an advantage. Survival of older brothers and sisters seemed ambiguous. There was some evidence for a survival advantage but there was also evidence for rivalry and so greater risk. In general, although there were advantages from family support these seemed to differ widely with the patterns of relationships found in different societies and were not as great as anthropologists had first thought.
It is interesting to think how these trends may translate to modern, urban societies but we have left the simple life so far behind that this is only an intellectual game. There seems general agreement that part of the premature culling of men occurs because we begin to suffer from increased incidence of cardio-vascular problems, and from some cancers in late middle-age whereas women experience these a decade later. There is also agreement that the gender-gap in survival is shrinking, and speculation that this is because better medical management of chronic conditions and advances in preventative medicine are beginning to make up for innate differences in vulnerability. The unpleasant flip side of this is that as women become better represented in demanding professions they experience more stresses and the biological damages associated with them. Cardiovascular problems in women are becoming more common and are appearing earlier. Perhaps advances in geriatric medicine and the direct relationship of age to affluence in rich societies may counter the evolutionary push. It may also be that as direct and rapid genetic manipulation prolong repair and maintenance in late life evolutionary pressures will be bypassed and we will eventually achieve at least an equal, if haphazard mortality.
1.Perls, T. and Fretts, R. (1998). Scientific American http://www.sciam.com/1998/0698womens/0698perls.html.
2.Blastland, M. & Spiegelhalter, D. (2013) The Norm Chronicles. Profile Books, London.
3. Sear, R. and Mace, R, ( 2008). Who keeps children alive ? A review of the effects of kin on child survival. Evolution and Human Behavior , 10, 1-18.