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Why is the Postfertile Longevity Exhibited by Humans so Unusual?

Humans are an unusually long-lived species when compared to other mammals of a similar size, and even in comparison to our near relative primates. Further, we exhibit an extended period of life following loss of fertility, a rare form of life history that is only observed in a few other species. The grandmother hypothesis is one of the possible explanations for the evolution of extended longevity without fertility; it is a selection effect based on the ability of older individuals to assist in the survival of their descendants. Given the existence of such a mechanism, however, why is it not more widespread?

Data on historical agricultural populations and modern hunter-gatherers show that these groups enjoy significant postfertile periods. Taking an evolutionary approach, the Grandmother Hypothesis proposes that this reproductive inactivity is in fact adaptive. With the sacrifice of continued reproduction, an individual may increase their inclusive fitness by decreasing the interbirth intervals of their offspring. The care that would otherwise be put into one's own children can now be put into weaned (and increasingly independent) grandchildren, allowing their own offspring to reproduce again sooner. Otherwise put, the cost of a reduced relatedness coefficient may be outweighed by an increase in total number of grandchildren resulting from the diverted care.

A valid objection to the Grandmother Hypothesis, however, is if grandmothering can result in a higher fitness, why are significant postfertile life stages so rare? Among vertebrates in the wild, only humans, Globicephala macrorhynchus (pilot whales) and Orcinus orca (resident killer whales), have a significant proportion of individuals with such a life history. In this study, we present a model to investigate this objection. Our model assumes only that individuals transition through various life stages and that there is an average time to conception and gestation. In one of those stages, individuals have the option to provide care for a certain number of their grandchildren thereby allowing their own offspring to reproduce again sooner.

By comparing inclusive fitnesses of individuals that provide intergenerational care with those that instead continue to reproduce into old age, we arrive at a necessary condition for grandmothering to be an evolutionarily stable strategy (ESS). This condition, or stability threshold, relates the number of grandchildren that care must be given to with the ratio of the length of the first two life stages. It tells us nothing about when or how grandmothering may arise initially in a population, but places restrictions on when it will persist. We then make the observation that if a grandmother is to provide care for even one set of grandchildren, their expected postfertile stage must be sufficiently long. More precisely, for grandmothering to be adaptive, it must be the case that postfertile life exceeds the time taken to raise a weaned child to independence. If this were not the case, grandmothers would not be able to shorten their offspring's time between births by caring for some infants themselves. In this way, we derive an eligibility threshold that tells us when grandmothering is a strategy with any possible evolutionary advantage. These eligibility and stability criteria must both be satisfied for grandmothering to evolve and then, most importantly for our purposes, to persist.

Our analyses show that there is conflict between the stability and eligibility thresholds. As it becomes increasingly easier to meet one of them, it becomes increasingly harder to fulfill the other and vice versa. This conflict is, at its core, a grandparent-grandoffspring conflict analogous to parent-offspring conflicts. The result of this is that there is a narrow range over which we should expect grandmothering to evolve and then to persist. In other words, we should in fact expect grandmothering to be rare.

Link: https://dx.doi.org/10.1002/ece3.2958

Comments

I'm not too convinced by any of this. Why ask this particular question when a better alternative question is why do (human) females live longer than (human) males, but have a shorter reproductive span? You could then look at how the female reproductive system is more fragile than the male's and see how that varies across species. Any adaptation that extends lifespan would logically also extend reproductive span, so really all we are doing here is looking at the reasons why that link might break down in certain species. I find it a stretch that any species would have an adaptation to shorten an individuals reproductive span relative to its total lifespan.

Posted by: Mark at May 23rd, 2017 7:14 AM

Humans are quite unique in that childbirth is very dangerous, as human head sizes increased due to an intellectual arms race until they couldn't really get any bigger without putting the mothers at even more risk.

So it is entirely possible that the alegbra of not maintaining fertility in women and instead being a helpful non fertile grandmother is different for humans than almost all other species. Also consider that from at least a couple of million years ago humans have been the apex predator.

Posted by: Jim at May 24th, 2017 9:38 AM

The apex predator with the big head...but so damned stupid as to be in the process of destroying it's own home and other species that it co-habitates with?

Can't seem to control it's own reproductive rate.

Problem being that there is only 1 apex predator that's doing all the talking. The apparently really dumb one.

* this is not a personal attack...I'm "attacking" the whole brainless species with the really big head. LOL.

Posted by: harold at May 31st, 2017 6:30 AM

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