(Mwenza Blell’s post from The WILEY FREE ONLINE LIBRARY.)
The grandmother hypothesis is a hypothesis to
explain the existence of menopause in human life history by identifying the
adaptive value of extended kin networking. It builds on the previously
postulated "mother hypothesis" which states that as mothers age, the
costs of reproducing become greater, and energy devoted to those activities
would be better spent helping her offspring in their reproductive efforts.
It suggests that by redirecting their energy onto
those of their offspring, grandmothers can better ensure the survival of their
genes through younger generations. By providing sustenance and support to their
kin, grandmothers not only ensure that their genetic interests are met, but
they also enhance their social networks which could translate into better
immediate resource acquisition. This could extend past kin into larger
community networks and benefit wider group fitness.
The grandmother hypothesis is an adaptationist explanation for the fact that the human female life span extends beyond the period of fertility. Human female reproductive life spans are variable in length but reproductive senescence occurs much faster than somatic aging.
Menopause,
permanent cessation of menses, effectively represents the end of the
reproductive life span and is a universal for human females who live into old
age. A third of the average human female life span is postmenopause.
Since
fertility declines typically begin in the decades before menopause, the
grandmother hypothesis refers more generally to the end of reproductive
capacity rather than the end of menstrual bleeding per se. Such long female
postreproductive life spans are rare in the animal kingdom and, apart from
humans, have only been confirmed to exist outside of captivity in a few
species, mainly cetaceans. (Only Toothed whales and Elephants.)
Human females' postmenopausal longevity has been considered to represent an evolutionary conundrum because it has been reasoned that a shorter reproductive life span would impair fitness and therefore would not be expected to have evolved. Since Medawar first suggested that grandparents could influence their fitness by their actions toward their grandchildren, many have followed this line of thinking, including Hawkes, O'Connell, and Blurton Jones who used fieldwork data to explore this possibility.
They
collected seasonal foraging data among the Hadza of northern Tanzania, an
egalitarian hunter-gatherer group living in a semiarid environment, and found
that postreproductive females in the group had high productivity in foraging,
particularly of tubers, a challenging but valuable food to obtain in the
context.
They
reasoned that a long postreproductive life span in women might have evolved as
a consequence of the sharing of foraged food in humans' hominin ancestors.
Relying on Hamilton's inclusive fitness theory, they suggested that older
females could gain greater fitness by foraging intensively and sharing food
with their adult daughters. This provisioning was expected to offer sufficient
resources for adult daughters to reap fitness benefits, whether by increasing
the number of offspring or by ensuring the survival to adulthood of existing
offspring.
For the
grandmother, the consequence of contributing these resources was expected to be
an inclusive fitness benefit since she would share genes with her
grandchildren. In this same publication, Hawkes, O'Connell, and Blurton Jones
suggest, based on published data from the Kalahari, that the same effect could
be had from grandmothers babysitting grandchildren while their adult daughters
foraged.
