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Grandmothering
By Sarah Ford
What is the Grandmothering Hypothesis?
• The theory to explain the existence of
menopause in humans’ evolutionary
history by identifying the value of
extended kin networking.
• Builds from the ”mother hypothesis” that
the costs of reproducing become greater
as the mother ages and energy should be
further devoted to ensuring the survival
of her existing offspring rather in their
reproductive efforts.
Kin Selection
• Serves as an adaptive strategy for
related individuals to exhibit altruistic
behaviors in order to ensure the survival
of their own genetic material being
passed on.
• Hamilton’s rule: RB > C
• Evidence emerged around 1.7 to 1.8
million years ago demonstrated in food
sharing and female foraging practices
due to climate changes.
Parental Investment
• Trivers defines parental investment is any
investment that a parent contributes to an
offspring that will increase the offspring’s
survival.
• Females are the more investing sex due to
100% maternal certainty and tending to be
the sex that does the majority of the
childrearing.
• Paternal uncertainty is much higher;
therefore, they are the less investing sex.
The Grandmother Effect
• Due to parental investment, females invest more into their children
due to the inter birth interval (IBI). Which limits the number of
children a woman can have.
• IBI - The period of time between births is longer for humans than some
species- perhaps due to the extended childhood - which is unique to
humans..
• IBI - longer in larger species and shorter for smaller primates.
• Grandmothers are able to help facilitate shorter IBIs for mothers by
helping the mother with childrearing. Grandmothers are alloparents that
can provide provision and protection for their grandchildren which helps
the mother reproduce more children. This benefits the grandmother
because her genetic material is more likely to be passed on.
• It has also been found that grandmothering can decrease senescence
(Hawkes & et al., 2004).
Origins of Grandmothering
and Menopause
• Menopause is the cessation of ovulation
which occurs in a female human’s life around
45 to 50 years of age. It is estimated that
60% of a woman’s life is spent in her postreproductive life span.
• Extended PRLS is unique to humans in this
sense, because typically female nonhuman
primates do not live for an extended amount
of time after they stop reproducing.
• Evidence for menopause has been exhibited
in rhesus macaques and Japanese macaques
as well as other species such as guppies and
whales.
Origins of Grandmothering
• Evidence for grandmothering dates back to 1.9 million yrs ago
with Homo Erectus. As we have learned H. Erectus were the first
of our ancestors to begin hunting for “big game” when they
moved into colder drier climates.
• We began eating meat helped us evolve larger brains by promoting
more cooperation and dependence on our social groups.
• Larger brains have lead to slower maturation and increases in
extended childhood.
• Grandmothers were perhaps able to facilitate the evolution of
human life span by providing for the children.
Caspari and Lee Tooth Study
• Rachel Caspari and Sang-Hee Lee
examined the teeth of
Australopithecines, early Homos,
Neanderthals, and Early Upper
Paleolithic homos.
• They determined how old the teeth
were based on wear seriation and the
eruption of molars.
• Found that there was an increase in
longevity amongst all the groups, which
indicated survivorship amongst older
adults in human evolution.
• The greatest increase was with modern
homos in Upper Paleolithic era, where
there was a larger amount of older
adults than young adults in the death
distribution.
Table 1. Sample summary: Sample sizes and OY
ratio for each group
Old
37
Young
316
Total
353
OY
0.12
Early Homos
Neanderthals
Early Upper
Paleolithic
42
37
50
166
96
24
208
113
74
0.25
0.39
2.08
All groups
(combined)
166
602
768
0.28
Australopithecines
The Hazda
• In the 1980’s, Kristen Hawkes and James O’Connell studied
the Hazda, a traditional hunting-gathering tribe in Tanzania,
Africa. They are known for their foraging with tubers.
• Hawkes discovered that mothers who were caring for their
infants spent less time foraging and contributed less to the
nutrition of their weaned children. They also noticed older
women were spending their days collecting tubers and other
food for their grandchildren.
• Hawkes suggested that grandmothers increase their inclusive
fitness by doing this decrease weaning ages of their
daughter’s children thus shortening IBI.
• Grandmothers increase the survival of their young weaned
grandchildren by supplying them with food when their
mother is busy taking care of a new infant.
Longevity and
Grandmothering
• Kim, Coxworth, and
Hawkes examined
findings from three
hunter-gatherer
groups: Dobe! Kung,
Ache, and Hazda.
• Female reproductive
output as a function of
expected life span.
Conclusion
• Grandmothering has made us uniquely human,
because it allowed for ”a whole array of social
capacities that are then the foundation for the
evolution of other distinctly human traits,
including pair bonding, bigger brains, learning
new skills and our tendency for cooperation”
• Grandmothering has developed from “big game
hunting” which has allowed for our increase in
brain size. Increase in brain size has led to
slower developing infants and the need for
alloparenting, social dependence, and pair
bonding amongst groups.
• Grandmothers allow for longevity by
decreasing senescence and allowing for natural
selection to allow for older adult genetic
material to be passed on.
• Increase in longevity has allowed for the
assimilation of culture in human evolution over
time.
• https://www.youtube.com/watch?v=xzV9DIL_v
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