Consider them nature’s version of eHarmony: grandmas may be responsible for the formation of pair bonds between men and women, according to research published online in Monday’s edition of the journal Proceedings of the National Academy of Sciences (PNAS).
The study was led by University of Utah anthropologist Kristen Hawkes, who previously came up with the “grandmother hypothesis” to credit grandmothering for our long human lifespan, and used computer simulations to show that it also results in greater number of older, fertile males.
This, in turn, led to the tendency among males to guard a female mate from potential competition and to form a “pair bond” with her instead of mating with a multitude of partners. Grandmothers, she and her colleagues concluded, played an essential role in the development of pair bonds.
“We have much longer lifespans than to our closest evolutionary cousins (great apes) although female fertility ends at about the same age in us and them,” Hawkes told redOrbit via email. “The grandmother hypothesis proposes that human postmenopausal longevity evolved when our ancestors began depending on foods that youngsters couldn’t manage for themselves.”
“With kids dependent after weaning, grandmothers’ help feeding them allowed mothers to have next babies sooner. If a grandmother lived longer, they could help more so their descendants lived longer, “she added. “Longer survival swelled the numbers of old males competing to mate with the still fertile females. That male bias in the fertile ages makes guarding a female a more successful way to father offspring than continually seeking another mate.”
Findings contradict theory based on males providing food
This proposed link between grandmothers and pair bonding contradicts the traditional view that pair bonding was the result of male hunters providing food to females and their children, and that in return they received paternity over offspring so that they could have descendants, she said.
According to Hawkes, the grandma hypothesis counters that the reason that females are able to have babies earlier is not because of the male providing for them, but because the grandmothers help feed weaned children, which helped increase longevity. Thus, her findings suggest that not only do prehistoric grandmas deserve credit for longevity, but committed relationships also.
“This connects human habits of mate guarding and pair bonding (different from our closest evolutionary cousins) with our grandmothering life history,” she told redOrbit. “Most mammals have female-biased sex ratios in the fertile ages because mortality is higher in males.”
“Mortality is higher in men than women too, but that difference is more than overwhelmed by fertility continuing in old men,” Hawkes added. “Mate guarding is found in other animals when mating sex ratios are male-biased. Explanations are the same: when the number of competitors increases, males do better to guard a current mate than continually seek new ones.”
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