Open Arms and Stocked Fridge: The Evolutionary Strength of Grandmothers

I don’t know where I would be without my abuela. As my mother’s mother, she is so much more than just a convenient and free babysitter. Sure, abuela took care of me when mom had to work, but she also sang with me on our walks home after school. She’s the one I call when I need help with a recipe, though my palomilla steak will never be as good as hers. Sometimes, she serves as referee between my mother and I, reminding my mom “you too were young once!”. Her arms are always open and her fridge is always stocked.

While I know my abuela is better than the rest (obviously), I also realize that this special bond between grandchild and grandmother is not unique. Whether we call her granny, nana, or meemaw, our grandmother holds a special place in our hearts, in our lives…and in our history as a species.

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¡Ay Abuela! – The author with her abuela. Now that grandmothers live long enough to help raise their grandchildren through reproductive maturity, they can help raise great-grandbabies…though the author isn’t expecting any time soon.

As a species, we  sometimes forget that we are the products of animal evolution and that evolution favors organisms that can make as many babies as possible (cue the Marvin Gaye). Evolutionary biologists refer to baby-making capabilities as an individual’s fitness. Being the most fit means you have the most offspring and more copies of your genes in the next generation. Because fitness is central to the theory of evolution, Grandmothers are sort of an evolutionary conundrum.

Most animals reproduce until they die, maxing out their fitness. This general rule of lifelong fertility holds true for human males. Human females, however, live long into their post-reproductive years (menopause). Living past menopause is unique to only 3 species in the world:  humans and two species of whales.This long postmenopausal lifespan distinguishes humans from our primate relatives.

Why do human females invest in longevity past reproduction? If evolution favors fitness (i.e. to have as many babies as possible), how did living past your baby-making years evolve? In simple terms, the question we are asking here is, “If grannies aren’t increasing their fitness by making babies, why do they even exist?”

You may not realize it, but you probably already know the answer: it’s because they are awesome! Open arms and stocked fridge, right?

While this might seem a bit trivial, years of research and theory support what some evolutionary biologists call the Grandmother Hypothesis. Grandmothers, that is, mothers who live long into their post-reproductive years, provided a key resource granting ancient humans an evolutionary advantage: the ultimate babysitter. Mind blowing, I know.

In case you haven’t already noticed, human babies are pretty helpless. They need round the clock care for way longer than most other mammals, including our primate relatives. Parental care is costly, both in time and resources. Grandmothers help their daughters take care of their offspring.

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Though chimpanzees have complex mother-offspring relationships, they do not breed cooperatively across generations like humans do. Image Credit: Buffa via flikr

This social behavior is also unique among primates– humans breed cooperatively across generations where other primates do not. In chimpanzee families for example, a mother and daughter might be taking care of their own infant offspring at the exact same time; the grandmother will not help her daughter raise young. By helping take care of their grandchildren, grandmothers also allow for their daughters to have more children. That is, mothers don’t have to wait for their offspring to become independent before they can reproduce again.

Remember those nights you spent at Nana’s so mom and dad could go out on a date? Yeah, chances are they actually stayed home. The whole night.

By helping their daughters have and raise more offspring, grandmothers are increasing their daughters fitness. This is a really important point: mothers who live long enough past their reproductive years to become grandmothers have more evolutionary fit daughters. In having more fit daughters, grandmothers are passing on their long-life genes (the same genes that helped them live past their fertile age) to a greater portion of the population. And generation after generation, this unique social interaction contributed to the evolution of human longevity in general.

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Three Generations – the author (left) with her mother (center) and her amazing abuela (right). Families of women like this one have the potential to influence human fitness and longevity through multiple generations of mothering and grandmothering.

Now that human lifespans are climbing to the 100-year mark and crossing multiple generations, grandmothers are able to help raise their grandchildren through reproductive maturity and can even contribute to their granddaughter’s fitness by helping raise great-grandchildren.

Thanks abuela!

mz_aboutauthorMichelle Ziadie is a Ph.D. student in the Department of Genetics at UGA working on the evolution of maternal effects and undergraduate evolution education. A native of Miami, Florida with caribbean roots, Michelle never turns down an opportunity to engage with spicy food or spicy music. When she’s not dancing the night away with friends or getting lost in the Georgia wilderness, Michelle can be found sipping a café Cubano and watching Teenage Mutant Ninja Turtles with her kitten, Thomas. More from Michelle Ziadie.