fb-pixelReproductive choice is at the core of being a female mammal - The Boston Globe Skip to main content

Reproductive choice is at the core of being a female mammal

Evolutionary biology helps explain why women won’t stop fighting for control over their bodies.

Female bottlenose dolphins can't select their sexual partner, but they have evolved a way to choose which of their mates' sperm fertilizes their eggs.Patrick O’Brien/Kiawah Island Real Estate/Patrick O’Brien/Kiawah Island

I can’t predict the legal fate of abortion in the coming years, but there is one thing I am certain of from my studies of the animal world as an evolutionary biologist. Women won’t stop fighting for control over their bodies and will continue making reproductive choices no matter the obstacles. These are deeply ingrained and vital instincts for female mammals, akin to defending yourself when being physically attacked.

Reproductive choice is one of the most ancient and fundamental features of our biology, and it is built into all aspects of mammalian reproductive physiology and behavior.

Most relevant to the abortion debate, female mammals make choices about which embryos and fetuses to keep. In our mammalian ancestors, the lining of the pregnant uterus evolved to sense and reject unhealthy embryos as they try to implant. Embryo screening became more sophisticated in humans and other primates, which partially explains how an estimated 40 to 60 percent of human embryos are lost after fertilization. (It may also explain why we menstruate.)

But embryo and fetus choice is not only about offspring health. More broadly, it is about what is best over the long term for the mother, who invests significant time and energy in pregnancy, lactation, and caregiving. In pregnant coypu rodents, the mother’s reproductive system senses if a litter is too small and spontaneously aborts the litter in response. This physiological reaction likely evolved because coypu mothers that aborted small litters early and soon became pregnant with larger ones had more offspring in the long run than those who didn’t. In many polygamous rodents and primates, males are infanticidal, killing the unrelated offspring of females with which they want to mate. In the extended presence of an unfamiliar male during early pregnancy, females in some rodent species spontaneously terminate their pregnancies. They have evolved to cut their losses as early as possible to avoid investing time and resources in offspring that will probably not survive after birth.


Maternal choices about offspring continue after their birth. When a house mouse delivers a litter that is too large, rather than distributing milk and care to all the pups, she will only give to the strongest ones. Tamarin monkeys usually give birth to twins or triplets, and other family members help the mother raise them. But if a mother doesn’t have help, she will abandon one or both babies in the first few days of life. The mother instinctively knows that without help from family, the babies will not survive, so she saves her energy for future offspring that may have a better chance of survival. Behaviors like these evolved because mothers who considered their current situation and the survival probability of each offspring in their decisions had more offspring survive than those who didn’t.


If a tamarin mother doesn’t have help from family members, she will abandon one or both of her babies in the first few days after giving birth. David L. Ryan/Globe Staff

Females make other choices that are also vital to their reproductive success. One of the first choices made is about which egg to ovulate. Female mammals and birds produce all the eggs they will ever have during fetal development. Humans produce about 7 million eggs, ovulate fewer than 500 over reproductive life, and run out at menopause. Where do the rest go? Our reproductive systems actively dispose of many because they contain errors and don’t pass through egg quality control, a system that likely evolved hundreds of millions of years ago. The female body has evolved to choose the healthiest eggs for ovulation.


Female mammals and birds also choose preferred mates for the fertilization of their eggs, which determines the paternal genes their offspring will inherit and, in some species, the amount of support a mother will receive. These factors influence the survival and future success of a female’s offspring. When males thwart female choice, females may evolve new behaviors and even new anatomy to maintain or regain that freedom of choice. Most elephant seals mate on land in a harem controlled by a brutish “beachmaster,” but females in one colony on Marion Island, in the Indian Ocean, learned to mate in the open water — not a trivial task — with males of their choosing. Female bottlenose dolphins have found another way: The dolphin vagina evolved elaborate flaps and folds that give a female some freedom of choice. Not the choice of sexual partner — male dolphins are aggressive and unrelenting — but of whose sperm she’ll allow to fertilize her eggs. Females may position themselves during copulation, and contract or relax their vaginal muscles, to steer wanted sperm toward their eggs and unwanted sperm toward one of the dead-end folds.

These are just a few of the many examples in nature illustrating the choices that female animals are constantly making about reproduction and the lengths they go to make these choices. Females have evolved to factor their age, health status, availability of resources, amount of support, qualities of the male, needs of older offspring, and survival prospects of new offspring into their decisions. Otherwise, they leave too much of their reproductive fate up to chance, and up to the males in their species. Males often have different interests and strategies for reproductive success, which may involve sexual intimidation, coercion, and other methods to control female reproduction.


In the reversal of Roe v. Wade, the Supreme Court’s majority decided that abortion isn’t deeply rooted in the history and traditions of the country, so it shouldn’t be protected by the Constitution. But in fact, female reproductive choice is one of the most deeply rooted “traditions” in nature, one that can be traced back hundreds of millions of years.

Deena Emera is a senior scientist in the Center for Reproductive Longevity and Equality at the Buck Institute for Research on Aging. She is the author of the upcoming book “A Modern Guide to the Female Body: An Evolutionary Look at How and Why the Female Form Came to Be.” Follow her on Twitter @deenaemera.