Reproductive choice is central to being a female mammal

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I can’t predict the legal fate of abortion in the years to come, but there is one thing I know for sure from my studies of the animal world as an evolutionary biologist. Women will not stop fighting for control of their bodies and will continue to make reproductive choices regardless of the obstacles. These are deep-rooted and vital instincts for female mammals, akin to defending themselves when physically attacked.

Reproductive choice is one of the oldest and most fundamental features of our biology, and it is embedded in all aspects of mammalian reproductive physiology and behavior.

More 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 detect and reject unhealthy embryos as they attempt to implant. Embryo screening has become more sophisticated in humans and other primates, which partly explains how around 40-60% of human embryos are lost after fertilization. (It may also explain why we menstruate.)

But the choice of embryo and fetus is not only about the health of the offspring. More broadly, it is about what is best in the long term for the mother, who invests a lot of time and energy in pregnancy, breastfeeding and care. In pregnant coypu rodents, the mother’s reproductive system detects if a litter is too small and spontaneously aborts the litter in response. This physiological response likely evolved because nutria mothers who aborted small litters early and quickly became pregnant with larger ones had more long-term offspring than those who did not. In many polygamous rodents and primates, males are infanticidal, killing unrelated offspring of females they want to mate with. In the prolonged presence of an unknown male during early pregnancy, females of some rodent species spontaneously terminate their pregnancy. They evolved to cut their losses as early as possible to avoid investing time and resources in offspring that are unlikely to survive after birth.

Maternal choices about offspring continue after they are born. When a house mouse gives birth to too large a litter, rather than distributing milk and care to all the puppies, it will only give it to the stronger ones. Tamarind monkeys usually give birth to twins or triplets, and other family members help the mother raise them. But if a mother does not have help, she will give up one or both babies in the first days of life. The mother instinctively knows that without the help of the family the babies will not survive, so she saves her energy for future offspring who may have a better chance of survival. Such behaviors evolved because mothers who factored their current circumstances and each offspring’s likelihood of survival into their decisions had more surviving offspring than those who did not.

If a mother tamarin does not have the help of family members, she will abandon one or both of her babies within 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 concerns the egg to ovulate. Female mammals and birds produce all the eggs they will have during fetal development. Humans produce about 7 million eggs, ovulate fewer than 500 during reproductive life, and become exhausted at menopause. Where are the others going? Our reproductive systems actively eliminate many of them because they contain errors and do not pass through egg quality control, a system that probably evolved hundreds of millions of years ago. The female body evolved to choose the healthiest eggs for ovulation.

Female mammals and birds also choose preferred partners for fertilizing their eggs, which determines which 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 men thwart women’s choice, women may develop 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 brutal “beach master”, but females at a colony on Marion Island in the Indian Ocean learned to mate by full water – which is not a trivial task – with the males of their choice. Female bottlenose dolphins have found another way: dolphin vaginas have developed elaborate flaps and folds that give the female some freedom of choice. No choice of sexual partner – male dolphins are aggressive and relentless – but whose sperm will be allowed to fertilize his eggs. Females can position themselves during copulation and contract or relax their vaginal muscles, to direct wanted sperm to their eggs and unwanted sperm to one of the dead-end folds.

These are just a few of the many examples in nature illustrating the reproductive choices females constantly make and the effort they put into making those choices. Females evolved to take into account their age, health status, availability of resources, amount of support, qualities of the male, needs of older offspring, and survival prospects of new offspring in their decisions. Otherwise, they leave too much of their reproductive fate to chance and to the males of 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 majority of the Supreme Court decided that abortion is not deeply rooted in the country’s history and traditions, so it should not be constitutionally protected. But in fact, female reproductive choice is one of the most deeply rooted “traditions” in nature, a tradition that dates back hundreds of millions of years.

Deena Emera is a Principal Investigator at the Center for Reproductive Longevity and Equality at the Buck Institute for Research on Aging. She is the author of the forthcoming 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.

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