Monkey Twins and Big-Brained Babies
Callitrichid twins - hips don't lie - crucial childcare
Two for one
Many mammals have litters — multiple births at the same time. Large herbivores and primates are exceptions, usually giving birth to ‘singletons’.
But there’s an exception to the exception. A family of small monkeys called Callitrichidae (marmosets and tamarins) do something special: over 80% of their births are fraternal twins. Pretty close twins too, as they swap genetic material and brain cells in the womb.
We’re not sure why this happens in callitrichids. For primates, they tend to be small with a high infant mortality rate. Two babies for the price of one might help here. But it could also be related to their pair-bonding and alloparenting — help in childcare from other members of the (family) group. My female readers will swoon when I say that callitrichids are, on average, the only group1 of primates (yes, including humans) in which mom and dad contribute equally to after-birth childcare. Let me make it even more swoon-worthy. Despite being quite flexible in terms of mating system, many wild marmoset and tamarin groups include one breeding female with multiple male partners that help her raise the kids. Callitrichids have alpha females.
In contrast, about 2 - 4% of human births involve twins. For the Yoruba people in Western Africa, 4 - 5% of births are twins, and this number rises to around 10% in specific Indian and Brazilian villages. Having fraternal twins has a genetic component that increases the chances of hyper-ovulation (releasing more than one egg at a time). As far as we currently know, there is no genetic component to having identical twins.
In vitro fertilization messes with those numbers, though. In the early days of IVF, almost a quarter(!) of IVF births involved twins. Today, better procedures and technologies2 have brought that down to ~5 - 6%.
One human baby is challenging enough, thank you.
(Childbearing) hips don’t lie
For our body size, humans have an unexpectedly long gestation period of nine months. Even then, our babies plop into the world relatively underdeveloped, even compared to other primates. If human babies were born at an equal stage of development as a baby chimp, pregnancies would last about twice as long. I imagine any mother reading this snorting, “No thanks.” Pregnancy takes a substantial amount of energy3 and it’s a serious strain on the body. Moms, I salute you. Now imagine carrying twins (if you have, I’d love to hear your experience).
Despite the strain of pregnancy on the mother, human newborns are entirely helpless (altricial is the technical term). A baby giraffe tumbles out, shakes its head, and starts bumbling around. Compare that to a human baby, who will basically sleep, cry, eat, fill diapers, and blow snot bubbles for months on end. In predator terms, a creature like that is known as a snack.
So why don’t human babies come out more developed? Plop. “Welcome to the world, little Bobby, I’m mom. Here’s your lunch, off to school now.” But no, it’s a prolonged stay in the damp miasma of diaperville first.
The brain is the problem.
Look at a newborn baby. Look at the relative size of its head. A baby is basically half head, and even that is barely developed. Our complex brains need lots of space, and there’s only so much volume a woman can push through her birth canal. Women’s pelvises are already small relative to the size of a baby’s head (compared to other primates). In preparation for birth, pelvic ligaments soften and the baby’s skull is still not entirely fused. All that to literally squeeze through.
Here we meet a story of many conflicting demands. Walking upright has shaped our pelvis — relatively narrow (compared to the baby’s head), with a circular birth canal requiring fetal rotation during birth. This is the obstetrical dilemma hypothesis: bipedalism vs easy births, pick one. Another hypothesis for why human babies are born soon in development is the energetics of gestation and growth (EGG) hypothesis. Here too, the brain is the problem. The brain, after all, is the hungriest organ in the body. So, a brainy fetus is a hungry little parasite. At some point, the energetic demands of the growing fetus take too much of a toll on the mother’s body. When that happens, the EGG hypothesis suggests, labor begins and birth soon follows.
Human children are born (relatively) early in their development, either because of the hips or the hungry internal hippo. As with many things in biology, I suspect it’s not either/or; both hypotheses can be partially true.
Let’s skip ahead to menopause and grandchildren.
Better care, sooner
Only a handful of mammals - humans and (at least) five species of whale - experience menopause in the wild4.
The existence of menopause is often framed within the grandmother hypothesis that suggests that menopause is an adaptation that allows older women to help their grandchildren (and thus their genes) survive. This may be an overly simplistic story. Menopause might be an exaptation, or a byproduct that later became useful. The whales and humans may have evolved longer lifespans without the reproductive lifespan catching up, perhaps because of the higher chance of birth defects in old age, perhaps simply because of physiological constraints. Then, because grandma was around anyway, those grandmas that helped their children raise their kids were more evolutionarily ‘successful’.
Whatever the case, grandparents are often a source of great support and frustration for new parents. They help out, but are also eager to give advice, whether the new parents want it or not. Actually, everyone seems to feel entitled to give new parents advice, almost as if we instinctively know that the first 1,000 days in life are crucial in shaping,
… the development of brain circuits that lead to linguistic, cognitive, and socio-emotional abilities, all of which are predictors of later-life labor market outcomes.
That doesn’t mean childhood adversity dooms you to a life of misery or that a good childhood will protect you from all that life can throw at you. It implies that early life can load the dice, even if we still have to roll them. At a population level, in terms of policy, this also implies that investing in childcare is one of the best investments we, as a society, can make. Because it’s all about money (sarcasm alert), consider that the UN estimates that,
For every $1 spent on early childhood development interventions, the return on investment can be as high as $13.
And yet, we’re failing.
Across the Western world (from the US to Germany to Belgium), we are facing a childcare crisis. Daycare centers are understaffed, and the staff that’s there is overworked and underpaid. The average wage for a childcare worker in the US is an annual income of $32,070. On the other side of the coin, parents struggle with finding affordable childcare due to the rising costs following dwindling staff and decreased government funding. Over half(!) of working moms are thinking about leaving work because it’s too hard to find proper childcare.
Given the developmental importance of early childhood, investing in better, more affordable childcare and the people that make it possible as well as more support for those who choose to be stay-at-home parents seems like an obvious win. And yet, as journalist Jennifer Jay Palumbo writes in her Forbes piece,
The childcare crisis is more than just a series of isolated challenges; it's a systemic issue that requires comprehensive, multi-faceted solutions. From increasing government funding and support for childcare facilities to restructuring workplace policies to accommodate the needs of working parents, a concerted effort is necessary.
Collectively investing in the early years of life is not only better for the kids, but it’s a strategy for a brighter, healthier society.
Why aren’t we?
Thanks for reading. Go hug a kid. Preferably your own or a relative’s — would be awkward otherwise. (Disclaimer: Subtle Sparks is *not* recommending going out and hugging random children. All criminal charges are the reader’s responsibility.)
After the hug, click some buttons together…
Of course, there are great human dads, hence the ‘on average’. Owl monkeys might make a claim for primate dad of the year too.
If you want a glimpse at possible futures of human reproduction, I wrote an essay about the topic for Clarkesworld Magazine.
About 50,000 kcal if you were wondering. That’s (a lot) more than models previously suggested. Why the discrepancy? Previous models only calculated the ‘direct’ costs, aka the costs of the tissues in the fetus. But, as it turns out, up to 96% of the energy costs are indirect (growing a placenta, providing nutrients, etc).
The ‘in the wild’ qualifier may be important. A recent study that examined the lack of data for many species in the wild and observations in captive species suggests that many female mammals could enter menopause if they would live long enough in the wild.
What kind of society would we have if we organised life around care and connection, rather than productivity? How would human relationships even look like if the quality of care we share in created healthy, attuned humans?
Gah. There are spaces of possibility we can't even imagine.
Excellent article highlighting our greatest challenge to produce well adjusted and emotionally mature individuals. Ethan Crawley was only 12 when he walked into his school and killed 4 of his classmates and wounded 9 others. He was a latchkey child who drew bloody diagrams of dead children and wrote in his diary that he was on the edge, but nobody took any notice. We are the only species that has pronounced altricial development. Fetuses brains are too big for the birth canal, so nature devised altricial development from perinatal, new born, neonate, child, preteen adolescent through to adult, is the complete developmental arc, and children are very susceptible to stress especially chronic stress causes perseveration, and a inability to learn from mistakes and respond to social cues. Depressed children are more susceptible to amygdala hijack, and misreading people’s intentions, which may be linked to school shootings. It’s up to us to give our children the best possible chance to make the most of their lives.