Confused marmoset dads and shared paternity

How paternity certainty shapes father care

Last week, I was at a dull fundraising lunch for my university when I started chatting with the biology professor sitting next to me. I was delighted to discover that he studies the evolution of the mouse penis. We started talking about fathers and penises, as one does, and my new friend said, you must read about marmoset dads! That, of course, sent me down a research rabbit hole that got me through the rest of the lunch and beyond.

Cool Marmoset Fact #1: Many of them are chimeras

In ancient Greek mythology, the chimera is a mythical beast with the head of a lion and a goat, and the tail of a serpent. But chimeras are not just mythical, they are real creatures whose bodies contain cells from two different genetic sources. Chimerism is more common in marmosets and tamarins than any other species, but can pop up in tortoiseshell cats (about 1% are true chimeras) and even, very rarely, in humans.

In 2002, a mother named Lydia Fairchild applied for child support for her three kids, which required her ex to take a paternity test. Bizarrely, the test revealed that she had no genetic connection to her children. She was accused of welfare fraud and almost lost custody, even though the obstetrician who delivered her babies swore they were hers. It turned out that she was a genetic chimera, with cells from another DNA line in her ovaries. As in other cases of human chimeras, the double DNA probably came from a vanished twin – that is, a second fertilized egg that had spontaneously aborted in utero and whose cells had fused with hers. Fairchild’s vanished twin was the genetic mother of her children. I italicized that sentence for emphasis because vanishing twin syndrome always gives me the chills and makes me wonder – do I have a vanished twin and if so, what is she doing right now?

This is a real chimeric cat named Queen Domino

Back to marmosets. Twin pregnancies are very common in these strange little monkeys. Marmoset twins share a placental blood supply in utero. This allows stem cells from one twin to migrate into the other and creates genetic chimerism, in which a single marmoset can have cells in their body with more than one set of DNA. The marmoset becomes a hybrid of “self” and “non-self,” carrying their sibling’s DNA in their bodies along with their own.

A marmoset infant can therefore have more than one dad, because it’s possible for two different fertilized eggs to end up gestating at the same time and mix together chimerically in utero. Even more bizarrely, a marmoset’s twin brother can actually be the genetic father of his offspring (in the same way that Fairchild’s twin was her children’s “real” mother). Scientists have estimated that over half of male marmosets carry their brother’s sperm.” As a New York Times article about marmosets joked, “‘Well, I’ll be a monkey’s uncle’ has now taken on a new meaning.”

Cool Marmoset Fact #2: Dads Prefer Chimeras

Marmoset mothers and fathers seem to be able to distinguish between their chimeric and non-chimeric offspring, probably because their skin smells different. Scientists have observed that mothers seem to avoid their chimeric infants, and spend less time carrying them around. But fathers make up for this lack of maternal interest by investing more care into chimeric marmoset infants. Fathers spend more than twice as much time carrying chimeric infants around than they do carrying their non-chimeric infants.

Why are dads especially drawn to these mixed-up twins? Marmoset infants give off odors that helps parents recognize them, but a chimeric infant would smell both like itself and its twin sibling. If the chimeric infant’s father is a chimeric twin himself, the infant might actually be his brother’s kid. This mix of smells might improve the odds that the dad will recognize his own kin. To quote a study on paternal care in marmosets, “Chimeric offspring might match the father at more kin recognition alleles, elevating the perceived relatedness and thus, the confidence of paternity in fathers.” In other words, chimeric infants have more paternity-revealing smells that convince fathers that an infant is related to them, and not someone else. At the very least, fathers can tell that a marmoset baby is someone familiar, and not a complete stranger’s child.

Male marmosets are unusually good, attentive fathers in general, and its possible that the high rates of chimerism in infants makes males more willing to participate in the intensive cooperative care that marmoset infants require. If you can’t be sure which babies are yours, you’ll be nicer to all of them.

What Marmoset Confusion Tells Us About Human Fatherhood

Much like marmosets, it can be tricky for human fathers to tell which babies are theirs - not because of chimerism, but because we practice concealed ovulation, making it hard to know when a human woman is most fertile. Many species give off obvious “tells,” from unique smells to flushed hindquarters, but we human ladies keep it pretty subtle. As a result, men are left wondering about whether a particular sexual encounter has created a pregnancy. Human societies have developed all kinds of cultural and moral strictures to police women’s sexuality and increase men’s certainty about their offspring. But for human women, there’s an advantage to keeping paternity a little mysterious, because males are less likely to aggress against offspring that have a chance of being theirs. Like marmosets, we are cooperative breeders who require a lot of collective care to raise our young, and it’s better to have more nurturing hands on deck.

Some societies even believe that a child can have more than one father. “Partible paternity,” the belief that multiple fathers can contribute to a pregnancy, exists among indigenous groups that forage in the Amazon, like the Aché tribe in Paraguay. The Aché traditionally believe that if a woman has sex with more than one man before birth, all the men should be considered fathers, and they should divvy up parenting duties. Much like marmoset chimera twin brothers who might both contribute genes to an infant, Aché co-fathers are often brothers or cousins. They share fatherhood willingly, and children with more than one dad appear to enjoy a survival benefit. If the part-time dads are related to each other, shared care promotes their own genetic interests, too.

Partible paternity and marmoset chimera confusion both appear to have similar evolutionary purposes: they help keep men invested in infants that may or may not be theirs, and get more potential fathers involved in raising the next generation. This kind of research reminds us that childrearing is a collective occupation, not a solitary one. More caregivers can help ensure a child’s survival, and we need fathers on board.