Some Marmosets Have Three Biological Parents

Or, in other words, polyandrous chimera monkeys.

Now that I have your attention, let’s discuss this group of diminutive little primates and how they can throw most of what we assume about paternity out the window.


(Photo by Carmem Busko.)

Look at this common marmoset. Look at the disdain in its eyes. How flexible is your social system, exactly? Cause theirs is really, really flexible. Like they can go every which way. Monogamy? Yep. Polygyny (one male, many females)? Yep. Polyandry (one female, many males)…?

Hell yeah. Marmosets sometimes form sexual triads of one female and two males, and both males will help care for her young.

Now, polyandry is actually more common in the animal kingdom than you might think, and that is because it is usually referred to as female promiscuity (as opposed to ‘promiscuity for everyone!), because god damn are the science elite reluctant to give the husband-juggling ladies any recognition. But, for a couple of examples, female honeybees will mate with multiple males, while male drones have only got one shot; and female cats have an interesting arrangement where they attract every male for miles, mate with them, and then give birth to litters of kittens with different fathers.

The different fathers bit is important, because it’s not just an adaptation for mama cat to have genetic variability in her offspring (though I won’t deny that that’s probably a factor). The more important bit is to make sure none of the dads she’s mated with know which offspring are theirs- hell, they could all be from one guy, to be honest, but none of them will really know cause they all took the plunge, so to speak.

This is pretty important, because a lot of male mammals have the nasty tendency of killing offspring they don’t think are theirs. We all are probably familiar with the fact that male lions tend to do this, for example. But fewer people know how common it is in primates, like gorillas.

And it’s an inherent danger of having a polygynous harem system. It’s why in these species, the males tend to be pretty big and aggressive. If they fuck up, someone else is going to take their place and erase their genetic legacy from the earth.

You know, no pressure.

Ok, we know the males are all stressed out and snappy from this. But what about the ladies? Never mind the annoyance of having to share one dude with a bunch of their sisters (and yes, it is usually sisters; that helps curb female mating conflicts because at least it’ll be their nieces and nephews in the group- yes, there are inter-female mating conflicts!! All the time!!)- never mind that, it’s always their genetic progeny that takes the brunt of it when harems change hands.

And female mammals generally don’t have an issue determining maternity, being that their offspring comes kicking and screaming out of their body in one form or another. It’s hard to have a genetic whoopsie in the womb. (It IS possible, but we’ll get to that.)

So females know their offspring is theirs, and they’re hell-bent on defending it from those dick-ass males. One way of doing this is to completely flip the system, like hyenas, who have a strict no-access-without-permission rule with their psuedophalluses, forcing the males to rely on poetry and flowers and gold stars for good behavior if they ever want to get the v, but another strategy is polyandry.

Female Japanese macaques do this- as a species where male infanticide is common (and if you are a subordinate female, so is female infanticide- your sisters aren’t having any of that) they do their best to mate with as many males as they can possibly find, so that the raging male will stop, scratch his head, and think, Wait, did I bone this?

(And then the females go back to having copious amounts of gay sex, because you know.)

Ok, so polyandry can be a really good defensive technique against gene-greedy males. But let’s go back to talking about marmosets. Because marmosets are really not the infant-killing types. Quite the opposite, really. They are really really good parents. They have to be, because not only are marmoset babies almost always born in pairs, they are each freaking huge- like, up to one-third of mom’s body weight. Ow. And again, ow.

And then, even after they’re born, they need a lot of care. They’ve got to be clinging to someone 24/7 long after they’re weaned. And poor mom is exhausted. She needs some time off. She needs some goddamn sleep.

(Luckily marmoset babies are disgustingly cute, or I’m pretty sure that mama would’ve already chucked him to the forest floor in frustration.)

This is where dad- and papa- come in.

Now, I mentioned before that marmosets aren’t always polyandrous. This is true especially when they live in large groups. Usually there are only one to two breeding females in a group, and the rest of the members are their young adult offspring and unrelated males. In these large groups, the older kids can pick up a lot of the slack. But the kids aren’t always going to stay home and help raise their siblings. They’re gonna want to go look for their own opportunities to mate.

It’s in these little groups, newly starting out (and in groups that have lost most of their members) that polyandry tends to occur. The moms need the extra hands, and the way to get the males, usually two brothers, invested? Make ‘em think it’s their kid. The dads get extremely invested, to the point where a male marmoset in a mating trio doesn’t even have a hormonal response to a receptive female any more. He’s too busy cuddling his babies.

“Ok, that’s all fascinating and adorable, Koryos,” you say, honking your proboscis. “But you said something about three parents and alien babies and this is kind of a letdown.”

Alright, alright, put down your feelers, I’m getting to that bit now.

I said marmosets usually have babies in pairs, and this is true- but they aren’t identical twins, they are fraternal twins. AKA, they didn’t come from a zygote that split; they came from two different eggs that, theoretically, could have each been fertilized by different fathers. So each dad in the happy marmoset trio could theoretically have his own kid. But of course that’s up in the air.

No, when I say that marmoset kids can have three parents, I mean like… literally, they can be the genetic offspring of three different individuals.

You know what a chimera is? Not in mythology, a scientific chimera. It’s an organism that is made up from cells from two different individuals. Two completely different sets of DNA, residing in the same body.

Now, not all chimeras are that neatly divided; a lot of the time you just get a random mosaic of cells, like, your kidneys are from Albert and your left femur is from Johnson.

There are several human chimeras out there- most famously, in paternity cases where men have found their babies fathered by sperm that is genetically different from, say, their blood. Some intersexed people are actually chimeras with both male and female cells. (But even if you have both male and female cells, it doesn’t mean they’ll always both fight over your genitalia; it’s random which cells belong to which genomes, so you might have male cells in your liver and female cells in your ovaries.)

Soooo… marmosets. Have actually evolved to intentionally have chimeric offspring. Those fraternal twins actually swap cells in the womb as the embryos develop, giving rise to whole organ systems derived of their sibling’s DNA. And yes, this includes germ line cells that develop into gametes. A marmoset can father children with his brother’s sperm. And then this child, in the womb, will swap cells with its twin, that was fathered by yet another male- it is all a hot hot mess inside mama marmoset.

Speaking of which, I mentioned that the “I bore this child so therefore it is mine” mindset most female mammals have isn’t foolproof, and here’s how. Mama marmoset might actually be using her sister’s eggs. Or for that matter, mama marmoset might actually have a couple of her brother’s Y-chromosomes floating around in there and contribute it to her son. Like, who even are these babies?? Who do they belong too?? Why does he have uncle Randy’s eyebrows???

At least one study has shown that male marmosets can somehow pick up when a baby is a chimera (because they aren’t ALL chimeras, just most of them) and will prefer caring for those. Because you really never know if some of your genetic material is in there… somewhere.

Sources/More Info

Tamarin and marmoset mating strategies

Marmoset dads are too busy to cheat

Germ-line chimerism in marmosets

Female promiscuity as a countermeasure for infanticide in macaques

Girl-on-girl macaques

Polyandry in crickets

Polyandry in tamarins (a close cousin)


About Koryos

Writer, ethology enthusiast, axolotl herder. Might possibly just be a Lasiurus cinereus that types with its thumbs.
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One Comment

  1. This is wonderfully, awesomely weird. Thank you for this article!
    Would it be possible for a marmoset to have four genetic parents? It hinges on marmosets having two functional ovaries and a possibility that those are different genotypes, which I don’t know if it could happen or if both ovaries develop from the same cell. But if it can, then one twin could come from a mom-egg and the other from an aunt-egg, and be combined with different sperm. And then they swap and have four parents.

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