Self-love is important, but we mammals are stuck with sex

If Galentine's Day had an animal mascot, it would have to be one of the species whose females can reproduce without a mate. Almost all animals get more out of themselves in the traditional way, by combining eggs and sperm. But some have an alternative called parthenogenesis: no males are needed.

No matter how many romantically frustrated mammals wish they could really go it alone, a genetic quirk means we still need sexual reproduction. For now, parthenogenesis is for the birds (and the bees), the fish and the reptiles.

One of the most famous recent cases of parthenogenesis involved California condors, an endangered species. In 2013, Leona Chemnick, then a researcher at the San Diego Zoo Wildlife Alliance, discovered that two male chicks in the condor breeding program had DNA that did not match that of the fathers in their cages — or any other male. The chicks' DNA only matched that of their mothers.

Ms. Chemnick caught Oliver Ryder, the zoo's director of conservation genetics, on his way to his car and asked him about the strange data she saw. He explained to Mrs. Chemnick that such condor chicks must have come from eggs that were not fertilized by sperm.

“We literally walked to the parking lot and had a eureka moment,” said Dr. Ryder. “We didn't have time to dance or anything.”

By the time the two scientists and other colleagues published After their parthenogenesis finding in 2021, the two unusual chicks, or parthenotes, were long gone. They had both died young, almost two years old and almost eight. Their mothers both had many more offspring, conceived with their partners in the usual manner (despite headlines reporting virgin births).

Each condor conception is a miracle of a different kind. In 1982, when only 22 California condors remained on the planet, conservationists began capturing every bird and bringing them into captivity in a desperate attempt to save the species. In 2022 there were 561 birds, most of which lived freely in the wild.

A crucial part of growing that healthy condor population is tracking the birds' genetics, which made the discovery of the parthenote chicks possible. Since finding the first two, Dr. Ryder, his team discovered two more, although they died before hatching.

How their mothers made them is a bit murky.

Condors, like most animals, carry two copies of each gene: one copy from each parent. To create a sperm or egg cell, an animal must divide its genetic material in half. When egg and sperm meet during sexual reproduction, they combine their genes to create one completely new genome.

To create chicks without sperm, condor mothers must have duplicated the DNA from an egg. There are a few ways this could have happened, said Dr. Ryder, and his team conduct a deeper analysis that should solve the mystery.

Other birds, including chickens and turkeys, have also accomplished this feat. Then there are the reptiles, including Komodo dragons and others smart girls, which have been found to reproduce in this way. Last year, scientists reported parthenogenesis in an American crocodile. There are even some species of snakes and lizards that reproduce only through parthenogenesis and have given up sex completely.

Many insects and other invertebrates can reproduce without males. Certain sharks and other fish can do that too. One captive white-spotted bamboo shark carried several parthenuts, and one of them grew up have her own fatherless offspring.

At Shedd Aquarium in Chicago, a female zebra shark named Bubbles had two parthenote puppies in 2016, although both died shortly after release. Like the California condors, Bubbles surprised scientists with her quasi-virgin birth, because she was not alone at the time. She lived with two male sharks, who probably wouldn't have minded sharing their sperm.

No one knows whether a female can choose to reproduce herself – for example if her current reproductive options are unsatisfactory – or whether parthenogenesis occurs outside her control.

“It would be fascinating if they could knowingly decide to do that,” said Dr. Ryder.

Humans only noticed parthenogenesis when solo females had young or when researchers monitored a population's genes. Considering that many different branches of life have demonstrated this ability, many more species of female animals could secretly reproduce on their own.

“It's probably much more widespread than we think,” said Dr. Ryder.

However, scientists are convinced that no mammalian mother gives birth to fatherless babies. We are hampered by something called genomic imprinting.

To understand imprinting, we must remember that animals divide their paired genes in half to make a sperm or egg cell. Mammalian parents add another dimension to this process: they place chemical tags on certain clusters of genes. The tags make those genes unreadable, as if the genetic instructions have been crossed out with a black marker.

After a mammal's sperm and egg cells combine, those marked genes remain silent. This means that even if the offspring still has two copies of each gene, it can only use the copy from its mother or father, because the other copy is unreadable.

We can see imprinting in action when, for example, a lion and a tiger breed together in captivity. The resulting big cat looks different – ​​a bulky liger or a small tigon – depending on which species is the mother and which is the father. In the printed locations the hybrid is all lion or all tiger.

“It's really hard to understand why this process has evolved,” says Anne Ferguson-Smith, a developmental geneticist at the University of Cambridge.

Scientists have suggested that imprinting reflects a kind of evolutionary battle between the parents. That's because many imprinted genes influence growth. The father's modifications to the genome generally cause his offspring to grow larger, while the mother's changes keep the babies at a more manageable size.

Dr. However, Ferguson-Smith suspects the true story is more complex. Some imprinted genes influence the offspring's brains and behavior, or even the way they will care for their own young in the future.

Regardless of why we imprint our genome, the result is that mammalian sperm and eggs need each other.

If a mammal mother tried to make a baby like Bubbles the shark did, by doubling the genes from her own egg, her offspring would not develop. Genes she silenced would be completely absent. Other genes would be present at twice the usual dose, because the offspring would lack a father's usually silent copy. This can also cause serious problems, said Dr. Ferguson Smith.

So mammals are stuck with sex. But some scientists are experimenting with ways to save endangered animals whose dating pools are small or nonexistent.

Dr. For example, Ryder at the San Diego Zoo is involved in efforts to create embryos using frozen cells and then placing the cloned embryos into surrogates of closely related species. So far, he has helped create a clone of a black-footed ferret named Elizabeth Ann and two Przewalski's horses. The youngest cloned foal was born last year and given a name Olliein honor of Dr. Ryder.

The colleagues of Dr. Ryder also uses genetic technology to save the northern white rhino, a subspecies in serious trouble; only two are still alive. A few years ago, he said, researchers at the San Diego Zoo Wildlife Alliance took a step in that direction.

They coaxed frozen cells from northern white rhinos into stem cells. Ultimately, those stem cells could be turned into eggs and sperm. But first, as a test, the researchers told the cells to become heart muscle.

When Dr. Ryder saw the northern white rhino's heart cells beating in a dish, it was as good as a Valentine's card.