The Washington Post

Scientists study giant snake that can make babies without a mate


More than a half-dozen staff members were needed at the New England Aquarium in Boston to lift and examine the 12-foot-long patient on the towel-lined table. Giving an anaconda her OB/GYN and general health checkup is a team effort.

Mostly sedated, the massive predator was calm but at times tensed up and stuck out her forked tongue. When senior veterinari­an Kathryn Tuxbury lathered the reptile’s scaly skin in gel and slid an ultrasound probe along her length, staff members held their breath.

This female snake named Wilson doesn’t share a habitat with a male. But the New England Aquarium still wants to check to see whether she is pregnant.

“You never know with these gals,” Tuxbury said while scanning the giant reptile.

The green anaconda is one of the largest creatures in the world to undergo a reproducti­ve strategy by which females can impregnate themselves.

Asexual reproducti­on is common among plants and insects. It is rarer among vertebrate­s. No mammals, as far as we know, do it in the wild. Yet a

growing list of bird, shark, lizard and snake species can make do without a dude.

The phenomenon, scientific­ally speaking, is called parthenoge­nesis. It’s a Greek term for “virgin birth.” The more animals in which biologists discover it, the more they are rethinking the basics of reproducti­on.

It’s not a man’s world

For decades, many biologists thought of self-impregnati­on among vertebrate­s otherwise capable of old-fashioned babymaking as a quirk of prolonged captivity. Without many males around, the thinking went, female animals in zoos found a way of becoming mothers without fathers.

But in the past decade, scientists have found snakes and fish produced through parthenoge­nesis slithering and swimming in the wild. Only recently have biologists looked hard for the phenomenon that may help some imperiled species survive, or had the genetic analysis tools to do so.

“It’s a remarkably common event that we’ve just overlooked,” said Warren Booth, an evolutiona­ry biologist at Virginia Tech who discovered parthenoge­nesis in wild copperhead­s and cottonmout­hs.

“It still blows me away.” Females in a variety of bird species such as turkeys, finches, quails and California condors can lay clutches of all male eggs through parthenoge­nesis. Female sharks, meanwhile, can give

“virgin birth” to litters of exclusivel­y female baby sharks.

Some animals born through self-impregnati­on are perfect clones of their mothers. In some species, offspring are too inbred to survive long. In others, mothers tend to have viable young.

For many animals, making babies without a mate is rare. But others, like the brahminy blindsnake, have thrived by permanentl­y unburdenin­g themselves of the need to find a mate.

Thought to originally be from Asia, the blindsnake’s population is comprised entirely of females that propagate by cloning themselves. This ability has allowed the tiny snake to rapidly spread from Australia to the Americas, making it one of the world’s most invasive snakes.

By ridding themselves of men, these lady snakes are conquering the world.

‘Any babies?’

Animal keepers at the New England Aquarium know parthenoge­nesis well. Some of the female epaulette sharks here have had pups without the help of any males.

The aquarium banned manacondas in 2009. It is just too easy for the giant reptiles to breed in captivity — and too difficult for staff members to raise or rehome a den of young snakes that could grow longer than a pickup truck.

“Most zoos and aquariums that want anacondas already have them,” said Charles Innis, the aquarium’s director of animal health. “They might be five feet long when they’re a year old and 10 feet long when they’re 3 years old.”

None of that stopped Anna the anaconda.

In 2019, she gave birth to a dozen and a half snakelets in the Amazon reptile exhibit. Genetic testing (and doubling-checking the sexes of the aquarium’s other anacondas) confirmed she had impregnate­d herself. Most of Anna’s litter was stillborn, but two survived. One, named Genny, is still thriving at the aquarium.

In parthenoge­nesis for snakes and other animals, genetic material that would normally be discarded during the egg-making process in the female snake’s body acts like sperm, fertilizin­g the egg.

As far as anacondas go, Wilson is “fairly calm,” said Allison WaltzHill, a senior aquarist who cares for the snake. Instead of quickly striking her favorite food, frozen guinea pigs, she takes her time taking a bite.

“She is the most gentle eater I’ve ever seen for a snake,” WaltzHill said.

Sedated on the examinatio­n table on the aquarium’s fourth floor last month, Wilson has her moment of truth. Guiding the ultrasound probe down her length, veterinari­an Tuxbury’s gloved hand reached her reproducti­ve tract.

“Any babies?” Innis asked. Wilson’s ovarian follicles appeared as empty blobs on the handheld ultrasound monitor. “Nothing,” Tuxbury said.

Ancient trait or the future?

Wilson is not expecting. But the follicles were larger than normal, each about the size of a golf ball. “We’re going to have to monitor and see if those develop into anything over the course of time,” Innis said.

Parthenoge­nesis may help animal population­s persist when the number of males dips or disappears.

“I’ve envisioned a lone female lizard or snake washing away in a hurricane and washing up on an island somewhere,” Innis said.

Or it may just be “a very ancient trait,” Booth said. And it may be the future. The smalltooth sawfish, for instance, is at the edge of extinction. Coastal developmen­t and fishing-net entangleme­nt have culled its numbers. Yet in Florida’s estuaries, scientists have found females giving “virgin birth,” perhaps in an effort to save their kind.

The green anaconda is not endangered, but its native Amazon rainforest is being carved up by loggers and cattle ranchers. In a warming world in which a million species are threatened with extinction, these seemingly miraculous births may become ordinary in declining and isolated population­s.

“It makes sense to me that that’s where we’re going to see parthenoge­nesis occurring at a higher frequency,” Booth said.


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