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Many animals can give birth without mating

We think of virgin births as a miracle, and for humans they would be. But for

many animals, virgin births are a choice

By Melissa Hogenboom

16 December 2015

We have written before about the strange but spectacular phenomenon of virgin

births, or "parthenogenesis" as it's known.

Some animals are fully asexual and do not need a male to give birth: for

instance, some species of whiptail lizards. But there are also animals that can

mate with a male, but do not always do so, and they are the ones we are

considering.

Here we report four new cases published in the scientific literature in 2015.

They all point to the idea that, even in sexually-reproducing species, many

animals have long been able to go it alone.

She doesn't need, or particularly want, a male (Credit: David Brabiner/Alamy)

This stick insect doesn't always need a mate (Credit: David Brabiner/Alamy)

Stick insects

Female Australian giant prickly stick insects will mate with males when it

suits them, but they have found ways to repel them so they can have young

without any male interference.

In a study published in the journal Animal Behaviour in March 2015, scientists

examined why the females sometimes do without a male.

It was not that males are rare or absent, which is thought to be a key driver

for parthenogenesis in other species. Instead, the team proposed that sex can

be very costly for females, so they might prefer to take their chances alone if

they can.

They win sexual conflicts more frequently than females despite female

resistance

Female giant prickly stick insects will even fight off lustful males. First,

they emit an anti-aphrodisiac chemical to stave off temptation. If a male is

still keen, the female will curl her abdomen and kick her legs to repel him.

"Since females that have started reproducing parthenogenetically are no longer

attractive to males, such females appear to have the opportunity to continue to

reproduce exclusively via parthenogenesis," the team says.

All the offspring from parthenogenesis are female. So if the female stick

insects carry on reproducing alone, the males could be wiped out.

But for now the males still have a fighting chance. They "win sexual conflicts

more frequently than females despite female resistance," the team says.

This may help explain why parthenogenesis remains rare, even in species that

are capable of it. In such species, "males typically force females to mate".

This python mother had six healthy offspring, without a male (Credit: Kyle

Shepherd)

This python mother had six healthy offspring, without a male (Credit: Kyle

Shepherd)

Snakes

Parthenogenesis has been documented in several species of captive snakes, but

it was long thought to be something females only did when there were no males

around.

That changed in 2012, when Warren Booth of the University of Tulsa in Oklahoma,

US discovered that two litters of wild pit vipers had been born via

parthenogenesis.

These snakes are half clones of their mother, so they are highly inbred

It was the first time parthenogenesis had been documented in wild-caught

snakes, which presumably had access to males. One of the baby snakes has since

gone on to have healthy offspring.

This year another team noticed an instance of a pit viper virgin birth, but

this time the young did not survive. A captive female gave birth to one

stillborn snake and four undeveloped ova. Two years later, the same snake had

another virgin birth.

A python born by parthenogenesis (Credit: Kyle Shepherd)

Many snakes, like this python, are born by parthenogenesis (Credit: Kyle

Shepherd)

We don t know for sure why her offspring died, but the incident is telling. It

highlights that this form or reproduction can be far from ideal, says lead

author Mark Jordan of Indiana University Purdue University Fort Wayne in

Indiana, US.

"These snakes are half clones of their mother, so they are highly inbred," says

Jordan. "When parthenogenesis happens, there's a lot of mortality or lack of

development."

Nevertheless, Jordan says it is clear that reproducing this way has long been

"fundamental to their biology". "It's something they may use periodically in

situations where there are no males around to mate with, when populations are

low or if they are moving into new habitats."

The study was published in the Journal of Herpetology in March 2015.

The smalltooth sawfish is critically endangered (Credit: RD Grubbs)

Sawfish

2015 saw the first instance of a virgin birth in wild vertebrates that had

never even been caught.

The animal in question was the endangered smalltooth sawfish, which had never

previously been documented reproducing parthenogenetically. Virgin births have

been seen in sharks, which are related to sawfish, but only in captive sharks.

In the wild, it is much harder to know whether parthenogenesis has taken place.

The evidence came from genetic testing.

Many snakes are born by parthenogenesis (Credit: Chris Mattison/Alamy)

Seven healthy offspring had been born this way, a finding published in the

journal Current Biology in June 2015.

The discovery came about by chance. The sawfish population is dropping, so

ecologists were studying their genes to understand how this is affecting them.

"We were looking at how much genetic variation remains," says co-author Kevin

Feldheim of the Field Museum of Natural History in Chicago, Illinois, US.

A last-ditch effort for females to pass on their genes

The young sawfish were healthy and thriving, despite being inbred.

We do not know why the female smalltooth sawfish chose to undergo a virgin

birth. But it could be a survival strategy when population levels are low. "If

they can't find a mate, it's possible this mechanism kicks in as a last-ditch

effort for these females to pass on their genes," says Feldheim.

The team has now taken 130 further samples from wild smalltooth sawfish. They

are now analysing them to see how often they use parthenogenesis.

This lizard was thought to be all female (Credit: Sergio Marques de Souza)

Lizard

Strictly speaking lizards should not be on this list. We know that, in general,

the lizards that have virgin births are all female and asexual. They have no

choice but to reproduce alone.

But it turns out the story is not that simple. A study published in the Journal

of Herpetology in August 2015 reported that one lizard species, thought to be

all female, has males after all.

For this lizard parthenogenesis may be a successful strategy

Eight male Muller's tegus were discovered among 192 adults found in 34

different places in South America. It was the first time males of this species

have ever been found, even though it is abundant in several areas.

This suggests that some Muller's tegus reproduce sexually. However, the asexual

ones are thought to be strict about their no-males policy.

"We expect that parthenogenetic females do not cross with the males, but normal

females do," says lead author Sergio Marques de Souza of The University of S o

Paulo in Brazil. "In this sense, sexual and asexual lizards are distinct

evolutionary units, since we believe that there is no genetic exchange between

them."

Parthenogenesis may have appeared spontaneously in this lizard (Credit: Sergio

Marques de Souza)

The existence of these males may provide new clues into how the species became

parthenogenetic in the first place.

Muller's tegus have been doing it or rather, not doing it for four million

years

It is generally believed that parthenogenesis arises in lizards through

hybridization: when two related species mate, resulting in a new species. All

the offspring of these hybrids are then female.

Now that males have been found, it suggests this may not be the case. Instead

parthenogenesis could have arisen spontaneously due to environmental pressures,

says de Souza.

His analysis also suggests Muller's tegus have been doing it or rather, not

doing it for four million years. "It contradicts previous studies, which

proposed that parthenogenetic organisms have low genetic variation and,

consequently, low evolutionary success," says de Souza.

For this lizard at least, parthenogenesis may be a successful strategy.