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.
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".
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.
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.
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.
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.
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 São 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."
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.
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
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