Outside of humans and one species of armadillo, identical twins seem to be vanishingly rare. Now for the first time a dog has given birth to a pair
It seemed like a normal day for Kurt de Cramer, a vet in a busy veterinary practice in South Africa's Rant en Dal Animal Hospital in Mogale City.
He was performing a Caesarean section on a female Irish wolfhound, who showed distress and prolonged abdominal straining when she was due to give birth. For de Cramer it was a routine event; he averages about 900 Caesarean sections each year.
I immediately suspected that they might be identical twins
But something was different about this delivery. When he started the procedure, de Cramer noticed that the wolfhound had an unusual bulging by her uterus.
At first, he thought the lump was excess fluid surrounding a foetus. De Cramer painstakingly extracted this foetus from the bulge by making an incision into the dog's uterus.
That was when the real shock came. He found not one, but two foetuses. They were both attached with umbilical cords to the same placenta.
"When I realised that the puppies were of the same gender and that they had very similar markings, I also immediately suspected that they might be identical twins having originated from the splitting of an embryo," says de Cramer.
The twins looked very similar – but pups from the same litter often do
He could not stop and marvel at this peculiar discovery, because there were still five live puppies to deliver. These appeared as they normally do, each with their own placenta arranged in single file in their mother's uterus.
This was the first time de Cramer had delivered two living pups from one placenta in 26 years of practice.
He immediately called upon other reproductive specialists to confirm his suspicions.
The team, including Carolynne Joone of James Cook University in Townsville, Australia and Johan Nöthling of the University of Pretoria in South Africa, obtained blood samples when the twins were two weeks old.
"The twins looked very similar," says Joone. "But pups from the same litter often do, [and] there were small differences in the white markings on their paws, chests and the tips of their tails. I wasn't sure they were monozygotic [identical] at all initially."
However, the results confirmed what de Cramer had suspected: the dog had given birth to two identically genetic puppies. De Cramer had therefore delivered the first known instance of monozygotic twin dogs. The team outline their findings in the journal Reproduction in Domestic Animals.
The subtle differences in the markings on their fur initially caused some doubt to their status as identical.
This can be explained by the fact that, despite having the same DNA, the way individual genes are expressed can differ. "Human identical twins also have the same genes, but because those genes are expressed differently in each person, they have different freckle and fingerprint patterns," says de Cramer.
When the twins were six weeks old the team again took DNA samples, this time from their tissue rather than their blood. They also profiled the rest of the litter, "really just to have some sort of idea of how similar the pups in the litter were genetically", says Joone.
If the other pups from the same litter had very similar profiles, the team says they could not have convincingly concluded that the other two were identical.
Again, the results confirmed that the first two pups were genetically identical, whereas the others were only similar to the level that littermates usually are.
This is the first recorded case in the scientific literature, but does that mean it is a rare occurrence? Female dogs, known as bitches, often have several pups in each litter. Many of the pups often look strikingly similar, and DNA tests are not typically performed.
It has taken so long for us to find a monozygotic pair, so they are probably rare
It is hard to know, says Joone. "We are so lucky that this bitch ended up having a C-section," she explains, because otherwise it is unlikely her owners would have noticed. "Yes, there would have been one less placenta than there were pups, but as the bitch often eats the placentas the owner probably would have simply shrugged this off."
While this is the first confirmed instance of genetically-identical twins, it is impossible to say just how rare it is. "There have been rumours about twins in dogs before," says Joone. "We just happened to be lucky enough to be able to confirm it genetically."
It seems unlikely that identical twin puppies are particularly common. "It has taken so long for us to find a monozygotic pair, so they are probably rare," says Joone. "But so many of them will have been born naturally and blissfully unaware."
Non-identical multiples are common in many species, like dogs and rabbits. However, identical offspring are believed to be extremely rare. The exception is ourselves and nine-banded armadillos, which give birth to identical quadruplets, each with their own placenta.
People don't see these things unless they know to look
It is thought that identical twins are rare because, when two foetuses share one placenta, they do not get enough nutrients from the mother and are therefore less likely to survive.
For instance, identical twin foetuses have been reported in horses, but none have survived. A horse's placenta is not efficient enough to transport oxygen for two foetuses.
This means that when a twin foal dies in the womb, "the living one will abort due to his dead neighbour", says Jan Govaere of Ghent University, who in 2008 published a paper on the abortion of identical twin horses, one of which had died early on. "If a horse is carrying twins, most of them are lost within a month of gestation," he says.
For the same reason, it was surprising to de Cramer that the identical twin pups he delivered were healthy. He had only seen one other case of two foetuses sharing a placenta, back in 2014, and the foetuses were already dead when he performed a Caesarean section.
"It is even less likely for placenta-sharing puppies to survive, because of several complications relating to nutrient and oxygen supply from a single placenta having to do the job that is normally done by two placentas," he says.
Today the twin dogs are still doing well. They were slightly smaller at birth, but by the time they were six weeks old they had reached a similar size to the other pups in their litter.
As with many rare things in nature, "people don't see these things unless they know to look," says Joone. "Perhaps now they will, and more will appear."
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.