There are probably half a billion dogs on the planet. So it may seem impossible that no one has ever documented a case of canine twins, until now. A new paper published in the journal Reproduction in Domestic Animals documents the first genetically confirmed identical puppy twins ever delivered.
Melissa Hogenboom at the BBC reports that in 2014, South African veterinarian Kurt de Cramer was performing a Caesarean section on an Irish wolfhound at the Rant en Dal Animal Hospital in Mogale City, west of Johannesburg. At first, he thought one of the fetuses had excess fluid build up around it in its amniotic sac. But when he looked inside, he found two puppies with their umbilical cords connected to the same placenta, an indication that they might be twins.
After the procedure, de Cramer called in canine reproductive specialists to confirm his hunch. “The twins looked very similar,” Carolynne Joone of James Cook University in Australia tells Hogenboom. “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.”
The researchers took blood samples when the twins were two weeks old and tissue samples from the brothers and their five litter mates when they were six weeks old. The results showed that the pups, named Cullen and Romulus, were genetically identical.
Identical twins are rare in the animal kingdom. Hogenboom reports that when two fetuses attach to one placenta, one or both of them often do not receive enough oxygen, causing the animals to die. Humans are an exception, however, with a two percent twinning rate. Another exception: the nine-banded armadillo. For unknown reasons, armadillos only give birth to monozygotic quadruplets.
The human reproductive cycle may also lead to more twinning, reports David Cyranoski at Nature. In most animals, it’s obvious when they come into estrus. They mate quickly after ovulation, so fertilization takes place with a fresh egg. With humans, mating can occur throughout the ovulation cycle, meaning an older egg can be fertilized. One theory is that as an egg ages, its outer shell hardens. As the blastocyst develops, researchers think, it can fracture the outer shell and split the egg in two, causing twinning.
It’s likely that twinning in dogs is more common than previously thought. But most people don't look for the signs, like the presence of one less placenta than the number of puppies (the fact that the mother dogs often gobble up the placentas makes this even more difficult). De Cramer did once encounter twin pups one other time during his 26-year-career, but they were deceased within the womb.
Still, it’s likely they are out there. “It has taken so long for us to find a monozygotic pair, so they are probably rare,” Joone tells Hogenboom. “But so many of them will have been born naturally and blissfully unaware.”