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First Ever Confirmed Case of Puppy Identical Twins

Two puppies born in South Africa provide new evidence on the rareness of twinning.

They’re gonna need a bigger bowl.

The recent birth of two Irish wolfhound puppies has scientists excited, because—for the first time ever—the puppies have been confirmed through genetic analysis to be identical twins.

People have suspected for years that identical (monozygotic) twins are possible in domestic dogs, and there have been anecdotal reports, but there has never been a confirmed case in the scientific literature until now. And it turns out that although human twins are relatively common, identical twins are exceedingly rare in other species.

The identical twin puppies, named Cullen and Romulus, were delivered by vet Kurt de Cramer in South Africa's Rant en Dal Animal Hospital in Mogale City. He was performing a cesarean section on the mother, who was in distress. He found the two pups attached via umbilical cords to the same placenta, something he had never seen before in decades of practice.

Five other littermates were also delivered, each surrounded by its own placenta, as is usually the case in dogs.

He was struck by how similar Cullen and Romulus looked, although they still had some subtle differences in white markings. But blood samples confirmed that the twins were indeed identical, and had arisen from the splitting of a single fertilized egg. (Learn more about twins.)

The slight differences in coat patterns can be explained by the fact that although the pups have the same set of genes, how each gene gets expressed is still influenced by subtle environmental cues. That’s one reason why human identical twins don’t look exactly alike.

So de Cramer enlisted several scientists to help with analysis of the pups, including Carolynne Joone of James Cook University in Australia and Johan Nöthling of the University of Pretoria in South Africa. The team published their findings in the journal Reproduction in Domestic Animals last week.

The researchers point out that confirmed cases of identical twins are extremely rare in nature. It’s certainly possible that cases simply go undocumented, they warn, because scientists have only done genetic testing on a small sample of wild (and even domestic) animals. And at least one species, the nine-banded armadillo, regularly gives birth to identical quadruplets. (See photos of twins.)

Watch: Welcome to Twinsburg, home of the world's largest twin festival.

But the scientists suggest that twins may be rarer in nature than among human beings because two organisms living in a single placenta can lead to less nourishment for each individual. In the tough world of nature, that can result in young that are less hardy and prepared to face the wild. In people, that is less of an issue now, thanks to the benefits of society and intensive parental care.

In people, the natural rate of twins is roughly one in 80 births, with identical twins making up roughly a third of that. The rate of identical twins has tended to be relatively consistent across nations and cultures, although it may tend to run in some families. The rate of fraternal (or dizygotic) twins tends to rise with the use of certain fertility assistance drugs.

Identical twins develop after a single zygote, or fertilized egg, divides, creating two (or more) embryos. Fraternal twins develop from two (or more) fertilized eggs.

In people, identical twins do tend to form closer relationships than fraternal twins or other siblings, psychologist and twin expert Nancy L. Segal previously told National Geographic. Identical twins often do seem to share some similar habits and quirks, but there are many differences that develop as well, thanks to different environmental impacts and possibly other unknown factors.

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