Born in October 2015, Irish wolfhound puppies Cullen and Romulus are the first dogs to be genetically confirmed as identical twins. When you hear the words "twin puppies," you may think, "what else is new?" Dogs give birth to multiple puppies at a time, so technically every puppy in a litter is a twin. But littermates, by and large, are fraternal twins, which means they each came from a separate fertilized egg or zygote—thus, why fraternal twins are also known as dizygotic. Identical, or monozygotic twins, come from the same egg, and therefore share a placenta and, often, an amniotic sac.
According to scientists, this is the reason identical twins are probably rare in the animal kingdom. It's hard for two fetuses to get enough nutrients and oxygen from a single placenta, and as a result, most identical twins die in the womb. This doesn't explain why humans may give birth to more identical twins than other animals, but twinning researcher Judith Hall told Nature that our unique reproductive cycle may be the reason: while many other animals only mate during periods of ovulation, humans often ovulate long before mating, resulting in older eggs that may be more likely to split into identical twins. The only other animal believed to have more identical twins than humans do is the nine-banded armadillo, which always gives birth to identical quadruplets.
It should be said, however, that it's possible that identical twins aren't actually all that rare in the animal kingdom. Since confirmation of identical twins requires DNA analysis, we may see plenty of identical-twin animals without even knowing it. Hear the science of identical twins in the videos below.