Dogs are always born in litters and oftentimes look alike, so it is easy to assume that identical twins aren’t uncommon in puppies. But in reality, identical twins in dogs are incredibly rare.

Twins are two offspring produced by the same pregnancy. They can be either identical twins, meaning that they develop from one egg (monozygote), which splits and forms two embryos, or fraternal twins, meaning that they develop from two different eggs (dizygote). In fraternal twins, each twin is fertilized by its own sperm cell.

So technically, all dog litters with more than one puppy are considered fraternal, and if the mother has only two puppies, they would be fraternal twins, or triplets if there are three puppies, and so on.

Identical twins, although rare, do occur in other animal species, including horses, cattle, pigs, and even armadillos. In some of these species, the occurrence of identical twins often comes the concern of conjoined twins, most notably in cattle.

Instances of Identical Twins in Dogs

There has been historical documentation of identical twins in dogs, but until very recently, it has not been able to be genetically verified. That is, until a litter of Irish Wolfhounds in South Africa produced a litter including a set of brothers born in the same sack with attached umbilical cords.

Five other puppies were also born in the litter, each surrounded by its own placenta, as is typical.

At six weeks of age, a team of reproductive specialists performed DNA testing that confirmed these brothers to be genetically identical monozygotic (“one-egg”), twins. The results of the testing were published in the journal Reproduction in Domestic Animals.

Are Identical Puppies Actually identical?

Identical puppies, like identical humans, will sometimes have subtle differences in their markings and appearance. This is because even though the dogs have the same genes, those genes aren’t always expressed the same way.

Behaviorally, many twins seem to share similar personalities and habits, but there are still temperament differences as well, due to factors such as environment.

There isn’t widely documented research on the growth of the identical twin Irish Wolfhounds. It will be interesting to see how these brothers grow and how similarly their appearance, behavior, and health develop. If they went to different homes, it would also be interesting to see what part genetics and environment may play in the expression of their genetic make-up.

How Do We Know Identical Puppies Are Rare?

While identical puppies are known to be rare, it is possible that cases go undocumented. Only a small sample of wild and domestic animals go through genetic testing.

However, scientists hypothesize that the reason twins are rarer in animals than in humans is because sharing a placenta can lead to fewer nutrients for each animal.

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