By D. Caroline Coile, Ph.D.
Sure, dogs can be fraternal twins – or triplets, or octuplets and so on – but what about identical?
Unlike fraternal twins, which arise from two independently fertilized eggs, identical twins come from the same fertilized egg, which splits and becomes two separate embryos. Depending on when the split occurs, the twins may or may not share a single placenta. The earlier the split, the greater the chance they will each have their own placenta.
Some dog breeders have reported puppies born with only one placenta between them, but even that’s not foolproof. There’s always a chance that two puppies were born one after another and that one placenta became detached and was delivered later. It’s also possible two placentas could grow together and appear to be a single entity.
What about reports of dogs with identical markings? It could be coincidence. But to complicate matters, identical twins would not necessarily share the same markings anyway. That’s because certain characteristics, such as the size and placement of spots, are determined during development by random cell splits that occur after the fertilized egg split.
DNA tests are the only conclusive way to prove identical twin dogs, and no reports exist of dogs proven to be genetically identical. Conjoined animals are often identical twins. Reports of conjoined dogs would be consistent with the idea that they are identical twins because conjoined twins usually result from an incomplete split of a fertilized egg that would have produced identical twins. But although conjoined kittens are not that unusual, conjoined puppies are rarely if ever reported.
So for now at least, there’s no conclusive evidence that identical twins exist in dogs. But who needs science? You already knew that every dog is an individual!