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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.
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.
“For instance, identical twin foetuses have been reported in horses, but none have survived,” Hogenboom explains. “A horses placenta is not efficient enough to transport oxygen for two foetuses.”
Think about how many pups are born on the streets, or in the basement of their owner away from view. Dogs tend to eat their placentas once theyve given birth, which hides the evidence that tipped off de Cramer, and considering identical twin pups can differ in appearance, there might be a bunch of identical twin pups running around and we dont even know it.
But given the fact that no vet has ever delivered one – and de Cramer himself averages about 900 Caesarean sections every year – its safe to say that even if weve missed some, theyre still incredibly rare.
When de Cramer delivered the pups from the same placenta via a Caesarean section, he suspected he had the first monozygotic dogs known to science on his hands.
Twins can be either monozygotic (identical), meaning they develop from the same zygote (or egg cell), which is fertilised by the same sperm cell; or they can be dizygotic (fraternal), meaning they develop from two different egg cells, each fertilised by separate sperm cells.
Annie gives birth to 14 puppies (including twins)!!!
Two puppies born in October are in fact identical twins, a team of veterinarians reports. This is the first time that a genetically confirmed set of identical twin dogs has been reported in the scientific literature.
“What happened in this case … would have been the same thing that happens in a woman when she has identical twins,” Carolynne Joonè, a veterinarian and lecturer at James Cook University in Australia, and co-author of the report of the finding, told Live Science.That is, early during the mother dogs pregnancy, a fertilized egg split in two, creating two genetically identical embryos, Joonè said.
The Irish wolfhound puppies were first suspected to be twins when, during their birth last October in South Africa, veterinarian Kurt De Cramer observed that the two puppies had shared a single placenta. The puppies mother had been straining to give birth for several hours and was taken to De Cramer, who performed a Cesarean section, Joonè said.
De Cramer began cutting at the site of an unusual bulge in the uterus. From that incision, he removed one puppy, and then saw that there was another fetus within the same placenta. Very excited, De Cramer “put [the newborn puppies] on the table next to him, within the [operating] theater, and quickly got assistants to take photographs of these pups still connected to a single placenta,” before cleaning them up as usual, Joonè said.
The veterinarian also delivered the mothers other five puppies, each with its own placenta, the authors wrote in their paper. [Seeing Double: 8 Fascinating Facts About Twins]
De Cramer thought it possible that the placenta-sharing pups were monozygotic twins, Joonè said. To test this hypothesis, when the puppies were 2 weeks old, De Cramer, Joonè and another colleague, Johan Nöthling, a veterinarian and professor at the University of Pretoria, drew blood samples and sent them for genetic testing.
“They did look very alike, but they werent completely identical.” There were slight differences in the puppies white markings. But the DNA showed that the puppies had identical genes on 40 different markers that are commonly used in such testing.
A second DNA analysis, done with samples taken from cheek swabs, confirmed that the dogs were identical. The differences in white marking patterns are likely due to differences in gene expression between the two puppies, Joonè said.
Because this is the first documented case, researchers think that monozygotic twins in canines are rare, Joonè said. However, its also possible that such twins are born more frequently than thought, but go undetected, she added. [10 Things You Didnt Know About Dogs]
Its certainly not rare to see what, in an analogy to humans, could be called fraternal twinning in dogs, though.
“With dogs, they always have multiples. They always have twins, triplets, quadruplets and so on, but theyre all different eggs that have been fertilized [by different sperm],” Joonè explained. Human fraternal twins come from two separate eggs fertilized by two separate sperm.
As for what exactly causes a single fertilized egg, or embryo, to split into two embryos, whether in humans or in dogs, that remains mysterious, Joonè said. Twinning is “something that has been fascinating us for years,” she said.
Ashley P. Taylor is a writer based in Brooklyn, New York. As a science writer, she focuses on molecular biology and health, though she enjoys learning about experiments of all kinds. Ashleys work has appeared in Live Science, The New York Times blogs, The Scientist, Yale Medicine and PopularMechanics.com. Ashley studied biology at Oberlin College, worked in several labs and earned a masters degree in science journalism from New York Universitys Science, Health and Environmental Reporting Program.