By Dr. Becker
The average number of puppies in a litter depends on the mother’s breed, size and other factors, but typically includes two to six pups. Those puppies are technically fraternal twins, meaning they each developed from different eggs and have unique DNA. They may look similar and have similar behaviors, which is to be expected, since they’re siblings from the same litter.
In some cases, an egg may split and form two embryos, which leads to identical twins. Identical twins are known to occur, albeit rarely, in a number of species, including dogs, pigs, cattle, horses and armadillos,1 but it wasn’t until 2016 that a case was scientifically identified in dogs.
Irish Wolfhound Gives Birth to Identical Twin Puppies
A 4-year-old Irish wolfhound in South Africa was taken to a veterinarian after being in labor for two hours with no progress. The veterinarian, Kurt de Cramer, performed a caesarean section and soon noticed an “unusual bulging by her uterus.”2 It turned out there were two puppies attached to the same placenta, a very unusual finding that tipped de Cramer off that the puppies might be identical twins. Typically, each puppy in a litter has its own placenta.
"When I realized that the puppies were of the same gender and that they had very similar markings, I also immediately suspected that they might be identical twins having originated from the splitting of an embryo," he told BBC News.3
When the pups were 2 weeks old, they underwent genetic testing that revealed they are, in fact, identical twins, the first case that’s been genetically confirmed in a dog.4 Although the twins had slight differences in markings on their paws, chests and tips of their tails, this is likely the result of differing genetic expression.
"Human identical twins also have the same genes, but because those genes are expressed differently in each person, they have different freckle and fingerprint patterns," de Cramer explained.5 This is the first case of identical twins formally identified in a dog, but that doesn’t mean it’s the first time it’s ever happened.
Under ordinary circumstances, most dogs give birth naturally, and determining whether each puppy had its own placenta could be difficult, especially since dogs tend to eat the placentas soon after birth. Many dogs also have similar markings, which means the only way to determine if they were identical or fraternal twins would be via genetic testing. As Dr. Jerry Klein, chief veterinary officer of the American Kennel Club wrote:6
“It will be interesting to see how these brothers grow and how similarly their appearance, behavior, and health develop. If they do go 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.”
Thinking About Adopting Two Pups From the Same Litter?
If a litter of pups is brought into your local shelter, you may be tempted to take home a pair of littermates instead of just one. While this can work out fine, it’s important to be aware of the potential for littermate syndrome, which can lead to some undesirable behaviors.
For instance, two siblings adopted together may bond intensely with each other, such that one becomes the leader and the other the follower. The second-in-command dog may then look to its sibling as its leader rather than looking to you for social cues and commands.
In some cases, littermates may also develop separation anxiety when they’re separated for even short time periods or may become aggressive toward one another. Again, this doesn’t happen with every pair of littermates raised in the same home, but it happens often enough that some animal behaviorists, dog trainers and rescue professionals discourage dog guardians from acquiring puppy littermates.
If you already have a pair of littermates, take steps to treat them as individual dogs to help discourage an unhealthy emotional dependence from developing. You should, for example, enroll each in separate training sessions and give them separate spaces (i.e., crates) at home. At least some of the time, take each dog for walks independent of the other, and even engage them in play sessions at different times so they can each come into their own.
Are You Interested in Your Dog’s Genetic Background?
Whether you have a pair of littermates you suspect may be identical twins or a rescue dog that you’re intrigued to know the breed of, genetic testing for dogs is now available. DNA tests are available for varying costs that can tell you your dog’s breed, ancestry and risk of certain genetic diseases.
No matter what such tests reveal, keep in mind that your dog’s personality, health and behavior are the result of both nature (i.e., genetics) and nurture (i.e., environment), which means you can influence your dog’s genetic destiny for the better by providing appropriate nutrition, exercise and mental stimulation.
On a larger scale, a citizen science research project currently underway to understand how specific genes control the behavior and health of our canine companions is Darwin's Dogs, led by Elinor K. Karlsson, Ph.D., a canine geneticist and assistant professor at the University of Massachusetts Medical School.
Karlsson and her team are asking dog guardians to record their own observations of their pet's behavior and personality, and collect doggy DNA at home using mouth swabs provided by Darwin's Dogs.
There's no cost to the dog owner, and the researchers share any information they find, including how genetics may relate to certain behaviors and personality traits in dogs. As in humans, it’s likely that the now-confirmed existence of identical twins in dogs will reveal that, despite their identical appearance and DNA, each dog is an individual, a product of both her genes and her environment.