One new baby is handful, but what about two or more? One in thirty parents have their hands full with twins and about one in 10,000 juggle triplets or more, consistent with U.S. data from the Centers for Disease Control & Prevention.
But how unusual is human’s ability to bear multiple young at-once and the way often do other mammals have twins, triplets & quadruplets?
In many animals, multiple babies are the norm instead of the exception.
For instance, the average dog gives birth to 5 puppies in a litter, consistent with a 2011 study published in the journal Theriogenology. So, what is the difference between a litter & twins? ” It really just comes down to number,” said Charles Long, a reproductive physiologist at Texas A&M University. Whereas a set of twins is by definition 2 babies, litter-bearing animals nearly always have more.
A female animal have a litter, when she releases multiple eggs. When fertilized, these-eggs produce multiple embryos. Identical twins are the exception, but we will get to that later.
When a woman releases 2 eggs, we call the resulting offspring fraternal or non-identical twins. 3 eggs lead to fraternal triplets, 4 in fraternal quadruplets. Whether they are littermates or fraternal twins, these siblings share about half their DNA, the same as any other sibling pair.
Litter-bearing animals tend to give birth to more than 3 babies, on average, Long said. But the difference between litters & twins ends there.
Technically, there is no difference between a litter of 5 and a set of fraternal quintuplets, apart from the very fact that human quintuplets are exceptionally rare.
In 2017, the Centers for Disease Control & Prevention reported only 49 quintuplet and higher order births out of 3,855,500 total births.
On the other hand, some animals rarely give birth to litters and instead nearly-always bear two young with each pregnancy.
In these animals, two babies born at the same time, called twins, not littermates. However, these are usually fraternal, not identical twins. Sheep, goats & deer all regularly give birth to fraternal twins, Long said.
But what about identical twins?
These genetically identical siblings form, when one fertilized egg splits in two. They are rare in humans: about 3 to 4 in 1,000 human births leads to identical twins, consistent with the U.S. National Library of Medicine.
And as far as scientists can tell, they are less common in other animals. Veterinarians have identified identical puppies only once, in 2016.
In that rare example, a veterinarian performing a C-section on an Irish wolfhound happened to notice that two of the puppies shared a placenta. A genetic test confirmed that the 2 puppies shared all of their DNA. But scientists usually are not on the lookout for identical twins in other animals, which can be why they appear so uncommon.
In one animal, identical sibling are the rule, not a rarity.
Armadillos always-give birth to identical quadruplets. In other words, female armadillos ovulate one egg that subsequently splits-into four once fertilized.
Scientists are not sure why this happens or why it is unique to armadillos, Long said.
One possible explanation is having identical quadruplets prevents inbreeding. Armadillo siblings cannot possibly mate with one another, so they are forced to venture outside of their underground burrows to find out mates.
And other animals may give birth to identical twins at higher-rates than humans realize. “If a sheep has twin males or twin females, we do not go and test them to-see if they are identical,” Long said.
The uncanny resemblance shared-by identical twins is not a helpful clue, he added. “You know, sheep look alike.”