As a rule, humans give birth to one child at a time, and the appearance of twins (two or rarely more) does not happen so often. Animals more often bring several cubs at once. But are they twins? Can animals give birth to twins at all?

The Conversation asked professors at the University of Mississippi College of Veterinary Medicine. And here's what they said.

What are twins?

In humans, twins are defined as two or more children from the same pregnancy. They can be identical. This means that one sperm fertilized one egg, but it somehow split into two separate cells that later developed into two identical babies. They share the same DNA, which is why the two twins are virtually indistinguishable from each other.

Twins can also be non-identical. In such a case, two separate eggs are fertilized at the same time but separately. Such twins have a different set of genes, they are similar to a certain extent, but not identical. These can be, for example, a pale-skinned blond and a swarthy brunet, or ones of different sexes.

How does pregnancy happen in animals?

Many animal species give birth to several cubs at once, and the number of cubs is more or less the same in each litter. For example, a study that examined more than 10,000 litters of pedigreed dogs found that the average number of puppies depends on the size of the dog breed. Miniature breeds such as Chihuahuas and Toy Poodles (under 4.5 pounds) gave birth to 3.5 puppies at a time. And giants, such as mastiffs and German dogs (over 45 kg), gave birth to more than 7 puppies.

However, when a dog gives birth to two cubs only, people tend to refer to the two puppies as twins. But twins are the most common pregnancy result in goats, although mother goats can give birth to a single calf and have more litter. It is also quite common for sheep to give birth to twins, but more often they bring lambs one at a time.

Horses with a gestation period of 11 to 12 months and cows carrying calves for 9 to 10 months usually give birth to only one calf at a time, but twins can occur. However, there is a considerable risk of birth complications and small and weakened calves that are less likely to survive.

What does DNA say?

If we measure such a pregnancy in an animal by human standards, then yes, animals give birth to twins. The more difficult question is whether two animal cubs born together are identical or identical twins.

Cats and dogs ovulate to produce several eggs at the same time. All of them are fertilized by different sperm and so several embryos are formed. That is, in these animals, the cubs are twins, but not identical.

Biologists believe that identical twins in most animals are very rare. We know of the only case where DNA testing showed that pups from the same litter turned out to be identical twins. Since such testing is infrequent, no one knows exactly how often animals split one egg into two embryos and give birth to identical twins.

But so far we have been talking about domestic animals and livestock. In the wild, you can find slightly different types of fertilization and pregnancy. Thus, females of the nine-banded armadillo usually give birth to identical quadruplets. They produce a single egg, which divides into four embryos after fertilization, and thus the animal gives birth to four identical twins. A related species, the seven-banded armadillos has gone further, giving birth to seven to nine identical twins in a single pregnancy.

Scientists are still largely uncertain when it comes to twins in other species. They don't usually do DNA testing on animals and therefore don't know how often identical twins are born to them. Therefore, it is safe to say that this happens in nature, but there is also a lot of room for discovery regarding how often and in what species.

Earlier OBOZREVATEL spoke about the animal that has the shortest life. We also explained why nature deprived ir of the digestive system.

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