Having a new baby is an exciting experience for parents, although it also comes with a lot of work. When the new family member is a pair of babies, it would also bring a different feeling and challenges.
In the animal kingdom, it is actually the norm to deliver multiple babies at once. Meanwhile, multiple births in humans make up only about 3 in 100 pregnancies.
How are Twins Formed?
Twins refer to two offspring produced by the same pregnancy. They can be identical (monozygotic) or fraternal (dizygotic).
Identical twins are formed when a single sperm cell units with a single egg cell. The fertilized egg cell, called zygote, then splits into two separate cells that would later develop into identical individuals. Since they came from the same fertilized egg cell, identical twins have exactly the same genetic information, the same gender, and look exactly alike.
On the other hand, fraternal twins occur when a female releases more than one egg cell which gets fertilized individually at the same time. For fraternal twins, one can be male and the other can be female. Basically, fraternal twins are just similar to any set of siblings.
Multiple Animal Babies
Each animal species has its own standard number of babies per birth. For instance, a study on a group of purebred dogs revealed that the average number of puppies per litter differs according to the size of the dog breed. Miniature breed dogs that weigh less than 10 pounds (4.5 kilograms) deliver an average of 3.5 puppies per litter. Meanwhile, giant breed dogs that are more than 100 pounds (45 kilograms) bring forth an average of over seven babies per litter.
Having twins is also the most common pregnancy outcome in goats, although the mother goat can give birth to a single offspring. Sheep can also have twins, but single-born lambs are much more common. For horses and cows, they tend to deliver just one baby at a time, but twins may also occur.
A lot of animals can give birth to multiple offspring. A more complicated question, however, is whether these animal babies born together are identical or fraternal twins.
For female dogs and cats, ovulation involves releasing multiple eggs at one time. When individual egg cells are fertilized by distinct sperm cells from a male, multiple embryos are formed. As a result, the female gives birth to babies that are fraternal, even if they look very much the same.
Scientists believe that it is very rare for animals to produce identical twins. The tricky part, however, is that a lot of animal siblings look very similar, so experts would need to conduct a DNA test to confirm whether they share all their genes. So far, there is only one documented report of identical twin dogs which was verified by DNA testing. Still, biologists do not know for sure how frequently fertilized animal egg cells divide and develop into identical twin animal babies.
Reproduction is also different in various species of animals. For example, nine-banded armadillos typically give birth to identical quadruplets. After a female armadillo ovulates and the single egg is fertilized, it divides into four separate identical cells which grow into identical babies. On the other hand, a seven-banded armadillo can give birth from seven to nine identical offspring at a time.
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