Assuming you’re human, the likelihood of having twins is quite little. But, depending where you’re from, there’s a surprising amount of variation. In Central Africa, the twinning rate is almost 30 in every 1,000 births; in much of Asia and Latin America, the average is as low as eight per 1,000 births.
There are, of course, different types of twins. In this article we’ll see how different twins develop and find out about some fascinating examples from the animal kingdom!
Types of twins
The first distinction to make when discuss twins is whether they are identical or fraternal twins. What’s the difference? Here’s a quick refresher!
Fraternal twins, also known as dizygotic twins, are the most common type of twin. This is when two eggs are fertilised by two different sperm cells. So, in fact, fraternal twins are no more genetically similar than any other siblings.
Identical twins, or monozygotic twins, occur when a single egg is fertilised to form one zygote. This then divides to make two separate embryos. Giving birth to identical twins is not hereditary, so the rate of it happening is relatively constant all over the world.
Have you ever heard of mirror twins? This is a subset of identical twin, when the fertilised egg splits later (9-12 days). Mirror twin babies are essentially reflections of each other; they will have opposite features. For example, one will likely be right-handed and the other left-handed.
It’s also possible for the egg to split before fertilisation. If the resulting eggs are fertilised by two separate sperm cells, the twins will be half identical and half fraternal. These twins are called polar body twins.
Twins in the animal kingdom
So that’s humans covered, but what about twins in the animal kingdom? Keeping in mind these different categories, the answer should be fairly obvious. Whenever an animal gives birth to multiple young, the offspring are fraternal twins. Well, if not twins then fraternal triplets, quadruplets or quintuplets. You get my point!
Different animals have different size litters. Primates, like humans, rarely have more than one baby at a time. Rabbits tend to have litters of three or four, whereas foxes have a maximum of 10 or 11.
Naked mole-rats and the one-half rule
Interestingly, you can estimate the average litter size of a mammal by following the one-half rule. This rule is based on the observation that the size of a mammal’s litter is roughly half the number of mammaries the female has.
This ensures that there is space for all offspring to feed and, therefore, presents a clear evolutionary advantage. However, as with any rule, there are always exceptions.
The naked mole-rat, one of our all-time favourite forms of life, is one of these exceptions. The female has just 11 mammaries and can produced litters of up to 28 pups! How do they cope? Well, it turns out that baby naked mole-rats are just incredibly good at sharing.
Identical Irish Wolfhounds
This covers fraternal twinning, but what about identical ones? Monozygotic twins are extremely rare in the animal world. When two fetuses attach to one placenta, one or both of them tend not to get enough oxygen and don’t survive the pregnancy.
That’s not to say it can’t happen. The first ever case of identical twins in dogs was confirmed by scientists just a couple of years ago. Cullen and Romulus, twin Irish wolfhounds, were born in South Africa in 2016.
Initially there was some doubt about it, due to small differences in the markings of their fur. However, this is to be expected. Even though monozygotic twins have the same DNA, genes can be expressed differently. It’s the same reason why identical human twins have distinct fingerprint patterns.
Remarkably, there’s one animal out there that always has identical babies: the armadillo. Most members of the genus Dasypus, including the nine-banded armadillo, give birth to four monozygotic young.
This process, through which multiple embryos develop from a single fertilised egg, is called polyembryony.
Why does this happen in armadillos? A female armadillo can only carry one egg. Therefore, polyembryony is the only way for an armadillo to produce more than one offspring at a time. It’s simply an evolutionary adaptation in response to a natural limitation.