why most animals have litters of babies when they give birth, but when humans give birth to twins or triplets, it’s considered rare?
In: Biology
Animals have different strategies for continuing their species. Some have lots of babies in the hope that one or two will survive, some have only a few and work harder to make sure those few make it to adult hood. Humans spend a lot of time and energy making sure that their one kid is born and survives to adulthood and is prepared to live on their own. Wild dogs, cats, and many other mammals spend not quite as much time and energy making sure that the few young they have survive to adulthood, so they have more young to increase the chances that some survive. Some animals like fish don’t spend any time taking care of their young, so they have thousands to increase the chances that a few will survive.
Also, the longer you spend taking care of your young after it is born, the less developed it has to be when born. For example: baby fish are born tiny, but completely developed. They don’t need to be cared for. But human babies are not completely developed. They can’t even walk or crawl on their own, they need to be cared for.
The more care young requires, the more intelligence and empathy a species has to have to keep their young alive. Human children need 18 years of care, and we have to be very intelligent to manage that, and we also form significant familial bonds. Kittens only require about eight months of childhood before they are completely self sufficient and can breed, so cats don’t have to be as smart and they don’t often care about their young after they grow up.
The number of young you have is an evolutionary trade off of having more or less young, to sacrifice more or less effort in raising them, to increase your chances of having surviving offspring to carry your genes.
Welcome to the wonderful world of breeding strategies. In short, most animals can be categorized as as one of two breeding strategies.
Humans, elephants, great apes and others have a small number of offspring that take a relatively long time to raise and so require significant investment, but have a higher rate of survival.
The other, much more common strategy, is to have as many offspring as possible which require little or no additional resources from the parent. This could be a litter of eight or a clutch of THOUSANDS.
Evolutionarily speaking, less sophisticated species are successful because of shear numbers. (Compare the hundreds and thousands of offspring frogs have with the scores that rabbits have with the dozens that cats / dogs have). Generally not only is it more “expensive” to grow a sophisticated animal, but also advanced social behaviours (parenting and community in homo sapiens) have meant that it’s more successful to have a small number of very well looked after children. This capability in turn has led to the evolutionary pressure in humans to tend to bear one child at a time – it gives them maximum growth advantage.
Animals have different shapes in the uterus that allow animals to do this. Additionally, animals release more eggs than humans do. What’s interesting is that for a female cat, the eggs line up in the tubes of the uterus, and the kittens will be lined up.
Humans have big brains, meaning big heads, which makes it really hard to fit multiple babies in a creature that walks upright.
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