i have always wondered whyanimals like dogs and cats have litters at a time while humans tend to have a single children? sorry if this is a stupid question

koryos:

It is not at all a stupid question- it’s a question a lot of biologists have devoted a lot of time examining, after all, since it says a lot about an animal’s lifestyle.

The amount of offspring an animal has depends on several factors, but the main one is how many of them, on average, will survive to adulthood. For an animal like a mouse, for instance, which is basically food on four legs, it makes sense to have LOTS of babies. For an animal with tiny, tasty free-swimming larvae like a crab, it makes sense to have a HORDE of tiny babies.

For an animal like you- like a human- while it certainly isn’t the case that all human babies are bound to survive to adulthood, there’s a much higher chance of it. So instead of investing energy into having MANY babies, we invest energy into making our ONE baby be very big and strong. We can afford to do that if there’s less of a chance of it dying. If a mama mouse put all her time and effort into raising one pinky, it could very easily amount to nothing if pinky gets gobbled by a snake.

There is actually a name for these two strategies (and I should also mention that it is a spectrum, and those are the extreme ends)- r and K strategies. “r-selected” refers to organisms who have lots of babies, while “K-selected” refers to organisms who throw a lot of investment into only a couple at a time.

This graph shows what i mean. The y-axis is how many individuals of a species survive, and the x-axis is increasing age. So you can see that the amount of surviving oysters decreases sharply with age, versus the amount of humans.

r-selected organisms also have a number of other common traits, such as a short lifespan, small body size, and rapid sexual maturity. K-selected organisms are often the opposite, being large, slow-aging, and long lived. But these are not exclusive by any means; you can have very long lived animals like sea turtles, for instance, still producing great masses of eggs simply because the infant mortality is so high. Or you can have a tiny mouse-sized bat that lives 20 years to the mouse’s 2 and has one baby each season.

Basically, a lot of evolutionary factors go into an organism’s litter size.

Figures taken from these two pages, which both talk in more detail about K and r selection.