Why do some mammals have litters?

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Why: I don't know. I think I had a nightmare about a horse placenta. Dogs and cats have litters, but horses and human ladies usually only have 1 or 2 babies at a time - unless they were having trouble conceiving and selfishly implanted themselves with fertilization treatments instead of adopting abandoned babies who are already alive.

Answer: For one thing, a litter helps protect the animal from predation, aka getting eaten by other animals. It's not that the baby animals won't get eaten (because they probably will), but like if a snake eats 3 baby bunnies, but there are still 5 left, its parents didn't waste a whole breeding season.

Another reason is because it helps natural selection or whatever. The strongest and healthiest baby animals compete most successfully for food and space, often leaving the weaklings and runts to die from lack of care. If there are 8 nipples and 12 baby mice, a few might just starve to death. The mother doesn't interfere.

Biologically, an animal that can have a litter has a bicornuate (two-horned) uterus designed to handle multiple embryos. In a human, this is considered a defect and can cause problems with fertility.

Source: Wikipedia, MadSci Network

The More You Know: In human multiple births, there is a weaker "runt," but human parents take care to make sure that both or all babies get the same equal start. The more babies, however, the more likely it is that there will be a noticeable physical difference in the runt. I'm looking at you, Aaden.