system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The runt of the litter

Why, when dogs have puppies, is there always--or, at least, I think always-- a runt of the litter? What happens to cause this, and why do mother dogs always seem to ignore the runt?

Edit: Also, does this happen outside the realm of domesticated animals?
gpt
In a sense, you will always have a runt, which is generally defined as the smallest and weakest of the litter. So in any given litter, you'll have the most relatively weak one.

From there, survival of the fittest plays out. From the runt's view, they are at a disadvantage for when it comes to getting resources, such as milk from the mother. During feeding, they'll be pushed out by their stronger siblings, and in many other cases they will be beaten out (or even beaten down) in other fights. This cause a bit of a loop, too weak to get feed, not fed enough to become strong.

From the mother's view, it's still survival of the fittest. Or rather, the death of those at the bottom of the list. The runt has the weakest chance to begin with and it's only going to get weaker (assuming no other force kills off the competition or intervenes to strengthen the runt), so why waste your time and energy raising it?