Scientists still aren’t 100% sure why runts exist.

Meet Monkey. She was the tiniest of three kittens her feral mom gave birth to. By the time she came to live with us, along with two brothers, she was 1 pound and had to be hand-fed. Her brothers were more than twice her size and perfectly happy to gobble down whatever we gave them.

Monkey was the runt of the litter. Meaning, she was smaller and weaker than all the other kittens born to her mom at the same time, by a significant amount.

Today she’s one of our fattest cats; you’d never know she had once been so much smaller than her siblings. (In fact, she outweighs her brothers by a good four or so pounds!)

Cats aren’t the only animals that can give birth to runts. Many other species can have runts including dogs, rabbits, rats, pigs, and even bats. (For those who have read Charlotte’s Web, remember Wilbur was the runt of his family.)

What Causes Runts?

Scientists still aren’t 100% sure why runts exist. The popular belief that runts are the babies that were squished in the middle of the uterus has been discarded as a myth. What is clear, is that during their development they didn’t receive the same level of nutrition that their littermates did.

Dr. Margaret V. Root Kustritz puts forth a more modern theory in her book “The Dog Breeder’s Guide to Successful Breeding and Health Management.” Dr. Kustritz suggests runts are the result of “poor placentation.” This essentially means the embryo did not get an adequate supply of nutrients from the mother’s bloodstream during pregnancy, hindering its growth.

After birth the struggle for nutrition often continues because of their smaller size. Pushed aside by their bigger siblings and sometimes ignored by their mothers, these undersized babies are at an extreme disadvantage. (In the wild, runts often don’t survive to adulthood.)

But we wondered, do all litters have a runt? One of the puppies or kittens or piglets in the litter is going to be the smallest, but does that automatically make it a runt?

The answer is no.

Most obviously, a litter of one baby will have no runt. In litters of multiple babies, if all the babies are relatively the same size, healthy, and capable of competing for mom’s milk and her attention, there is no runt.