A runt is an unusually small animal. The term is often used specifically in reference to the smallest animal in a litter. Numerous animals typically bear a runt with each litter, perhaps most famously in the case of pigs, and the survival of runts has been a topic of scientific interest and study historically. The term is also sometimes used as a slang insult for especially small children, carrying connotations of weakness and inconsequence.

The origins of this word are rather interesting. The first appearance of “runt” in English was in 1501, in reference to an old or decaying tree stump, but no one is sure where the word came from in the first place. By 1549, people were using the term to talk about unusually small animals, and in 1841, runt in the sense of the smallest in a litter entered common usage.

The runt of a litter is often very noticeable, as he or she will be much smaller than the others in the litter. Many runts demonstrate a failure to thrive, as they are unable to compete with their siblings for food and warmth. If they survive, runts may grow up to be smaller than other animals of the same species, and they sometimes develop unique personality traits as a result of their stunted childhoods. However, when a runt is taken away from the litter and given supplemental food and attention, it may develop normally.

Some people theorize that the runt is designed to be a potentially sacrificial member of the litter. Under this theory, if the runt survives, so much the better, but if resources are scarce, the runt will not be missed. In some cases the runt may even be eaten by the parent animal or by its siblings as a response to limited resources or stress; this is a particularly infamous problem with pigs.

Many people have romantic associations with the runt of the litter, thinking of runts as small, helpless, and weak. Some people may choose to adopt a runt to give it a better start in life as a result, and the heroes of many children's books are runts, illustrating an ability to overcome adversity. The famous Wilbur of Charlotte's Web, for example, is a runt.