In a group of animals (usually a litter of animals born in multiple births), a runt is a member which is significantly smaller or weaker than the others.[1] Owing to its small size, a runt in a litter faces obvious disadvantage, including difficulties in competing with its siblings for survival and possible rejection by its mother. Therefore, in the wild, a runt is less likely to survive infancy.
Even among domestic animals, runts often face rejection. They may be placed under the direct care of an experienced animal breeder, although the animal's size and weakness coupled with the lack of natural parental care make this difficult. Some tamed animals are the result of reared runts.
Not all litters have runts. All animals in a litter will naturally vary slightly in size and weight, but the smallest is not considered a "runt" if it is healthy and close in weight to its littermates. It may be perfectly capable of competing with its siblings for nutrition and other resources. A runt is specifically an animal that suffered in utero from deprivation of nutrients by comparison to its siblings, or from a genetic defect, and thus is born underdeveloped or less fit than expected.
Research in a news journal about a runt puppy highlights important factors including maternal care, genetic factors, health concerns, and personality development associated with runt puppies. Maternal care is crucial as, in some cases, the runt may face difficulties competing with their larger siblings for nutrients. It is important that a runt receives its fair share of milk from their mothers so they can continue growing. Genetic factors play a role in why a puppy is born a runt; it could be because of fertilization process or placental issues. They may face health issues like a heart defect, cleft palate, and any organ defects. Getting a veterinary evaluation is crucial to address potential health issues. Not all runt puppies are weak or unhealthy. With proper care and attention, runts can have positive personality traits and be well socialized and happy.
Another animal species where the birth of runts is common is in pigs, especially in large litters where competition for resources is higher. Research has shown that genetic improvements have been made in pig breeding that have resulted in an increase in low birthweight piglets, known as runts. They face challenges in accessing colostrum and milk, which are a competitive environment in large litters. A study in the National Library of Medicine showed the effects of uniform litters of different birth weights on piglets’ survival and performance. When a litter of piglets are similar in size, runts have a better chance of survival since there is less competition between all of them. By understanding and improving litter uniformity, farmers and animal caregivers give the runt of the litter a chance for survival, reducing pre-weaning mortality.
Runts, whether puppies or kittens, need special attention and care, so they have the best survival rate. The Akron Beacon Journal mentioned runts are adopted faster from shelters as their small size and perceived vulnerability makes them appealing to potential adopters. While there are many risks once they are born, once they make it to 6-8 weeks the chance of survival is high, and the runt will most likely grow to full size. While domesticated runts have a higher likelihood of being wanted, this is not the case with runts of wild animals like pigs. In agricultural settings, when a runt pig is born, a farmer is most likely inclined to cull the animal as they will not be able to reach the proper size for meat production. In the wild, a runt's chance of survival is lower as only the strongest survive. Wild animals do not have the same opportunities for care as domesticated animals.
https://www.whole-dog-journal.com/puppies/what-is-the-runt-of-the-litter/
https://www.beaconjournal.com/story/lifestyle/home-garden/2012/10/06/as-pets-runts-can-be/10712851007/