A runt is the smallest, weakest newborn in a litter of animals. The term is most commonly used for puppies and kittens, but it applies to any species that gives birth to multiple offspring at once, including pigs, rabbits, and rodents. While the word “runt” is casual and widely understood, veterinarians use more precise language: low birth weight, small for gestational age, or intrauterine growth restriction. Whatever you call it, the runt faces real disadvantages from the moment it’s born.
How Veterinarians Identify a Runt
There’s no single weight cutoff that makes an animal a runt. Instead, vets look at several factors: how the newborn’s size compares to its littermates, its weight as a percentage of the mother’s body weight, breed-specific weight thresholds, and whether its body proportions differ from normal. A puppy-to-mother weight ratio can help flag underweight puppies in mixed breeds, while breed-specific thresholds work better for purebreds, since birth weights vary considerably even among large and giant breeds of similar adult size.
In practical terms, the runt is usually obvious. It’s noticeably smaller than the rest, may appear thinner, and often struggles to compete for a nursing spot. Some runts are only slightly smaller and perfectly healthy. Others are dramatically undersized, which signals a more serious problem.
Why Some Newborns Are So Small
A runt’s small size can stem from several different causes, and the reason matters for the animal’s long-term outlook. The most common explanation is poor placental positioning. In species like dogs and pigs, each fetus attaches to the uterine wall via its own placenta. A fetus that implants in a less blood-rich area of the uterus receives fewer nutrients throughout gestation, resulting in restricted growth. This is essentially a nutritional disadvantage that begins before birth.
Other causes include congenital defects (heart abnormalities, organ malformations), infections passed from the mother during pregnancy, or genetic conditions that impair growth. A runt whose small size is purely due to a less favorable spot in the uterus has a much better chance of catching up than one born with a structural heart defect or other underlying condition. Unfortunately, the cause isn’t always obvious at birth, which is part of why runts carry higher risk overall.
The Dangerous First 48 Hours
The biggest threats to a runt’s survival come down to two interconnected problems: getting too cold and running out of energy. Newborn animals can’t regulate their own body temperature, and the smallest ones lose heat fastest because of their higher surface-area-to-weight ratio. That temperature drop triggers a cascade. Cold newborns burn through their limited energy reserves trying to stay warm, which causes blood sugar to plummet. Among growth-restricted newborns, nearly 60% of those who become hypothermic develop at least one episode of dangerously low blood sugar.
Low blood sugar in a newborn is not just uncomfortable. It causes progressive weakness, reduced nursing, and if uncorrected, organ failure and death. The cycle is vicious: a cold newborn nurses poorly, which means less energy intake, which means less ability to generate heat, which makes the cold worse. Runts are especially vulnerable because they have smaller glycogen reserves (the stored sugar that acts as an emergency fuel supply) and less body fat for insulation. Hypothermic newborns are transferred to emergency care at more than four times the rate of those who maintain normal temperature.
This is also why runts often get pushed aside by larger, stronger siblings. A slightly weaker newborn that can’t fight for a good nursing position falls behind quickly, and the gap widens with every missed feeding.
Fading Puppy Syndrome
The worst-case scenario for vulnerable newborns is fading puppy syndrome, a condition where seemingly healthy puppies decline rapidly and die within the first few weeks. In one prospective study tracking 165 puppies, fading puppy syndrome affected about 13% of them, with a 100% mortality rate. The symptoms are frustratingly vague: decreased nursing, progressive weakness, and eventual death. By the time it’s noticeable, treatment is rarely successful.
Runts aren’t the only puppies affected by fading syndrome, but their pre-existing disadvantages (lower birth weight, weaker nursing, less energy reserve) put them at higher risk. Research has linked the condition to early disruptions in gut bacteria, suggesting the problem may begin before any visible symptoms appear. This is one reason why close monitoring of the smallest puppies in a litter is so important from day one.
How to Care for a Runt
If you’re raising a litter that includes a runt, the two priorities are warmth and nutrition.
For temperature, the environment where newborn puppies are kept should be 85°F to 90°F for the first four days of life. You can gradually lower this to 80°F by day seven to ten, and to about 72°F by the end of the fourth week. Heating pads and heat lamps work, but use caution: newborns can’t move away from a heat source the way adult animals can, so burns are a real risk. Place heating elements to one side so puppies can shift away if needed.
For feeding, watch whether the runt is nursing effectively and gaining weight daily. If it’s being pushed off the nipple by larger siblings, you may need to hold other puppies back and give the runt dedicated nursing time. When the mother’s milk supply isn’t enough or the runt can’t latch well, supplemental feeding with a commercial milk replacer fills the gap. Warm the formula to 95°F to 100°F (roughly skin temperature on your inner forearm) before feeding. Puppies under two weeks old need to eat every three to four hours. Between two and four weeks, every six to eight hours is sufficient.
If the mother can’t nurse at all, whether from illness, infection, or death, the entire litter needs full replacement feeding or a foster mother. Without intervention, puppies can die within 24 to 48 hours. Hand-reared newborns also need help with elimination during the first couple of weeks. Gently wiping the genital area with a warm, damp cloth after feeding mimics the mother’s licking and stimulates them to urinate and pass stool.
Do Runts Stay Small?
This depends entirely on why the animal was small in the first place. A runt whose growth was restricted by poor placental positioning but is otherwise healthy often experiences significant catch-up growth once it’s eating well on its own. These animals may end up slightly smaller than their siblings as adults, or they may close the gap entirely. There’s no guaranteed timeline, but most of the catching up happens in the first several months of life as the nutritional playing field levels out.
Runts born with congenital problems, chronic health issues, or genetic growth disorders are more likely to remain undersized. For purebred dogs, breed standards provide a useful reference point. If an adult dog falls well below the expected weight and height range for its breed, that’s a sign the runt status had lasting effects. For mixed breeds, the comparison is harder to make since there’s no standard to measure against.
Many runts that survive the critical newborn period go on to live completely normal, healthy lives. The idea that runts are permanently fragile or sickly is largely a myth. The real danger is concentrated in those first days and weeks, when size disadvantages have life-or-death consequences. Once a runt makes it through that window and is eating, growing, and maintaining body temperature independently, the prognosis improves dramatically.
Temperament and Behavior
There’s a popular belief that runts grow up to be either especially timid or compensationally feisty. Neither claim has strong scientific backing. No peer-reviewed research has established a reliable link between runt status at birth and specific personality traits in adulthood. A dog or cat’s temperament is shaped by genetics, early socialization, and life experiences, not by how much it weighed on day one.
That said, runts that are separated early from the litter for intensive care, or that miss out on normal sibling interactions during the first weeks, may have gaps in socialization that affect behavior later. This isn’t a runt-specific trait. It’s the same effect seen in any animal that misses key developmental windows, regardless of birth weight.