Runt

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A runt is an animal that is unusually small for its species. They may also be called low birth weight, intrauterine growth restriction (IUGR), or small for gestational age (SGA). What counts as a runt can depend on the situation: it may be smaller than littermates, a certain percent of the mother’s weight, a breed- or species-specific weight, or unusual body proportions.

Runts face more health problems than normal siblings. They’re more likely to get sick, to die as newborns, and to have trouble staying warm, carrying energy, and growing. In farming, runt calves or piglets also mean economic losses: runt cows may produce less milk and have fertility problems; IUGR piglets can have muscle changes that affect meat; adult IUGR sows may have smaller litters and lighter piglets; lambs born light may lead to poorer fleece.

SGA has been studied a lot in pigs because they have high preweaning mortality and because pigs are used in science. Runts come from a mix of genes, the in‑utero environment, and how the mother is cared for. When many piglets are born, there may not be enough teats, birth can take longer, and some babies get less oxygen during birth. Other causes include viral infections like circovirus, poor maternal nutrition, or a small or poorly placed placenta. In cattle and sheep, hot weather during pregnancy can raise the risk. For dairy cows, being pregnant while lactating can limit nutrients for the fetus. First-time mothers are more at risk. In dogs, larger litters tend to bring more low-weight puppies. In cats, younger mothers often have lighter kittens, and a stillbirth can raise the chance of low birth weight.

For pet owners and vets, some tools help identify at-risk puppies. In dogs, watching Apgar scores at birth and tracking puppy weight relative to the mother can improve early growth. But predicting which puppies will be runt is hard because dog breeds vary greatly in size. Some methods use the puppy-to-mother weight ratio or breed-specific birth weight thresholds.

In farming, heavy birthing effort is sometimes used to help but isn’t practical for large operations. A better strategy is to improve the mother’s nutrition and avoid breeding for IUGR. In cows, runt calves are less productive; some suggest directing SGA calves to veal and focusing on average-sized, productive calves.