Rats exhibit a wide range of fur colors, leading many to wonder if they typically have white bellies. The science of rat coloration involves a fascinating spectrum of genetic and environmental influences.
The Spectrum of Rat Coloration
Many rats, especially those bred in captivity, exhibit white bellies. This trait is common in domesticated or “fancy” rats, which display a wide array of colors and patterns due to selective breeding. For example, a “Berkshire” rat has a colored top coat with a white belly and feet. A “hooded” pattern features a white body with a colored head and a strip down the spine.
Wild rats, conversely, generally have more uniform, camouflaged colors to blend into their natural environments. The common wild Norway rat often has a brown body with a lighter underbelly, ranging from white to cream or silver-gray. Some wild species, like the chestnut white-bellied rat, are named for their bright chestnut upper-coat and white under-coat.
Beyond belly patterns, rat coloration includes numerous other possibilities. Domesticated rats come in solid colors like black, blue, champagne, or mink. Other patterns include “Irish” rats with a white chest triangle, “Dalmatian” rats with white bodies and black spots, and “Siamese” rats with light bodies and darker points on their extremities, similar to Siamese cats. These diverse patterns result from inhibited or enhanced pigment production.
Why Rat Colors Vary: Genetics and Environment
The diverse fur colors and patterns in rats are primarily determined by genetics, with specific genes controlling pigment production and distribution. Two main pigments, eumelanin (black/brown) and phaeomelanin (yellow/red), create the overall coat appearance. Different gene locations, or loci, dictate how these pigments are expressed, leading to color and pattern variations.
For wild rats, natural selection significantly shapes their coloration. Their typical “agouti” coat, characterized by individual hairs banded with multiple colors like black, gray, brown, and orange, provides effective camouflage. This banded appearance, or “ticking,” creates fur depth, helping them blend into varied natural backgrounds and avoid predators. Lighter bellies also offer a survival advantage by making the rat less visible against the sky when viewed from below.
In domesticated rats, selective breeding has expanded color and pattern diversity. Breeders intentionally select rats with specific aesthetic traits, including white bellies, to create new varieties. The presence of white patches, known as piebaldness, links to genes like the “hooded” locus. Studies show that selecting for tameness can increase depigmentation, leading to more rats with white bellies over generations.