Rats, whether found in the wild or kept as companion animals, exhibit a broad range of colors, extending beyond the typical gray or brown most people imagine. A rat’s coat color is a direct result of its species and environment, or in the case of pet rats, decades of selective breeding. This variation provides clues about the rat’s origin, genetic makeup, and natural camouflage.
The Colors of Wild Rats
The two most common wild rats globally are the Norway rat (Rattus norvegicus) and the roof rat (Rattus rattus). Their coloration serves the primary purpose of camouflage. The Norway rat, often called the brown rat, typically displays coarse, grizzled brownish-gray fur on its back with a lighter gray or whitish belly. This coloring allows it to blend into environments where it burrows, such as sewers, basements, and ground-level structures.
The roof rat, sometimes known as the black rat, generally features a darker palette, with sleek fur ranging from dark gray to solid black or deep chocolate brown. This darker coat is advantageous for this agile climber, which often nests in high places like trees, attics, and upper building levels. While habitat dictates these distinct color patterns, both species can display slight variations of tan, buff, or reddish-brown depending on local genetics.
Pure white or albino rats are virtually nonexistent in wild populations. A lack of pigment would compromise their ability to hide from predators, drastically reducing survival chances. The wild color standard is characterized by the agouti pattern. In this pattern, each individual hair shaft is banded with two or more colors, typically dark at the base and lighter near the tip, creating a grizzled appearance.
The Spectrum of Domestic Rat Colors
The domestic pet rat, or “fancy rat,” is descended entirely from the Norway rat, but its color range has been exponentially expanded through selective breeding. This intervention has produced an array of coat colors and patterns not present in their wild ancestors. Colors are often grouped into categories, such as “self,” meaning the entire body is one uniform color, or “agouti,” where the hairs are banded.
“Self” colors include deep shades like Black, Blue, and Chocolate, as well as lighter hues such as Beige, Champagne, and Lilac. Blue, for example, is a dilute version of black, appearing as a bluish-silver. Over 40 distinct colors are officially recognized by organizations like the American Fancy Rat & Mouse Association (AFRMA).
Beyond solid colors, domestic rats display complex patterns and markings separate from the base color. Eye color can also vary, ranging from common black to ruby, pink, or even odd-eyed (where each eye is a different color). Intricate patterns include:
- Hooded: Color only on the head and shoulders, with a stripe running down the spine.
- Berkshire: A colored top with a white belly, feet, and tail tip.
- Blaze: A white stripe on the face.
- Dalmatian: Splashes of color across a white body.
The Genetics Behind Rat Coloration
The vast spectrum of rat colors is governed by two fundamental pigment types: eumelanin (responsible for black and brown colors) and pheomelanin (which produces yellow and red hues). The distribution and concentration of these pigments within the hair shaft are controlled by different genes located at specific positions, or loci, on the rat’s chromosomes.
The Agouti locus determines the distribution of yellow pigment and controls the banding pattern on the hair shaft. Rats with the dominant wild-type allele exhibit the agouti pattern, while those with the recessive allele have a solid-colored, non-agouti coat. The C locus, or Albino locus, is another major factor, regulating the synthesis of all melanin by controlling the enzyme tyrosinase.
A full mutation at the C locus results in a complete absence of both pigments, producing a pink-eyed white or albino rat. Partial mutations at this locus lead to temperature-sensitive colors, such as Siamese or Himalayan, where pigment only develops in the cooler extremities of the rat’s body. Selective breeding has isolated and combined these genetic mutations, enabling breeders to produce colors like Champagne, which is a black-based rat with a specific dilution gene that turns the coat light tan.