Roof rats aren’t just gray. Our line carries several naturally-arising coat colors and markings we track across generations. Here’s the genetics, in plain language.
Three coats, seen up close
For the genetics behind these coat types, follow MC1R and black coats, RAB38 and blonde pigmentation, and ASIP and the agouti pathway. Each section explains what the roof-rat observations and the cited experiments tell us.
A blonde face in a woven basket, an agouti face looking out of a hammock, and a black rat curled in a hand: these portraits bring the coat descriptions to life. Start with the broad colour, then look at the face, back and sides. The photographs beside the questions below offer another angle on each coat. Together with the video, they give you a closer look at the rats behind the genetics.
How black, agouti and blonde coats arise
MC1R and the black coat. Researchers investigating black-rat colour variation compared the Mc1r gene in animals with agouti and melanistic coats. Their sample included 61 specimens from Japan and four from Pakistan, spanning Rattus rattus and its close relative Rattus tanezumi. In the sampled R. rattus, a change at one position in the DNA was associated with the dominant melanistic trait. That substitution changes one amino acid in the receptor from glutamic acid to lysine. This is the variant discussed in the black-rat genetics literature as E94K. [2]
The same study found 11 versions of the gene sequence. They fell into two main groups corresponding to the two species, and the melanism-associated substitution appeared in a single R. rattus version. The researchers interpreted this pattern as suggesting a relatively recent origin of that melanistic variant. [2]
In our colony, grey-backed E/e rats can have agouti sides and bellies, while E/E rats are jet black. E/e coats can rust with age, and the same interaction is seen on the blonde background. Our MC1R deep dive brings together these colony observations and the genetics. It is the place to explore the difference between the back and the sides, the contrast between the two-copy and one-copy forms, and the changes a breeder can follow as an animal grows older.
RAB38 and pigment delivery. Inside pigment cells, the proteins involved in making pigment have to reach melanosomes. Experiments with mouse pigment cells showed that Rab38 and the related Rab32 help transport melanogenic enzymes, including tyrosinase and TYRP1. The researchers found these Rab proteins on mature melanosomes and on nearby vesicles carrying the enzymes. Their results placed Rab38 and Rab32 in the delivery route between the cell’s Golgi sorting system and the melanosomes. [3]
The experiments also revealed a useful partnership. In the mouse cells studied, Rab32 could compensate for defective Rab38, allowing nearly normal pigmentation. When the researchers also reduced Rab32, pigmentation fell sharply. Tyrosinase was misdirected and degraded after leaving the Golgi. This gives a concrete explanation for why pigment transport matters: making the enzyme is one step, and delivering it to its working destination is another. [3]
The earlier discovery in “chocolate” mice connected a brown coat with a mutation in Rab38, which encodes a small GTP-binding protein involved in TYRP1 sorting. Their coat resembled that of Tyrp1-mutant mice, and TYRP1 delivery to melanosomes was impaired. [4]
In Norway rats, another study identified the Ruby locus as Rab38. Fawn-hooded and Tester-Moriyama rats carried the same mutation, and their shared genetic background pointed to a common ancestor within a Long Evans sub-strain. [5] These mouse and Norway-rat studies explain the comparative research behind the gene. For our blonde roof rats, follow the RAB38 deep page, which connects that research with the colony’s blonde line and the questions it makes possible to study.
ASIP and the agouti pathway. Agouti signalling protein is another important part of pigmentation biology. A study of black-rat species-complex lineage-II animals on Okinawa compared 38 agouti specimens with four melanistic specimens. The researchers found a change in Asip predicted to disrupt the protein’s function. It occurred in the melanistic animals and was absent from the agouti sample, making it a proposed cause of melanism in that population. [6]
That finding adds a second route to the black-rat colour story: the MC1R-associated melanism studied in R. rattus, and the proposed ASIP-associated melanism in the Okinawan lineage-II sample. The distinction helps explain why scientific accounts name both the gene and the population when describing a coat variant. [2][6]
Follow each gene into its supporting explanation: MC1R and the pigment switch, the glossary’s Ruby/RAB38 source note, and the primary ASIP study from Okinawa.
Looking closely at a coat
The basket photographs show a blonde coat from the front and the side. The hammock and towel portraits bring an agouti face close to the camera. For the black coat, the curled rat gives a view across the back, while the shelf photograph shows the animals upright. Compare the areas each view makes visible: the forehead and cheeks in a portrait, the back and flanks in a side view, and the tail where it is in frame.
When keeping your own coat record, photograph the whole rat and add close views of the features that interest you. Record the animal’s identity and the date with each set. A back view and a side view are especially useful when following the colony’s grey-backed and rusting coats described above. Keeping dated images together lets you return to the same animal’s earlier appearance as you follow it through life.
Watch black and agouti roof rats
Watch the rats explore, turn and climb, bringing their backs, sides and tails into view. The video introduces black coats and white tail tips among the family. Follow it with the questions below for more about the colours and markings we track in the colony.
FAQ questions. The 11 answers below cover common coat questions. For MC1R/E/e, Rab32/TYRP1 and ASIP in the guide above, use the coat guide contents.
On this page
- What colors do roof rats come in?
- How did you get a blonde or near-white roof rat?
- Are you breeding for color?
- Despite the name, aren’t most roof rats actually brown?
- Is the black (melanistic) coat dominant?
- Does coat color affect temperament?
- What does the agouti coat look like?
- What is the white tail-tip marking?
- How many breeding lines do you keep?
- How do the genes work: what makes a roof rat agouti, black, or blonde?
- Are blonde roof rats more sensitive to light?
What colors do roof rats come in?
In the wild, two: agouti (banded brown, the most common here) and black. In our line we also have a recessive blonde (amber/cream) and a white tail-tip marking, all of which arose naturally and we now track.
How did you get a blonde or near-white roof rat?
It’s a Rab38-null mutation, the same trafficking defect behind fawn-colored Norway rats. It appears randomly and doesn’t meaningfully affect their health or behavior.
Are you breeding for color?
Yes, we track and breed for the colors, but each is driven by specific genes that arose naturally in the line, not by crossing in other species.
Despite the name, aren’t most roof rats actually brown?
Right. In the USA, “black rats” (the other name for roof rats) are usually agouti (brown), not black. Black is one variant; agouti is the common wild coat.
Is the black (melanistic) coat dominant?
Yes, melanistic black is dominant, so a black × agouti pairing produces, on average, about half black pups.
Does coat color affect temperament?
We’ve noticed rats with the white tail-tip marking tend to be calmer and friendlier, though we’re not certain whether the color itself is linked to temperament, or whether it simply rode along with the traits we selected for. The calmer-with-color pattern shows up in other domesticated species too.[1]
What does the agouti coat look like?
Agouti is the banded brown wild coat. Agouti roof rats can have dark bellies or light, almost-white bellies; we also have solid blacks and the blonde variant.
What is the white tail-tip marking?
An incomplete-dominance marking we track in the line, present as just a few hairs in some rats and a bold white tip in others.
How many breeding lines do you keep?
Being the only active breeder in North America is itself a challenge, we maintain a large enough population, rotate pairings, and select carefully to keep the line healthy and genetically diverse.
How do the genes work: what makes a roof rat agouti, black, or blonde?
Two genes drive the coat: MC1R (call it M) and Rab38 (R), each switched on (+) or off (−). Agouti is the wild combination; a gain-of-function MC1R gives black, though it’s only “sort of” dominant, so a single copy often lets the agouti “rust through,” leaving a hint of brown on the sides; and a Rab38-null (loss-of-function) gives the blonde/amber color. That Rab38 mutation is well characterized, it makes Norway rats “fawn,” mice “chocolate,” and our roof rats blonde, and we’ve confirmed it by PCR testing of hair samples.
Are blonde roof rats more sensitive to light?
They do seem more bothered by bright light than the other colors, the MC1R gene affects more than just coat color. Their eyes aren’t as pink or ruby as a fawn (amber) Norway rat’s, though.
See the coat variants up close in the gallery on The Project, and more on our signature colour on the blonde rat page.
Thinking about keeping or breeding roof rats yourself? See Successor Breeders Wanted and What It Takes, or get in touch.
References
- [1] Trut L. “Early Canid Domestication: The Farm-Fox Experiment.” American Scientist 1999;87(2):160. DOI.
- [2] Kambe Y et al. “Origin of agouti-melanistic polymorphism in wild black rats (Rattus rattus) inferred from Mc1r gene sequences.” Zoological Science 2011;28:560-567. PubMed.
- [3] Wasmeier C et al. “Rab38 and Rab32 control post-Golgi trafficking of melanogenic enzymes.” Journal of Cell Biology 2006;175(2):271-281. PubMed.
- [4] Loftus SK et al. “Mutation of melanosome protein RAB38 in chocolate mice.” Proceedings of the National Academy of Sciences 2002;99(7):4471-4476. PubMed.
- [5] Oiso N et al. “The rat Ruby (R) locus is Rab38: identical mutations in Fawn-hooded and Tester-Moriyama rats derived from an ancestral Long Evans rat sub-strain.” Mammalian Genome 2004;15(4):307-314. PubMed.
- [6] Sasamori S, Yoshida MA, Suzuki H, et al. “Potential causative mutation for melanism in rats identified in the agouti signaling protein gene (Asip) of the Rattus rattus species complex on Okinawa Island, Japan.” Zoological Science 2017;34(6):513-522. PubMed.