Sinaloan pocket mouse

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The Sinaloan pocket mouse (Chaetodipus pernix) is a small rodent found only in Mexico. It is listed as Least Concern for conservation.

Two subspecies exist: Chaetodipus pernix pernix and Chaetodipus pernix rostratus. All are native to Mexico, where the mice live in coastal areas.

Habitat and range
This species mainly lives on the coastal plain of Sinaloa state, extending from southern Sonora to northern Nayarit. It favors sandy soils with small trees, shrubs, and cacti. Sinaloan pocket mice hide under low bushes, dense vines, and grasses. Unlike its rock-dwelling relatives, it does not inhabit rocky areas.

Size and appearance
The Sinaloan pocket mouse is one of the smallest in its genus. It measures less than 200 millimeters in total length. It has a narrow skull, an elongated nose, and medium-sized ears. The tail is thin and longer than the body. Its coat color is variable but usually yellowish-brown, darker on the back, and lighter on the sides and belly.

Diet and reproduction
These mice feed on grains and seeds. They typically have an average litter size of about seven young.

Coat color variation and why it happens
Coat color is a genetic polymorphism, meaning different colors occur due to genetic differences. The environment also shapes which colors are favored, because coat color affects survival.

Crypsis and temperature regulation
There are two main camouflage strategies: camouflage (matching the background) and countershading (darker on top, lighter on the bottom). The Sinaloan pocket mouse often has a yellow-brown coat with dark-tipped hairs to blend in with sandy, sunlit habitats. In contrast, some rock-dwelling relatives have gray or black coats to blend with rocks.

Coat color can also help with temperature regulation. Lighter colors reflect sunlight and help prevent overheating in hot, sunny environments, while darker colors absorb more heat.

Genetics of coat color
Two pigments determine color: eumelanin (black/brown) and pheomelanin (red/yellow). A receptor gene called MC1R promotes eumelanin production, while the agouti signaling protein reduces eumelanin and promotes pheomelanin. Environmental factors can influence how these genes are expressed, leading to different coat colors in different places.