Endemic to the Mentawai Islands (Siberut, Sipora, North and South Pagai)
The fraternal squirrel is a tree squirrel endemic to Indonesia, found only in the Mentawai Archipelago (Siberut, Sipora, North Pagai and South Pagai) off the west coast of Sumatra (Wikipedia, general species account).
Description: Wildlife of Sumatra guide, Wildlife of Everywhere (wildlifeofeverywhere.com); general species information paraphrased from Wikipedia (CC BY-SA)
Mammals of the World