Found across two small islands in the Gulf of California, Santa Cruz and San Diego, separated by a sea channel that has sufficed to keep both populations genetically isolated for generations. Like many small-island mammals in the Gulf of California, it is noticeably larger than its mainland relatives—a pattern known as insular gigantism that recurs in several rodent species on these same islands. It takes shelter among the arid scrub vegetation and rocks of both islands, a tiny habitat that constitutes the species' entire global distribution. Its scientific name refers precisely to this distribution shared between two neighbouring islands, linked by their common evolutionary origin but separated by the surrounding sea.
A medium-sized rodent for its genus, noticeably larger than other mainland Peromyscus species, with greyish-brown fur on the back and white on the belly. It differs from other Gulf of California species in its larger size—a case of insular gigantism—and its exclusive distribution to Santa Cruz and San Diego islands.
Small to medium track, 1.6-2 cm, with 4 fore toes and 5 hind toes, somewhat larger than those of other mainland Peromyscus species due to the superior body size of this island species. Small, elongated droppings, 4-6 mm, left scattered along surface runways through the island's arid scrub vegetation. It digs shallow burrows beneath rocks or roots, with the entrance semi-concealed among dense vegetation.
Arid scrub on desert islands, in lowlands. Endemic to Santa Cruz and San Diego islands in the Gulf of California, Mexico, with no natural occurrence outside these two islands.
Omnivorous, feeding on seeds, leaves and invertebrates from the arid scrubland of the islands it inhabits.
Solitary and nocturnal, with a home range centred around shelters among rocks and dense island vegetation.
Its restriction to two tiny islands, combined with the constant risk of predator introduction, leaves its populations perpetually vulnerable despite the lack of serious direct threats documented to date.
Risk of cats, rats or other predators being introduced to Santa Cruz and San Diego islands, alongside the species' extreme geographical restriction to a tiny island territory.
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