It may no longer exist in the wild: recent genetic studies suggest that the introduced House Mouse might have completely replaced this species on Ángel de la Guarda Island, with no confirmed record of a genuine individual for years. It occurred across several islands in the Gulf of California, each population exhibiting slightly different traits after generations of isolation on islands separated by open sea. Its potential disappearance illustrates one of the least visible yet most common extinction mechanisms on islands: not direct predation, but silent displacement by an introduced species occupying the same ecological niche. It is one of the most frequently cited cases of a recent potential extinction of a Mexican mammal through competition with invasive fauna.
A small rodent with greyish-brown dorsal fur and a white belly, large eyes and well-developed ears. In appearance it is remarkably similar to the introduced House Mouse with which it is nowadays confused on its former islands. It is distinguished from that introduced mouse by cranial and genetic details, a separation that can no longer be confirmed through direct observation in most of its historical localities.
Small track, 1.3-1.8 cm, with 4 fore toes and 5 hind toes, practically indistinguishable in the field from that of the introduced House Mouse now occupying much of its former habitat. Droppings small and elongated, 3-5 mm long, equally difficult to attribute with certainty without capturing the specimen for genetic analysis.
Arid scrub on desert islands in the Gulf of California, in lowlands. Historically distributed across several islands in the Gulf of California, including Ángel de la Guarda, off the coast of Baja California, Mexico.
Omnivorous, feeding on seeds, leaves and invertebrates from the arid island scrub it inhabited.
Solitary and nocturnal, with a home range centred on shelters among rocks and dense vegetation on its native islands.
The presence of genuine La Guarda Deermouse individuals has not been confirmed in most of its historical localities in recent years, with evidence indicating it has been replaced by the introduced House Mouse; it is unknown whether any pure population survives on any of the less studied islands.
Competition and potential ecological replacement by the introduced House Mouse on islands in the Gulf of California, alongside the risk of predation by feral cats.
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