The Big Mexican Small-eared Shrew is the largest shrew of the genus Cryptotis in Mexico. This difference, though small on paper, is evident when handled, especially compared to its tiny relatives. It lives hidden in the damp soil of the mountain cloud forests of Oaxaca. This type of forest forms only under very specific conditions of constant humidity and is increasingly restricted in area. Like all shrews, it has such an accelerated metabolism that it must eat several times its own weight daily to avoid starvation within hours. This biological requirement keeps it in almost permanent activity. Its relatively large size within its genus makes it a good example of how the diversification of shrews in the Mexican mountains has produced very distinct species from a single body plan.
The Big Mexican Small-eared Shrew is relatively large for its genus. Its fur is dark brown, almost black, with no distinctive markings. It has a long, mobile snout ending in a flexible proboscis, and tiny eyes barely visible among the fur. It differs from other Broad-clawed Shrews of the Oaxaca mountains mainly by its larger size and by cranial details that often require capture and direct measurement for reliable identification.
Tiny track, barely 1-1.2 cm long, rarely visible except on very fine substrates like mud or ash. Tiny, dark droppings, a few millimetres long, with insect chitin remains visible under magnification, left scattered along its paths under damp leaf litter. It builds surface galleries under leaf litter and spherical grass nests in hollows under fallen logs. Its presence is revealed by a sharp,
Cloud forest and montane mesic forest, with damp soil and abundant leaf litter, generally between 1500 and 3000 m altitude. Endemic to the Sierra Madre de Oaxaca, in southern Mexico, with no natural presence outside these mountains.
Insectivorous, the Big Mexican Small-eared Shrew feeds on insects, earthworms, and other invertebrates from damp soil. It catches these thanks to a very high metabolism that requires it to feed very frequently.
The Big Mexican Small-eared Shrew is continuously active in short periods throughout the day and night, driven by its very high metabolism. It exhibits solitary and territorial behaviour. Its strong dependence on the constant humidity of cloud forest greatly limits its ability to move between habitat fragments separated by drier areas.
Its populations are considered to be in decline due to the continued loss of the cloud forest it depends on. This is an already diminished mountain ecosystem, which continues to lose area due to deforestation in the Sierra Madre de Oaxaca.
Deforestation and fragmentation of cloud forest due to agricultural and livestock expansion, and the altitudinal shift of the humidity conditions necessary for this ecosystem, associated with climate change.
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