One of the major environmental concerns in the world is the loss of biological diversity due to anthropogenic activities. Of special concern is the conservation of endemic species that are particularly vulnerable to extinction. The Mearns's squirrel (Tamiasciurus mearnsi) is endemic to the Sierra de San Pedro Mártir, Baja California. Federally listed as threatened in Mexico and as endangered by the International Union for Conservation of Nature (IUCN), little is known about the ecology of this southernmost Tamiasciurus. Interestingly, Mearns's squirrels exhibit deviations from common behaviors observed in other congeners such as lack of leaf nests (dreys) and larderhoards (middens), suggesting potentially unique adaptations. Herein, I reviewed the diversity and conservation status of the arboreal squirrels of Mexico. Using radio-telemetry and satellite imagery, I examined if the lack of dreys and middens may be associated with differences observed in nesting behavior, home-range dynamics, and life-history and behavioral tactics between Mearns's squirrels and other arboreal squirrels.Mexico harbors 14 species of arboreal squirrels, of which four are endemic, with the states of Chiapas and San Luis Potosí possessing the greatest diversity. Unfortunately, high deforestation rates in Mexico, and a dearth of information on their ecology, pose serious threats to the persistence of this squirrel diversity. Mearns's squirrels apparently are obligate secondary cavity-nesters with specific nesting requirements and their population possibly limited by the low occurrence of tree cavities in their habitat. The species seems to have lost the territorial behavior that is characteristic of the genus Tamiasciurus. Home-range dynamics of Mearns's squirrels are similar to nonterritorial Sciurus squirrels. Although reproduction and survival are similar to other Tamiasciurus, the species is heavier and apparently larger while exhibiting important variations in their behavior compared to other congeners. The lack of dreys and middens appears to be associated with the unique tactics adopted by Mearns's squirrels to persist in the Sierra de San Pedro Mártir. The species provides an important opportunity to learn more about geographic variation in nesting behavior and the evolution of territoriality. Large trees and snags that facilitate cavity formation are critical for the conservation of this species.