Mexican Gray Squirrel, Sciurus aurogaster nigrescens

Mexican Gray Squirrel, Sciurus aureogaster nigrescens. Photographs taken in Parque National Huatulco, Huatulco, Oaxaca, March 2021. Photographs and identifications courtesy of Marina Sutormina, Stockholm, Sweden. Note: we believe this to be the nigrescens subspecies based on the location the photograph was taken.

The Mexican Gray Squirrel, Sciurus aureogaster nigrescens, (Cuvier, 1829) is one of two subspecies of Mexican Gray Squirrel, both of which are found in Mexico. The aurogaster subspecies is found in eastern Mexico and the nigrescens subspecies resides in central and western Mexico and southern Guatemala. Both subspecies are referred to as the Mexican Gray Squirrel. They are a member of the Sciuridae family of Squirrels which has two hundred eighty-five members placed in fifty-eight genera. The are twenty-eight global species of the Sciurus Genus. They are also known as the Red-bellied Squirrel. The Mexican Gray Squirrel is known in Mexico as La Ardilla Gris Mexicana and Ardilla Vientre Rojo.

Mexican Gray Squirrels can vary significantly in their color and pattern. They have grayish salt and pepper fur on their dorsal side that may be broken by nape or rump patches, a shoulder and costal patch, or combinations of those patterns. Their ventral side may vary from white, to orange, to deep chestnut. The orange color of their belly and flank may spread dorsally over their forelimbs and shoulders to form an hour-glass shaped marking over their back. In northern populations, a melanistic phase is common. Most melanistic individuals have either completely black or gray dorsal pelage with a reddish tinge on the ventral side of their belly and rump. Their tails are bushy and dark gray in color with with a white outline. Slight sexual dimorphism is displayed with males growing slightly larger than females, averaging 560 grams (1 lb 2 oz) in weight. They measure 23 cm (9.1 inches) to 31 cm (12 inches) in snout-to-vent length, with their tails measuring slightly shorter than their body length. 

Mexican Gray Squirrels are diurnal and generally solitary. In areas where they have been  introduced, they rarely come to the ground and spend most of their time in the tree canopy. In Mexico, however, individuals can be observed using the forest floor year round, especially during the dry season. Mexican Gray Squirrels build leaf nests in tree branches approximately 5 m (16 feet) to 15 m (49 feet) above the ground. These nests consist of a base of interwoven twigs containing an inner cup of tightly packed singular leaves, covered by a dome of interlaced leaves and small twigs. It is believed that the twigs and leaves are taken from the immediate area surrounding the nest. When available, artificial nest boxes are utilized. However, the artificial nest boxes are exposed and uncovered, unlike their natural form. Mexican Gray Squirrels are territorial, establishing and defending individual home ranges. The home ranges of females are typically smaller than those of the males. Four distinct vocalizations have been identified for this species; the call of apprehension, the call of danger, the mating call, and the squeal of death. The call of apprehension is a low-pitched barking when they are at a safe distance but within sight of an intruder. The call of danger consists of rapid high-pitched barking calls. The mating call includes numerous rapid squeaking and clucking sounds. Finally, the squeal of death contains several sharp, piercing, high-pitched squeals. During the apprehensive and danger calls, their tail is held forward over their back in an “S” position and rapidly moved forward and backward. Chasing behaviors have been observed between individuals and may be involved in reproduction. Their chasing behavior consists of multiple individuals chasing a single individual while producing numerous squeaking and clicking sounds over a small area of the forest. While the reproduction of the Mexican Gray Squirrel is poorly studied and documented, it is known that breeding occurs year-round and reproduction is viviparous. They are polygynandrous, with each male mating with multiple females, and each female mating with multiple males. Each female gives birth to 2 to 4 young during the dry season. There is little information regarding the longevity of Mexican Gray Squirrels and  their lifespans are not documented. 

Mexican Gray Squirrels are shy and elusive, avoiding contact with humans. However, they will exhibit aggressive behavior toward intruders and predators. Lowland populations consume grains, seeds, and fruits. Highland populations consume acorns, bark, pine nuts, stems, and wood. They will also raid commercial orchards for cacao, corn, green figs, plums, tamarind, and zapote. In turn, they are preyed upon by Bobcats, Lynx  rufus, relying on their cryptic color for defense. In introduced populations, there is little predation pressure. Rather, competition for food and denning sites with native fox squirrels, black rats, raccoons, frugivorous birds, owls, and other species appears to be the limiting factor on population density. 

Mexican Gray Squirrels occupy forested habitats including thorn scrub, deciduous and evergreen forest, dry pine-oak woodland, secondary forest, and plantations. They are most commonly found in dry woodlands or forest, especially in those bordering agricultural areas. In addition, they have been known to occupy urban areas. 

The Mexican Gray Squirrel may be confused with the Mexican Fox Squirrel, Sciurus nayaritensis (larger in size with grizzled tan and black fur on their dorsal side and a black outline to their tail).  

From a conservation perspective, the Mexican Gray Squirrel is considered to be of Least Concern due to their wide distribution, presumed large population size, and tolerance of a board range of habitats. No major threats have been identified for this species. In some areas, they are hunted for food by native populations or to prevent damage to corn and other crops. While hunting may cause a decline in local populations, it is not considered a major threat.