The Mexican gray squirrel (Sciurus aureogaster, F. Cuvier), also known as the red-bellied squirrel, is known in Spanish as ardilla gris mexicana or ardilla vientre rojo. The species consists of two subspecies S. a. aureogaster and S.a. nigrescens, which are native to Mexico and Southern Guatemala. The Mexican gray squirrel was introduced to the Florida Keys in 1938 and is considered an invasive species.
Taxonomy Sciurus aureogaster was named in 1829 by Frédéric Cuvier. It is classified as part of the subgenus Sciurus within the broader Sciurus genus of tree squirrels, alongside several other North American and Eurasian species. Due to the species' considerable variety in fur color and patterns across its range, over sixty distinct scientific names have been applied to specimens since its original naming. Consequently, while the possibility that additional subspecies might be present within the species has been raised by taxonomists such as Guy Musser, at present only two subspecies are universally recognized:
S. a. aureogaster: Occurs in eastern Mexico. Typically has orange to chestnut ventral fur, small postauricular patches, and a white tail accented with black or chestnut. S. a. nigrescens: Occurs in the Pacific coast of Mexico. Typically has white to chestnut ventral fur, patchy dorsal fur, large postauricular patches, and a gray to chestnut tail. A 2020 paper published on the taxonomy of Sciurinae, which proposed a split of Sciurus into multiple genera, grouped the Mexican gray squirrel alongside four other Central American taxa, Collie's squirrel, Deppe's squirrel, the variegated squirrel, and the Yucatan squirrel, within the resurrected genus Echinosciurus. The genus had originally been created for the binomial name Echinosciurus hypopyrrhus, one of the junior synonyms for S. aureogaster.
Description Mexican gray squirrels have considerable individual and geographic variance in fur color and pattern. In general, their dorsal fur is pale to gray, but frequently marked with patches of other colors on the neck, shoulders, back, and sides. Belly fur ranges through white, orange, and chestnut. The tail tends to have the same colors as the belly. Partial and complete melanism is common. In Mexico, the Mexican gray squirrel typically has a light to dark gray dorsal side and a ventral side ranging from white, to orange, to a deep chestnut color. However, in northern native populations, it is common for individuals to have all black or gray dorsal fur with a slight coloration of red on the belly and rump. In Elliott Key, the common phenotype is entirely black fur.
Ecology and behavior The Mexican gray squirrel is an arboreal species, spending most of its time in trees, including the trees where it nests. They are also most active during the daytime after sunrise and before sunset. They are very agile creatures that can make long jumps from tree to tree with extreme precision. They spend the majority of their time at heights of 10–20 metres (33–66 ft) above ground level, but will forage at higher and lower heights, including on the ground. They frequently feed while hanging from a branch by their hind limbs. Mexican gray squirrels are typically silent except when frightened or mating. In these contexts, they make four distinct calls: a low-pitched bark made in response to a distant threat, a high-pitched bark made in response to a nearby threat, a high-pitched squeal made when grabbed or handled directly, and a series of rapid squeals and clicks made during mating chases Mexican gray squirrels are mostly solitary, and are typically only encountered in groups when attracted by abundant food. They are not however territorial, and their ranges typically overlap. These squirrels are extremely shy toward humans. In the introduced population on Elliott Key, Florida, they typically remain in trees except when foraging or reproducing. Chasing behavior has been observed in this species and is thought to be associated with mating, with multiple squirrels chasing a single individual while producing squeaking and clicking sounds.
Diet Mexican gray squirrels feed primarily on acorns, with other seeds, fruits, fungi, insects, and nestlings rounding out their diet. In lowland environments they typically feed on fruit and seeds, including acorns, pine seeds, and dogwood seeds. In highland habitats, they feed on acorns and pinecones. Squirrels in all terrains, depending on the season, eat fruit including mangos, figs, plums, tamarind pods, chico zapote, and corn. Wild mastic, papaya, coconut, sea grape, thatch palm, blackbead, and poisonwood berries are also a common part of the squirrel's diet. While these berries, seeds, and fruits are not available in the winter or spring months, the squirrels find buds and twigs of mahogany, gumbo limbo, and sea grape. During food scarcity, squirrels can clip branches and eat the phloem, or the inner bark of the wild mastic. Near farms or urban areas, the squirrels feast on corn, crops, and plants available to them. The introduced population in Florida originally fed on seeds and fruits from wild mastic (Sideroxylon foetidissimum), papaya (Carica papaya), seagrape (Coccoloba uvifera), thatch palm (Thrinax parviflora), coconut (Cocos nucifera), and Australian pine (Casuarina equisetifolia). Beginning in the 1980s, nonative species such as the papaya, coconut, and Australian pine were removed from the island, which caused the squirrels to refocus their diet on the remaining native species.
Ecosystem role and predators In Mexico, the Mexican gray squirrel is preyed upon by bobcats, as well as by humans for human consumption. For this reason, they use their fur for camouflage. In the Florida Keys, there is little predation pressure, as they have no natural predators in introduced areas. Instead, they compete with other native species such as rodents, birds, and fox squirrels for food and denning sites. The primary predators of Mexican gray squirrels are birds of prey, felids, procyonids, and snakes.
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