Sciurus aureogaster F. Cuvier, 1829
Ardilla vientre rojo (Castilian), Mexican gray squirrel (English), Red-bellied squirrel (English)
The normal gray-red color phase of Sciurus aureogaster (from which the common name Mexican red-bellied squirrel is derived), typically consists of a salt-and-pepper dorsal pelage combined with a bright rufous belly and flank, although the appearance of this squirrel is highly variable. The rufous color of the flank spreads dorsally over the area of the forelimbs and shoulder to form a striking hour-glass shaped marking over the back. A second color phase, as seen in the introduced Florida population, is totally black with no other markings. Most of these melanistic squirrels have a reddish tint to the underfur on the back and rump (Brown and McGuire 1975).
The Mexican red-bellied squirrel (Sciurus aureogaster) is diurnal, arboreal (rarely coming to the ground), and solitary (Emmons 1990)
Brown and McGuire (1975) found that the introduced red-bellied squirrels studied in Elliott Key County Park, Florida, were extremely shy and elusive, seldom allowing an observer to approach closely. Unlike eastern gray squirrels (Sciurus carolinensis) in public parks, red-bellied squirrels never approached campers or took food, even after long exposure to humans. Few campers who used the picnic area adjacent to the study plot ever even saw the squirrels even though population densities were high in the forest near the campsites. Observations on Elliott Key indicated that this species might be described as an "early-day" diurnal tree squirrel, because most movements were confined to the period starting one-half hour after sunrise until about one hour before noon. An occasional squirrel was observed during the afternoon or early evening hours, but not frequently. Red-bellied squirrels rarely came to the ground and spent most of their time in the interlocking mass of limbs of the canopy. They are thus more arboreal than eastern gray squirrels, which forage a great deal on the ground.
McGuire and Brown (1973) describe the feeding behavior of the introduced red-bellied squirrels on Elliott Key, Florida, foraging for coconuts, a favored food.
Red-bellied squirrels are usually silent, but sometimes make rasping trills and harsh chatters (Reid 2006).
In its native range in Mexico and Guatemala, the Mexican red-bellied squirrel (Sciurus aureogaster) might be confused with Deppe's squirrel (Sciurus deppei) or the variegated squirrel (S. variegatoides) (Emmons 1990). Deppe's squirrel is much smaller than the red-bellied squirrel and has a more slender tail. The variegated squirrel is highly variable in appearance, but compared to the red-bellied squirrel it has a more coarsely patterned coat, prominent pale patches behind the ears (absent in the red-bellied squirrel) that are white in pale individuals and tawny to orange in dark individuals, and a geographic distribution that is largely or entirely to the south of that of the red-bellied squirrel.
In the red-bellied squirrel's introduced range on Elliott, Sand, and Adams Keys in Florida, U.S.A., there are no other squirrels, although eastern gray squirrels (Sciurus carolinensis) do occur on Key Largo and on the adjacent mainland (Reid 2006).
The red-bellied squirrel is highly variable in appearance. Upperparts are finely grizzled gray, brownish, black, or intermediate in color; the nape of the neck, and/or shoulders, and/or rump sometimes show a patch of orange or chestnut (or, in melanistic individuals, darker black). The tail is black frosted with white above, gray or reddish medially below. Underparts are pure white, deep chestnut red, or any intermediate color. Animals from the Isthmus of Tehuantepec northward up the Pacific coast and to Veracruz have a bright chestnut rump or nape patches. Animals from Veracruz and Puebla northward are often melanistic. (Emmons 1990)
The Mexican red-bellied squirrel (Sciurus aureogaster) is found in a wide range of habitats ranging from lowland tropical evergreen broadleaf forest to Pine-Oak woodland and thorn scrub on the Mexican Plateau, but may be absent from intermediate cloud forests, where Deppe's squirrel (Sciurus deppei) is found (Emmons 1990).
In a study of an introduced population of Sciurus aureogaster on Elliott Key, Florida, Brown and McGuire (1975) found evidence of little predation pressure on the population. However, competition for food and denning sites by black rats, raccoons, frugivorous birds, owls, and other species appears to be a primary limiting factor on the island's squirrel density. According to Brown and McGuire, the only native mammals found living on the island were the marsh rabbit (Sylvilagus palustris), bobcat (Lynx rufus), and raccoon (Procyon lotor), in addition to a thriving exotic population of black rats (Rattus rattus). (In a study of bobcat diets in Mexico (Aranda et al. 2002), Sciurus aureogaster comprised 4.9% of the diet at a site in central Mexico.)
The Mexican red-bellied squirrel (Sciurus aureogaster) is native to southern Mexico from Nayarit and Nuevo Leon south to Chiapas and the highlands of Guatemala, occurring up to about 3,000 m elevation (Emmons 1990).
In 1938, a resident of Elliott Key in Dade County, Florida, imported two pairs of red-bellied squirrels from eastern Mexico and released them at two points on the island. The species adapted readily to the dense tropical forest present on the eight-mile long island and gradually increased in numbers (Brown and McGuire 1975). First reported by Albert Schwartz (Schwartz 1952, cited in Brown and McGuire 1969), this squirrel was abundant on Elliott Key by 1969 (Brown and Mcguire 1969). Brown (1969, cited in Brown and McGuire 1975) reported that the original introduction of S. aureogaster from Mexico in 1938 consisted of four squirrels, one of which was melanistic and the other three of which were the normal gray-red phase, although it is not known whether all four squirrels contributed to the current population, as the two pairs were apparently released at different times and about two miles apart.
Red-bellied squirrels have dispersed to Adams Key and Sands Key, and an individual was captured swimming to Old Rhodes Key (Layne 1997, cited in Koprowski et al. 2005); all of these islands are adjacent to Elliott Key at straight-line distances of <800 m (Koprowski et al. 2005). In 1992, the storm surge and winds from Hurricane Andrew submerged Elliott Key, raising some question about whether the resident red-bellied squirrel population had persisted (Layne 1997, cited in Koprowski et al. 2005). A survey in March of 2005, however, documented both fresh nests and living squirrels, showing that this population did indeed survive the devastation of Hurricane Andrew (Koprowski et al. 2005).
In a study of an introduced population of Sciurus aureogaster on Elliott Key, Florida, Brown and McGuire (1975) estimated the mean population density in a mature subtropical hammock to be 2.47 squirrels per hectare. Two color phases of this species were present, one totally black and one gray with a red belly, with the two forms present in nearly equal numbers, although the melanistic (black) forms may appear to be more common as a result of their greater visibility (Brown and McGuire 1975). Average home range size was estimated to be 2.3 hectares for males and 0.9 hectares for females.
In a study of an introduced population of Sciurus aureogaster on Elliott Key, Florida, Brown and McGuire (1975) found that reproduction occurred intermittently throughout the year with no tendency for a synchronized breeding season. Based on limited data from four nestling litters, one or two young appeared to be the normal litter size for this species.
In a study of an introduced population of Sciurus aureogaster on Elliott Key, Florida, Brown and McGuire (1975) found that this squirrel was largely frugivorous. Squirrels were observed utilizing the fruits or seeds of sixteen different species of tropical trees and shrubs including wild mastic, papaya, coconut, sea grape, and thatch palm. The most heavily utilized seeds were those of coconut (Cocos nucifera), and Australian pine (Casuarina equisetifolia). These trees provided a rich and constant food supply throughout the year, but are very restricted in distribution on Elliott Key. They occur mainly along the marine shorelines and were accessible only to squirrels inhabiting areas along the beach.
Interestingly, in their study of this introduced Florida population, Brown and McGuire (1975) found that none of the squirrels they live-trapped or collected during the study had ectoparasites. Schwartz (1952, cited in Brown and McGuire 1975) previously reported that the three specimens he examined on Elliott Key in 1951 were also free of external parasites. It is possible that the small initial group of red-bellied squirrels which colonized the island in 1938 lacked external parasites. The numerous ectoparasites of eastern gray squirrels on the Florida mainland apparently have not bridged the eight-mile wide barrier of Biscayne Bay.
Tilmant (1980, cited in Koprowski et al. 2005) reported that the red-bellied squirrel used leaves of palms in construction of their nests and suggested that they might pose a threat to the endangered Sargent’s palm (Pseudophoenix sargentii).