Analyses of fur color patterns, morphometric data derived from external, cranial, and dental dimensions, and distributions of collection sites for voucher specimens form the basis for a taxonomic revision of Sulawesi's endemic squirrel fauna. Eight species of tree squirrels in Rubrisciurus and Prosciurillus and two species of ground squirrels in Hyosciurus are recognized. All are diurnal and inhabit primary forest formations. Diet consists of fruit, nuts, seeds, and arthropods. Rubrisciurus rubriventer, the largest in body size, forages on the ground and in the lower canopy layer, is found throughout Sulawesi where primary forest persists, and occurs through an altitudinal range embracing tropical lowland evergreen and lower montane rain forests; it is absent from upper montane rain forest. Five species of arboreal squirrels comprise the Prosciurillus leucomus group, a cluster of species occupying the upper forest canopy: P. leucomus, known only from lowland and montane habitats in the northern peninsula and one offshore island; P. alstoni, recorded from lowland tropical evergreen rain forest in the eastern section of Sulawesi's central core, the east-central and southeastern arms, and two southeastern islands; P. weberi, represented by a few specimens from the coastal lowlands of the southern core of Sulawesi; P. topapuensis, endemic to the western mountain block in Sulawesi's central core and occurring along an altitudinal gradient from lowland evergreen rain forest to upper montane rain forest; and P. rosenbergii, the only species of squirrel collected on islands in the Sangihe Archipelago north of the northeastern tip of the northern peninsula. The Prosciurillus murinus group contains two species of small body size: P. murinus, found throughout Sulawesi and in all forest formations, from the coastal lowlands to mountaintops, and a forager in the lower canopy layers; and P. abstrusus, known only from montane forest habitats on Pegunungan Mekongga in the southeastern peninsula. Of two species of ground squirrels, Hyosciurus heinrichi occupies montane forest habitats in the western mountain block of Sulawesi's central core. It is altitudinally parapatric to H. ileile, which inhabits lowland evergreen and lower montane rain forests in the western mountain block and northeastern lowlands of central Sulawesi, and montane forest on the northern peninsula.
A slightly revised classification of Sciuridae is provided in which a new tribe, Exilisciurini, is proposed for the Bornean and Philippine Exilisciurus. Previously published results of morphological and molecular analyses point to Rubrisciurus, Prosciurillus, and Hyosciurus as a monophyletic cluster, the Hyosciurina, nested within a larger clade, the Nannosciurini, which along with Exilisciurini n. tribe and Funambulini, comprise the Nannosciurinae, one of the three subfamilies constituting Sciuridae, and one that contains most of the Indomalayan genera. The present diversity of species endemic to Sulawesi was derived from an ancient lineage that crossed a sea barrier from the Sunda Shelf to Sulawesi during the late Miocene.
Eight new species of hoplopleurid sucking lice (Insecta, Anoplura) are described as parasitizing 8 of the 10 species of squirrels endemic to