Tree Squirrel Species Research Articles

Long‐term scatterhoarding by Eurasian red squirrels (Sciurus vulgaris)

Hoarding behavious, seed species hoarded and hoarding intensity by red squirrels (Sclurus vulgaris L.) are described for two population; one in a coniferous woodland with few deciduous trees, the other in a mixed deciduous woodland with some conifers. Seasonal variation in the recovery of hoards and the importance of hoarded food with regard to a squirrel's annual energy demands are discussed. The importance of having hoards is compared among tree squirrel species.

マレーシア産樹上性リス類の空間利用と森林構造

In this paper, three-dimensional space use patterns, interspecific interactions, anti-predator vocalizations, and social structures were compared among three species of tree squirrels (Callosciurus caniceps, C. notatus, and C. nigroviUatus) in lowland forests of West Malaysia. These sympatric species had each different space demands in respect of tree height or forest structure, and interspecific interactions were infrequent. Therefore, they were able to coexist in the tropical forests that consist of tall trees and have complex habitat structures. Anti-predator vocalizations and social structures differed among the three species, and this may be correlated to the habitat structure of each species: C. caniceps predominated in a bushy area and more subtly used auditory communication in predator avoidance than did the other two species.

Evolutionary Genetics of New World Tree Squirrels (Tribe Sciurini)

Protein variation was analyzed electrophoretically in six species of New World tree squirrels (tribe Sciurini) representing four of five extant genera. Genetic variation was either low (Microsciurus flaviventer—percent polymorphism = 3.7, average individual heterozygosity = 0.031) or absent (Sciurillus pusillus) in populations for which samples were large enough to permit analysis. Parsimony and locus-by-locus analyses of the protein data supported the close evolutionary affinity of the genera Sciurus and Microsciurus. A probability analysis of allelic distribution indicated that Tamiasciurus may be more closely related to the Sciurus–Microsciurus clade than is Sciurillus. This result supports retention of red squirrels, Tamiasciurus, within the assemblage of New World tree squirrels (Sciurini) and indicates that the high level of morphological differentiation seen in Neotropical pygmy squirrels, Sciurillus, is accompanied by a high level of genetic differentiation.

Vocalizations in Response to Predators in Three Species of Malaysian Callosciurus (Sciuridae)

Anti-predator vocalizations were studied in three species of tree squirrel, Callosciurus , in Ulu Gombak, Selangor Darul Ehsan, Malaysia. In the study area (ca. 10 ha), C. notatus was dominant in areas of low tree density, C. nigrovittatus in dense forests with tall trees, and C. caniceps in shrubs. All three species produced different calls to different predators; terrestrial carnivores, raptors, and snakes. Each signal caused conspecific individuals to adopt a different escape strategy. On hearing the repetitious, staccato barks toward carnivores, neighboring individuals ran up trees. A single bark was produced when raptors flew in the distance, which made conspecifics freeze. However, squirrels produced a rattle sound toward raptors in cases of close encounter. Squirrels moved toward the sound source of squeaks to snakes, and mobbed in a group. Calling frequency, intensity, duration, and sound structure of each call were compared among three species, and related to habitat selection.

Role of Sound Communication in Mating of Malaysian Callosciurus (Sciuridae)

Mating behavior was compared between two species of tree squirrels, Callosciurus caniceps and C. notatus , in Ulu Gombak, Selangor Darul Ehsan, Malaysia. In the mating bouts of C. caniceps , six to eight males assembled within the home range of an estrous female early in the morning. The female mated with four to six males during mating bouts of ca. 6 h. Each male produced a barking sound for 12–35 min after copulation, during which time the female and the other males stopped moving. The other males frequently gave precopulatory calls and waited for a chance to mate when the mating males departed. In C. notatus , five to seven males assembled near an estrous female and two to four of them mated during 6–7 h of a mating bout. The postcopulatory barks continued for only 0.5–8.0 min. The precopulatory calls were low amplitude and were given infrequently. Chasing among males was observed repeatedly near the female. Males had to guard the female against frequent challenges by other males. C. caniceps predominated in a bushy area and more subtly used auditory communication in mating than did C. notatus in open forests.

Systematic relationship of parapatric tree squirrel species (Tamiasciurus) in the Pacific Northwest

Phenetic analysis of 20 cranial characters was used in an evaluation of the systematics of two Pacific Northwest tree squirrels, Tamiasciurus douglasii and T. hudsonicus. Although these species are predominantly allopatric, there exist at least three zones of sympatry along a parapatric border in British Columbia, Washington, and Oregon. Two populations of supposedly hybrid squirrels have been described, and these were evaluated in relation to allopatric populations of both species from throughout the area. Principal components analysis, clustering of taxonomic distance coefficients, and discriminant analysis showed a strong morphologic distinctness between the two species. These analyses also consistently placed "hybrid" specimens with one species or the other and gave no indication of intergradation. The Pleistocene glacial history of the Pacific Northwest is used to explain the divergence of the two species as well as the formation of the parapatric species border. In addition, postglacial character convergence in pelage coloration and squirrel size in forest areas allowing noncompetitive coexistence is used to explain the apparent hybrids and to account for the disjunct Vancouver Island distribution of T. douglasii-like T. hudsonicus. It is concluded that with the lack of hybridization and the presence of character convergence in zones of sympatry, reproductive isolation between T. douglasii and T. hudsonicus is complete.

Ecology and Resource Partitioning among Nine Species of African Rain Forest Squirrels

Nine species of sympatric rain forest tree squirrels were studied in the field and in captivity at Makokou, Gabon. The findings on resource partitioning among them include: (1) two species are restricted to special habitat types. The other seven which commonly co—occur in the mature rain forest formation represent a maximum of taxonomic diversity. (2) Differential use of the vertical vegetation column divides these seven species into four essentially arboreal and three ground—foraging forms. (3) The species occupying a given foraging level and habitat differ in body size by approximately Hutchinson's ratio. Body size can act to partition fruit resources by size and hardness. (4) The squirrels are frugivores or omnivores; there are quantitative differences in diet between some species. These differences vary seasonally and may only be important during the most difficult season. (5) The four largest species have short active periods. I suggest that the greater species richness in African compared to Nearctic forests is permitted by a more constant productivity of fruit and arthropods, and a higher plant species diversity in the African forest.

Spectral sensitivity curves of diurnal squirrels

Spectral sensitivity curves were constructed for four species of ground squirrel, two species of tree squirrel and one species of chipmunk using the stimulus intensity required to produce a constant etoctroretinogram as the criterion. All these species have an apparently pure-cone retina although the retinal structure of the two tree squirrels differs somewhat from that of the ground squirrels and chipmunk. There was no evidence of a Purkinje shift. The spectral sensitivity curves of all seven species were very similar. The curves showed two maxima, one at about 535 mμ, the other at about 490 mμ. The relative heights of these maxima varied from one experiment to another but the factors responsible for this variation could not be determined. Adaptation to blue light depressed the maximum at 490 mμ while adaptation to green or orange light depressed that at 535 mμ. Evidence was obtained that the mechanisms responsible for the two maxima were to some extent antagonistic. The possible nature of these two mechanisms is discussed in the light of recent work on the photosensitive substances extractable from the retina of the common gray squirrel. There was some evidence for a further mechanism maximally sensitive between 450 and 460 mμ at least in some of the species investigated.