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Adrienne
Sussman
Research Training Program, 2004
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INTRODUCTION Callosciurus prevostii is an arboreal squirrel found throughout the Sunda shelf, with a range from the Isthmus of Kra in the north to the islands of Borneo, Sumatra, and Sulawesi, as well as several smaller islands in the South China Sea. This area is known for its complex biogeographic history: although the islands of the shelf may have been connected by land bridges as recently as 9,500 years ago, the fauna on many islands show levels of divergence indicative of a much longer separation (Gorog, Sinaga, and Engstrom, 2004).
Current paleogeographic theories suggest that, although land bridges did exist until recently, vegetational barriers such as grass lands and savannah divided areas of rainforest, thus limiting the dispersal of forest dependent animals (Meijaard, 2003). C. prevostii shows a high level of variation throughout its range. Walker's Mammals of the World recognizes 21 subspecies of C. prevostii, each with a strikingly different pelage. Although all subspecies have a red underbelly, other coloration can range from plain black to elaborate patterns involving four or more colors. As would be predicted by Heaney's Island rule, there are also significant size differences between the squirrels on the various islands. Variations in coat color and size could reflect evolutionary distance between the populations, and, consequently, indicate the length of time the groups have been isolated. In this study, the morphological differences between eight subspecies of C. prevostii were using cranial measurements. MATERIALS AND METHODS
CONCLUSIONS • Size is an important factor in discriminating between the subspecies in canonical variate analysis. The correlation tests showed that when controlled for size, shape is the major discriminating factor. Differences in shape were significant enough among the larger squirrels for their skulls to be distinguishable from each other about 60% of the time. • The dwarf subspecies on the three smallest islands: C. p. armalis from Pulo Penebangan, C. p. mimiculus from Santa Barbe, and C. p. navigator from the Natuna islands, were an exceptional case; when compared to all groups, these skulls appeared very distinct from other subspecies, but when compared only to each other, very little differentiation was seen. These islands also had very little variance due to shape. These results may be due partly to the small sample sizes available from each island, but they seem to indicate the presence of traits that connect these three island populations but separate them from all other groups. In particular, the subspecies seem to have dwarfed in the same manner on all three islands. • The variation observed here in coat, size, and cranial morphology can be interpreted as having different systematic repercussions depending on which species concept one uses. Under the biological species concept, two subspecies such as C. p. prevostii and C. p. mimiculus, which rest at opposite ends of the size spectrum for this species, would most likely be physically incapable of interbreeding due to the size difference between them, complicating their taxonomic relationship. However, the three smallest subspecies (mimiculus, armalis, and navigator), probably independently dwarfed from their ancestral form, would be able to interbreed. Examining other species concepts, such as Wiens's evolutionary species concept, the subspecies may already be eligible for reclassification as distinct species, because of the geographic barriers that have prevented them from interbreeding for thousands of years. • In a preliminary genetic study, it was found that C. prevostii prevostii and C. prevostii pluto differ at 14 out of 373 base pairs in the cytochrome b sequence- a 4.2% divergence rate under the HKY model of evolution and gamma = 0.5. (Leonard, pers. comm..) Using the standard molecular clock of 2% Myr-1, this would place the divergence of these two subspecies at about 2.1 Mya. Under a more conservative molecular clock of 10% Myr-1, the latest possible divergence is placed at 0.42 Mya. (Gorog, Sinaga, Engstrom, 2004). In either case, these values suggest that separation between the subspecies on the Malay peninsula and the island of Borneo has existed at least since the mid Pleistocene. Further genetic work is needed to investigate the relationship between the different islands. More information may help explain the variation observed here, as well as contributing to a general understanding of the paleogeography of the Sunda shelf. Want to ask a question? Visit the Message Board Virtual
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To
analyze the differences between populations of C. prevostii,
we measured 147 of the skulls of the specimens housed at the National
Museum of Natural History. These specimens were chosen from 11 distinct
geographic locations (N= 8), and belonged to eight different subspecies.
Twenty-one landmark points were chosen on the ventral side of the skull,
and the three-dimensional coordinates of these points were taken three
times for each skull with a microscribe. From these coordinates, 33
distances were calculated, measuring portions of the skull where we
had observed variability. The mean distance for each measure was used
on each skull to perform several types of statistical analysis, including
canonical variate analysis, correlation analysis, and principle components
analysis. Each analysis was performed twice, first using the raw data
and then controlling for size. Size was eliminated by dividing each
distance by a factor representing overall skull size, defined as the
mean of all measures for a particular skull. 
