Microwear features were visible on many (73%) of the modern specimens examined. Figure 2 illustrates microwear features on molars of a representative tree squirrel (Callosciurus finlaysonii, Figure 2A-B) and a representative ground squirrel (Spermophilus variegatus, Figure 2C-D). Microwear features visible in the photographs include fine and coarse scratches as well as large pits. (Small pits are highly refractive under the light microscope and are difficult to photograph without causing a glare that masks other features.) These photos and the data of the Appendix demonstrate that microwear features can be documented from the epoxy casts of squirrel teeth. All microwear features were also visible on some of the fossil specimens (e.g., Figure 3).
The modern species show variable counts of pits and scratches (Figure 4). In Figure 4A, the species overlap considerably in the number of scratches, but the teeth of Spermophilus show a greater number of pits than do teeth from Callosciurus and Ratufa. This contrast is seen more clearly in Figure 4B, in which the average number of pits of the ground squirrels occupies the right side of the scatter-plot. In terms of the average number of pits, the ground squirrels exhibit a greater range of differences than do the tree squirrels. It is intriguing that the greatest number of pits was found on one specimen of Spermophilopsis leptodactylus, the species that occupies the most arid environment of the modern ground squirrels sampled.
Statistical analysis indicates that the extant tree squirrels and ground squirrels studied differ significantly in the frequency of pits. For the two continuous variables, the Kruskal-Wallis test shows a significant difference in numbers of pits and a marginally significant difference in numbers of scratches among all modern squirrel species (pits: X2= 18.94; p = 0.002; scratches: X2= 10.40; p = 0.064). This test demonstrates that the variation plotted in Figure 4A-B is statistically significant. According to the Mann-Whitney test, none of the frugivorous tree squirrels differ significantly from one another with respect to number of pits; nor do the ground squirrels differ from one another (Table 2A). However, all three tree squirrels differ significantly from S. annulatus, Ratufa differs significantly from S. variegatus, and the difference between C. finlaysonii and S. variegatus is marginally significant (at p = 0.079). Tree squirrels do not differ significantly from Spermophilopsis leptodactylus, most likely due to a sample size of one for the desert squirrel. With respect to numbers of scratches, some differences are seen among tree squirrels, among ground squirrels, and between tree-ground squirrel pairs (Table 2B). Also, the tree squirrels differ significantly from the ground squirrels in the frequency of specimens with large pits and gouges (large pits: X2=5, p<0.05; gouges: X2=8, p<0.005; Figure 4C). Altogether, these results suggest that the most diagnostic difference between the more frugivorous tree squirrels and the omnivorous ground squirrels is a higher number of pits in ground squirrels, possibly due to persistent grit in the forage of ground squirrels. The fact that S. leptodactylus, the desert squirrel from Iran, had the highest number of pits supports this inference, but further work is needed, since this species was represented by only one specimen.
The distinction in microwear patterns between the frugivorous tree squirrels and the ground squirrels differs from the microwear patterns observed in large mammals. Across primates, suids, and bovids, frugivorous species are generally characterized by a higher number of pits than scratches relative to folivorous species (Teaford and Walker 1984; Solounias and Semprebon 2002). This difference is probably due to the feeding by frugivores on hard-shelled fruits and seeds. The higher number of pits and high frequency of large pits and gouges in the ground squirrels studied (Figure 4) suggest that their diet is more abrasive, possibly due to more grit, seed-eating, or chitin from insect exoskeletons. In their analysis of the microwear of 50 extant ungulates, Solounias and Semprebon (2002) found that the average number of scratches was more useful in distinguishing among browsers, mixed feeders, and grazers than was the average number of pits. Our initial results for squirrels suggest that the average number of pits and coarse features will be useful in distinguishing frugivorous tree squirrels from ground squirrels. These results are compatible with Rensberger’s (1978) suggestion that striations on squirrel enamel are due to detritus but leave open the issue of whether the microwear features result from mastication of food or detritus on food items.
Solounias and Semprebon (2002) developed new categorical microwear variables (e.g., presence of large pits, etc.; see Appendix) to provide greater resolution of dietary categories among ungulates. For each species, they noted the percentage of specimens with particular microwear features. Species that routinely encounter grit in their diets, either because they live in dry habitats (pronghorn, camel) or because they root for food (bush pig), showed a high frequency of coarse features on their molars. Figure 4C suggests that these variables may be similarly useful among squirrel species. These patterns can be tested with an expanded dataset of squirrel species and individuals per species.
For the fossil squirrels, we found that isolated teeth can be successfully molded and cast to capture microwear features and that these features are preserved on some of the specimens. We examined four specimens of Spermophilus rexroadensis from the Pliocene of Kansas and found that their microwear features most closely resemble those of modern ground squirrels (Figure 4B). Furthermore, this fossil species, like the modern Spermophilopsis, differs significantly from modern tree squirrels in number of pits (See Table 2A). Two Eutamias specimens from the Miocene of Pakistan also yielded microwear. Based on the average number of pits, Eutamias plots in between the modern frugivorous tree squirrels and ground squirrels. At present, our limited sample of modern species does not permit us to interpret this pattern. Microwear analysis of modern and fossil squirrels is ongoing with larger samples and additional species.
Six of 23 fossil specimens examined, or 26%, had visible microwear features. The other teeth had either excessive wear, large dentine pits on the original teeth, problematic casts, or a sugary texture that obscured microwear. Wear and dentine pits are properties of the original teeth; modern teeth may have the same properties. The solution to these challenges is to examine additional specimens if they are available. The solution to problematic casts (e.g., air bubbles) is to recast the specimens; in some instances the improved cast did show microwear. The sugary texture occurred on fossil teeth from both the Miocene of Pakistan and the Pliocene of Kansas, as well as on small-mammal fossils from other sequences (Beranek, personal commun., 2004; Semprebon, personal commun., 2004). The frequency of this surficial texture on fossil teeth suggests the need to consider the post-mortem processes that might alter primary microwear features on small-mammal teeth. We briefly note two processes—acid etching and mineral precipitates.
During the retrieval of fossils, small-mammal remains are sometimes processed during screenwashing with a dilute acid solution to soften and break down a calcareous sediment matrix. In future work on fossil teeth, we need to consider whether acid might etch the surface of teeth or enlarge microscopic pits already present. Fortunately, the Siwalik collection includes rodents from both acid-washed and non-acid-washed localities, allowing us to examine this influence in future work. A similar concern with fossil-modern rodent comparisons is that many fossil rodents may have been the victims of predation, whereas modern museum specimens were collected in live-traps. Andrews (1990) compared the digestive modifications of small-mammal teeth by avian and mammalian predators. The effects of predator digestion ranged from none to near complete dissolution of the tooth. Among owls and hawks, digestion of rodent and insectivore molars ranged from 0 to 53% (Andrews 1990, table 3.12). Among canid, mustelid, and viverrid predators, digestion of rodent and insectivore molars ranged from 10 to 70%. The intensity of digestion was greater on isolated molars than on molars in dentaries. Andrews (1990) did not examine the effects of digestion at the scale of microwear features. Since the macroscopic effects of predator digestion were quite striking, the microscopic effects also need study in relation to microwear features. Acid etching may also have occurred in the soil—as a result of organic acids released by roots or microbes—after fossil specimens were buried, suggesting another line of investigation.
As fossil teeth lie in a sediment matrix for thousands to millions of years, they are susceptible to chemical interaction with sediment pore waters, including those that cement the sediment matrix during diagenesis. Calcareous, siliceous, and other common cementing agents may deposit a thin veneer of cement on fossils and other sedimentary particles. This process may explain the sugary texture seen on fossil teeth from different times and locations. We will investigate chemical and mechanical methods for removing this precipitate in ongoing work.