More microwear from South African australopithecines

Scott and colleagues (2005) examined dental microwear in some Swartkrans (A. robustus) and Sterkfontein (A. africanus) specimens. The interesting part of the study is the use of fractal analysis to quantify the complexity of scanned surfaces. They scanned a very tiny area of each tooth, around 200 micrometers on a side. Then they fed the scans through an algorithm to calculate texture.

The basic link to diet is the same as before: hard, brittle foods leave scars and pits, tough pliable foods leave directional marks like scratches.

Some results:

Fossil hominin results indicate that P. robustus (Asfc 4.29 2.150) has microwear textures more complex (chi-squared = 8.17, P < 0.005; Kruskal-Wallis test) and more variable in complexity (F = 16.82, P < 0.0005) than A. africanus (Asfc 1.686 +/- 0.52) (Fig. 2c, d). These results are consistent with the hypothesis that P. robustus incorporated more hard and brittle foods in its diet. However, some overlap in Asfc for the hominins (Fig. 3b) suggests that P. robustus was unlikely to have been a specialized hard-object feeder. It is more likely that hard, brittle foods were an occasional but important part of the diet. Previous studies have emphasized average differences between species rather than overlap, because low repeatability associated with observer error made assessments of within-species variability difficult.
In contrast, the microwear textures of Australopithecus africanus (epLsar1.8 0.0045 +/- 0.00163) show greater anisotropy (chi-squared = 3.84, P = 0.05; Kruskal-Wallis test) and epLsar variability (F = 7.38, P < 0.01) than P. robustus (epLsar1.8 0.0028 0.00060) (Fig. 2c, d). These data suggest a tougher diet on average for A. africanus compared with P. robustus, but one that is also more variable in its toughness (Scott et al. 2005:694).

The interesting thing is the overlap between the two samples. The authors also compared cebus and howler monkeys, finding extensive variation in both taxa, with minimal overlap in distributions (howlers are leaf-eaters, cebus eat a wider range of foods including some hard items). The two hominids overlap almost completely in "surface complexity" (i.e. whether they are pitted and scarred), with the main difference between the samples being an average greater complexity in A. robustus and an average greater anisotropy (i.e. grooving and scratching) in A. africanus. A third or so of each sample lie in the region of overlap in both variables.

From these measures, the diet variation within each species appears to be more extensive than the differences between them. The authors suggest this pattern of differences may represent a basically uniform diet with different fallback foods:

The greater variation in complexity for P. robustus and in anisotropy for A. africanus suggests that these species altered different components of their diet, but that there was probably substantial overlap in the fracture properties of their preferred foods. Thus, the clear differences between A. africanus and P. robustus microwear may relate, in part, to differences in critical dietary resources consumed only periodically during the year (Scott et al. 2005:695).

That would certainly be concordant with the stable isotope data. I guess it's a good thing for them that these two species weren't contemporaries.

References:

Scott RS, Ungar PS, Bergstrom TS, Brown CA, Grine FE, Teaford MF, Walker A. 2005. Dental microwear texture analysis shows within-species diet variability in fossil hominins. Nature 436:693-695. Full text (subscription required)