Referee 1's review:

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication, the manuscript has been revised in light of these comments and to address other editorial requirements.
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Authors: Ungar, Grine, Teaford
Title: Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei

In this paper Ungar et al examine the microscopic wear marks of teeth in perhaps one of the most robust jawed Hominoid, Paranthropus boisei. The authors use state of the art data acquisition and analysis of surface texture and basically report that P. boisei lacks features characteristic of both specialized hard object feeders and fibrous vegetation feeders. In general, the paper demonstrates a fascinating paradox of a massively robust phenotype that may not reflect its 'everyday' functional signature. This would indicate that relatively rare (but presumably severe) selective events may drive evolution of even the more extreme phenotypes.

The methods and analyses appear valid and I have only a few niggles that should be easily addressed.

1) The authors consider it unlikely that the lack of high complexity and anisotropy values in P. boisei is due to the 'last supper-effect' where the microwear just happens by chance to not correlate with the foods that have adaptively shaped the dentition. Yet, (e.g., figure 2) two out of the seven specimens show quite large anisotropy values. I wonder whether it might be useful to discuss which specimens show larger and smaller anisotropy values and whether this would help to interpret the feeding ecology better.

2) Page 5, last paragraph
Comment: What is Facet #9, is representative of the whole crown?

3) Page 6, first paragraph, last sentence "The environments in which they lived were dominated by grasslands...."
Comment: It would be perhaps better to state "The environments in which they lived are reconstructed to have been dominated by grasslands...."

4) Figure 1.
Comment: What are photosimulation montages? The figure caption would benefit from some further explanations.

5) Figure 2
Comment: Even though the axes are explained in the text, they should be stated in the caption as also the general point of the figure.

6) The author may wish to consider whether a picture, even a drawing, showing the massive skull and dentition of P. boisei would be informative.