Referee 2's Review:

The study titled "Tool use training in a species of rodents: emergences of an optimal motor strategy and functional understanding" by Okanoya et al. is a very interesting and original demonstration on how it is possible to train rodents in captivity to manipulate tools to reach a food reward. The authors show that tool use (so far studied only in birds and primates) goes beyond phylogenic factors and they provide a much-needed rodent model to study the neurobiological bases of higher cognitive functions.
The relevance of this paper is also in the very detailed training procedure. The results are nicely described with the aid of visual material.

Overall this work it is suitable for publication. Only a few issues/curiosities should be addressed or revised as follows:
Introduction.
First paragraph, last sentence: "What is need is a useful..." change to "What is needed is a useful...". Last paragraph, first line: "As such, lower primates have been only animal models..." change to "As such, lower primates have been the only animal models..."
Results.
Individual learning curves are interesting and they could be presented in Fig. 2 together with the average data instead of being in the supplementary information. How were the five animals selected for the study? Were there other animals discarded because qualified as "poor learners"? Which of the five animals was the female? Were there any gender differences? Were the four animals in the probe tests all males? Why one animal did not participate in the probe tests? Has age of the animal some influence on how fast they learn tool use? Will there be subsequent studies on age/sex effects on this behavior?
Discussion.
Second paragraph, second-to-last sentence: "The degus must be taking advantage of the some of the tool's physical properties..." change to "The degus must be taking advantage of some of the tool's physical properties...". Third paragraph, first sentence: "After extensively trained with the tool use.." may be better as "After extensive training on the tool use..". Third paragraph, second sentence: the word "inappropriate" could be changed to "irrelevant"?. Third paragraph, last sentence: "Results can be interpreted that degus might tried mental manipulation...before..." change to "Results can be interpreted that degus might try mental manipulation...before...".
Last paragraph, third sentence: it is not clear the meaning of: "Our existential proof...".
Materials and Methods.
It is very interesting and convenient that no food restriction is required for the training procedure.
Was it difficult to train the degus to use the forelimbs instead of the mouth? Were there many attempts to use the mouth in moving the rake? Is it the shape of the rake-handle that promotes forelimb use?
References.
Reference #9 contains some misspelling in the title.

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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.