Referee 1's Review:

This is an interesting study, using operant conditioning techniques to shape tool using behavior in degus. The final performances of the animals are impressive, but I think the authors are going too far when they state that the behaviors are comparable to those shown by nonhuman primates.

To elaborate on this point, although there appears to be a trend for the degus to use the functional rake over the nonfunctional ones, with one exception the data for all four individuals are not statistically signficant (it is a pity that only 10 trials were run in each condition). Also, before the generalization test a "reversed presentation" test was presented (although this is not mentioned in the longer Procedures section). The degus manipulated the tool "just like in training." What does this mean? Did they start to manipulate the tool immediately after it was placed on the table, or did they systematically wait until the reward was subsequently put in place? If it is the former, then this destroys the argument that what the animals are doing is similar to what primates do in tool use situations. We need data on these reversed tests.

In my version of the manuscript there are no figure legends, which makes it very difficult to decipher what is going on in some of the figures, especially 1A, and 3A and B.

In several places the writing needs to be tighter. This could be done with good editing, but in places the authors' use of the literature is rather slack. Starting with the Abstract, why the focus on corvids? The New Caledonian crow example of tool use is quite restricted, and I think that populations of Galapagos finches that use twigs to extract beetles from tree holes, and Egyptian vultures that use stones to break ostrich eggs are being unfairly neglected here. I don't see the need for most of the sentence starting "Although evolutional origin...". Instead I would simply state that "Most studies of animal tool use do not directly address..."

Introduction: It seems strange to state that tool use helps "determine human uniqueness" in the first sentence, but then continue to describe cases of tool use in animals. Beck's book has hundreds of examples of tool use by animals!

I object to the authors' use of "lower" primates. Use "nonhuman" instead.

Procedures: There needs to be more detail: In initial training, how was the reward given? By hand directly in front of the animal? Was any auditory cue given? How far was the reward from the tool at the start of each trial? What happened in cases of failure after 60 seconds? Were the food and reward removed? The placed again in the same configuration? What was the inter-trial interval?

References: Some of the titles wrongly have capital letters. I think that the recent paper on use of rake-like tools by capuchin monkeys (Fujita et al) should be cited.

Figures: Nicely presented, but those showing the trajectories of the tool need to be accompanied by good legends, which I don't have.

Again, if the authors want to claim that the degus' behavior is comparable to that of monkeys, we need more information about how they actually dealt with the problems presented. For example, if a degu's first attempt to bring the tool into the correct position for raking in the reward failed, what did it do? Did it immediately try again? How many "correct" solutions occurred following initial failure?

Discussion: First paragraph: I'm not sure about the authors' claim of a "conventional view" about tool use and intelligence. Who holds such a view? Certainly not Beck, who has written papers to argue that non-tool use behavior can be just as cognitively complex as tool-mediated behaviors.

I don't know much about rodent neuroanatomy, but is it accurate to say that they have a "prefrontal" cortex?

In conclusion, this is a well-written account of training up rodents to perform simple tool-use tasks. The study is good preparation for future physiological work, but I think that either more prudent conclusions are required before claiming that there are basically no differences between what the degus are doing and what monkeys do, or else the evidence for this needs to be considerably stronger.

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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.