Referee 1's review:

The authors have conducted a well-designed empirical test for the presence and strength of frequency and density dependent selection on female reproductive effort in voles. Their larger aim was to assess whether the selection pressures measured could explain the origin and maintenance of the variation in life-history strategies displayed by female bank voles.

All of the statistical results point to clear negative density dependent selection for the low RE strategy (higher fitness at low densities), as well as positive density/negative frequency dependent selection for the high RE strategy (high fitness at high density, but only when rare).

My main concern with the analysis is sample size. I believe the ‘story’ the authors are telling is supported by their data, but the small sample sizes make me wary of any strongly-worded statistical conclusions. Although all of the results are statistically significant, some treatment combinations only had sample sizes of two enclosures. I find error bars create the illusion of larger sample size, even though the small N is labeled on the figures. I suggest that some of the figures (figure 1a,b and figure 3) would be more appropriately presented as scatterplots including all the data points, as opposed to means and standard errors.

Additionally, I am not entirely convinced that the dashed line in figure 2 fits the open-circle data points as shown. I tried plotting the points myself and regenerating the line, but my line differed greatly from the one shown. Are there perhaps some overlapping data points obscuring the relationship?

The article itself is clearly-written, objective, but concise almost to the point of terseness. Consequently I think it would be mainly of interest to other specialists in the field of life-history evolution, but may not be easily accessible to non-specialists unfamiliar with the subject. Some additional general review of the literature of selection on life history traits in the introductory and discussion sections would help appeal to a more general audience, if desired. If possible I would also suggest altering the title of the manuscript to something more specific – including the name of the organism, perhaps. The current title is more suggestive of a general-review-paper than a specific experimental study of a particular system.

Convincing field/experimental studies measuring selection are rare and difficult to produce, this one definitely deserves to be published after some minor revision.

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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.