Ryan et al. take an interesting approach to test a provocative idea: does nares morphology across all baleen whales reflect a specialization towards prey that emit the kind of chemicals that would suggest these whales use stereo-olfaction to hunt? They make a compelling case for yes. This was an exciting, fun, and thorough manuscript to read, and I enthusiastically recommend it for publication in Biology Letters — it’s an ideal contribution in scope, length, and conclusions.

First, the authors provide the all-important frame: we don’t really know how the largest predators on the planet find their food, but it’s likely through olfaction; also, that should worry us given that ocean plastics emit the same chemical signature. The authors take advantage of data collection platforms in use today for behavioral ecology and energetics programs to focus on the nares: as many specialists would realize, these data are abundantly collected both today and can be found in the literature too. I really appreciated their focus on individual whales (as a way to maximize interspecific sampling) for which the specific nares and body width measurements could be collected. A thoroughly fun methods section to read! (Something I think any reviewer would rarely say). Also great job controlling for phylogeny, and checking the results for the spotty data on diet and UAS nares shots (Caperea and the mid-sized Balaenoptera). It’s really clear that the authors have thought through this study very carefully.

I thought that their conclusion that trophic status predicts nares spacing in baleen whales is profound, and delivers an important conclusion well worth disseminating to the readership at Biology Letters. I also appreciated that they provided nuance on their results with respect to stereo-olfaction -- something likely, but not yet fully demonstrated across all species. I think this particular item underscores the value of creatively collected data across a wide range of species -- it's hard work, and we still have a lot of in vivo data to collect, one day.

This is an excellent, short paper, and I could find essentially nothing wrong with it. I offer only two quick points for contemplation (truly comments, not things to fix), and some minor items below. This paper is ready for publication pending these very minor fixes, and I encourage the authors to prepare media packages with their best blowhole shots -- this is a story that will surely result in high media engagement.

Two quick of point of observation: First, do pinnipeds — say, Southern seals — use olfaction for DMS similarly? They lack a cribiform plate but map on trophic indices in the same way as many of the baleen whales in this dataset. It's less a mystery how they hunt, but the anatomical apparatus is worth a consideration. Second, on stereo-olfaction, did the authors note any right/left asymmetry in blowhole size in their dataset? There are potentially minor but significant differences in aperture size that could be discerned in the dataset, which may provide unexpected support for the stereo-olfaction hypothesis, even if narial closure is a bilateral event.

Minor, minor items.

• ll. 55-56. Suggested edits “Baleen whales (or Mysticeti)…trophic ecologies among vertebrates.” Suborder is just a Linnean rank that has little meaning (and worse comparability), same with “animal kingdom,” a frequently used set of words that are essentially free of phylogeny. Suggestion eschewing their use in the future.

• ll. 78-79, Same as above, what in the heck does a suborder even mean? Delete, doesn’t alter the precision of the taxonomic word “Odontoceti”

• l. 214, NARWs is insider baseball that most readers outside of modern whale ecology and conservation won't grasp. Use the abbreviation at l. 204.

• l.426, italicize brydei