Dear colleagues, JASA just published my Letter to the editor. It contains a few mathematical and conceptual perspectives on rhythmic structure in harbor seals’ vocalisations, and pinnipeds more in general. The article can be found here: http://asa.scitation.org/doi/full/10.1121/1.5021770 . If you would like to obtain a PDF, do not hesitate to contact me at: andrea.ravignani@gmail.com . Kind regards, Andrea Ravignani, A. (2018). Comment on “Temporal and spatial variation in harbor seal (Phoca vitulina L.) roar calls from southern Scandinavia” [J. Acoust. Soc. Am. 141, 1824-1834 (2017)]. The Journal of the Acoustical Society of America. 143(1):504-508. Abstract. In their recent article, Sabinsky and colleagues investigated heterogeneity in harbor seals' vocalizations. The authors found seasonal and geographical variation in acoustic parameters, warning readers that recording conditions might account for some of their results. This paper expands on the temporal aspect of the encountered heterogeneity in harbor seals' vocalizations. Temporal information is the least susceptible to variable recording conditions. Hence geographical and seasonal variability in roar timing constitutes the most robust finding in the target article. In pinnipeds, evidence of timing and rhythm in the millisecond range—as opposed to circadian and seasonal rhythms—has theoretical and interdisciplinary relevance. In fact, the study of rhythm and timing in harbor seals is particularly decisive to support or confute a cross-species hypothesis, causally linking the evolution of vocal production learning and rhythm. The results by Sabinsky and colleagues can shed light on current scientific questions beyond pinniped bioacoustics, and help formulate empirically testable predictions. Andrea Ravignani Research Dpt., Sealcentre Pieterburen AI-Lab, Vrije Universiteit Brussel ravignani.wordpress.com