Morphology of nares associated with stereo-olfaction in baleen whales

The sensory mechanisms used by baleen whales (Mysticeti) for locating ephemeral, dense prey patches in vast marine habitats are poorly understood. Baleen whales have a functional olfactory system with paired rather than single blowholes (nares), potentially enabling stereo-olfaction. Dimethyl sulfide (DMS) is an odorous gas emitted by phytoplankton in response to grazing by zooplankton. Some seabirds use DMS to locate prey, but this ability has not been demonstrated in whales. For all 15 extant species of baleen whale, nares morphometrics (imagery from unoccupied aerial systems, UAS) was related to published trophic level indices using Bayesian phylogenetic mixed modelling. A significant negative relationship was found between nares-width and whale trophic level (β = -0.07, Lower 95% CI = -0.12, Upper 95% CI = -0.02), corresponding with a 36% increase in nares-width from highest to lowest trophic level. Thus, species with nasal morphology best suited to stereo-olfaction are more zooplanktivorous. These findings provide evidence that some baleen whale species may be able to localise odorants e.g., DMS. Our results helps direct future behavioural trials of olfaction in baleen whales, by highlighting the most appropriate species to study. This is a research priority, given the potential for DMS-mediated plastic ingestion by whales.

Methods
Morphometric measurements were made for free-ranging whales mostly using UAS or aircraft-based imagery, including stills from video footage. The selection criteria for images were that they were vertical, in focus and clearly showed the nares. Altitude was typically between 10 – 60 m, however accurate altimetry data were not available for all flights and therefore relative measurements (i.e., pixel counts) were used. Because images were compiled from a range of different studies, the UAS, camera, camera sensor, lens focal length, altitude and thus pixel resolution will have varied among sampling events . Similarly, lens distortion will vary in its effect on whale measurements due to differences among UAVs. To mitigate for possible effects from image distortion, only images where the subject of interest (head) was in the center of the frame (≥ 30% from all frame edges) were used . 

Absolute measurements were not generated in this study, rather those previously published (total body length) estimates were used (tables S1 & S2). To compile a dataset of all 15 extant species of baleen whales, additional images for those rarely documented species were sourced from publications and reports . Two of these photographs were taken from a ship or underwater, but all clearly showed the nares. For each individual whale, the clearest available overhead image (i.e., photograph or video still) was used to measure the maximum width (pixels) of the open or closed nares and the maximum width of the body (pixels) at the nares using the straight-line tool in ImageJ Analyze and Measure (https://imagej.nih.gov/ij/index.html).

The maximum-nares-width measurements could vary depending on what point during the respiration cycle that the measurement was made, from fully closed through to fully open. We considered this by assigning two simplified nares states which were assigned to each set of measurements: open (ranging from fully to partially open: bubbles, vapour or respiratory tract visible) and closed (no bubbles, vapour or respiratory tract visible). The aim was to obtain a set of measurements for both states for each whale, which was not always possible.

To assign a trophic level to each whale species, we used available estimates based on diet from the SeaLifeBase database. There were no available estimates in the database for pygmy right (Caperea marginata), Rice’s (B. ricei) and Omura’s (B. omurai) whales. Instead, we determined the preferred prey for these whale species from published sources and applied trophic level estimates of the preferred prey plus 1. For phylogeny, we used the dated mammal phylogeny from Upham et al. which was truncated to include only those species present in our dataset. Because Rice's is missing from Upham et al., we added it as a sister species, Eden’s whale (B. edeni), based on its current phylogenetic positioning.