DOI: 10.5061/dryad.rn8pk0pjb
title: Morphology of nares associated with stereo-olfaction in baleen whales

description: 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.
description: 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
 (<em>Caperea marginata</em>), Rice’s
 (<em>B. ricei</em>) and Omura’s (<em>B.
 omurai</em>) 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 (<em>B.
 edeni</em>), based on its current phylogenetic
 positioning.
description: # Morphology of nares associated with stereo-olfaction in baleen whales
 The dataset contains one
 .csv file and the necessary code to reproduce our results using the data
 therein. The .csv file contains nares measurements, made using UAV (drone)
 imagery with some additional underwater, ship-bourne and airplane-bourne
 images for rarely observed whale species. This .csv data file also
 contains trophic index information derived from the literature and the
 citations to these sources are listed therein. Both .csv and .tre files
 described below are required to produce the Bayesian phylogenetic mixed
 model presented in the paper. We controlled for the expected similarity in
 trait values due to common descent among species using a distance matrix
 of the phylogenetic distance between these species. The code for this
 phylogenetic tree is provided in the .tre file, as described in the
 methods section of the paper. R (R Core Team, 2022) code to run the
 analysis is included as a .pdf file which also includes the sensitivity
 analysis, whereby nares measurements made from non-UAV platforms listed
 above are considered, in order to rule out the possibility that they are
 overly influential on the final model fit. ## Description of the data and
 file structure ### whale\_blowhole\_measures\_1\_12\_2023.csv The data
 comprise the following rows with the following defined column headers,
 i.e. row 1 in order of occurence: 1. *species*: Categorical variable.
 Binomial species name. 2. *sp_common*: Categorical variable. Common
 species name. 3. *contributor*: Categorical variable. Initials of the UAV
 data contributor. Full names can be found in the author list or
 acknowledgements section of the paper. 4. *samplingfolder*: Categorical
 variable. For filing purposes, used by the authors to track original file
 location. 5. *individual*: UID, unique identifier. Assigned to each
 independent sample, i.e. each whale. 6. *bl*: Continuous variable. Body
 length (m), from previous published studies (See methods for details). 7.
 *state*: Categorical variable. Can be considered a binary variable which
 indicates whether the nares were *open* or *closed* when the corresponding
 measurements were made (See methods for definition of each state). 8.
 *bottom_spacing*: Continuous variable. Distance (pixels) between a pair of
 nares at the caudal end of the nares. 9. *tl_fin*: Continuous variable.
 Trophic index assigned to each species (See methods for details). 10.
 *tl_ref*: Categorical variable. ISBN numbers, DOI numbers or weblinks to
 the source of *tl_fin*. Note that parent source, indicated as SeaLifeBase
 is found at 11. *width_at_nares*:
 Continuous variable. Width (pixels) of the whale's head in line with
 the caudal-end of the nares. 12. *bottom_width_ratio*: Continuous
 variable. Ratio of *bottom_spacing* to *width_at_nares*. ###
 MamPhy\_BDvr\_Completed\_5911sp\_topoCons\_NDexp\_v2\_tree0000\_(1).tre
 Phylogentic tree file derived using results by Upham *et al.* (2019) in a
 text-based format. See methods section in the paper for how this has been
 truncated. The accompanying R code is used to read-in the phylogeny data
 using the *read.tree* command. ## Code/Software ###
 Ryan\_et\_al\_code\_fig\_redacted.pdf R code was written and annotated by
 Kevin Healy using R version 4.2.2 (R Core Team, 2022). It uses the
 accompanying .csv and .tre files described above to reproduce the Bayesian
 phylogenetic mixed models (BPMM) presented in the paper, using the
 *MCMCglmm* package (Hadfield, 2010). Resulting figures have been redacted
 from the pdf for copyright reasons, but they will be produced when the
 code is run by users in R. ## References Hadfield JD. 2010 MCMC Methods
 for Multi-Response Generalized Linear Mixed Models: The **MCMCglmm** *R*
 Package. *J Stat Softw* **33**. (doi:10.18637/jss.v033.i02) R Core Team
 2022 R: A language and environment for statistical computing. Version
 4.2.2. R Foundation for Statistical Computing, Vienna, Austria. URL
 . Upham NS, Esselstyn JA, Jetz W. 2019
 Inferring the mammal tree: Species-level sets of phylogenies for questions
 in ecology, evolution, and conservation. *PLoS Biol* **17**, e3000494.
 (doi:10.1371/journal.pbio.3000494)