Specimens of Opportunity Reveal Novel Information on the Sowerby's Beaked Whale (Mesoplodon bidens)
Conservation science requires quickly acquiring information and taking action in order to protect species at risk of extinction. Elusive wildlife, however, present challenges to effective conservation measures because it is often difficult to collect enough data on these species to recognize their conservation needs and implement management plans. As a result, researchers must identify alternative means of gathering sufficient data on these species. Specimens of opportunity, such as museum specimens, provide a way to improve knowledge on these species, and these specimens have already proven valuable by increasing information on biodiversity, habitat and range, and population structure in many species. Beaked whales (family Ziphiidae) are a prime example of elusive species, and as a result little is known about their biology and ecology. This speciose group of cetaceans is challenging to locate and distinguish in situ due to the elusive behavior and similar appearance amongst species. Thus, specimens of opportunity may be the most efficacious means of gathering enough information on beaked whales to make informed conservation decisions. In this study I utilized specimens of opportunity from museum and research institutions to increase knowledge and generate data on the Sowerby’s beaked whale (Mesoplodon bidens). First described in 1804, little is known about this species’ basic biology or ecology. I employed snowball sampling to identify museums with specimens and collected data on 180 specimens from 24 museum and research institutions. I collected data on specimen collection date and location, sex, age class, and 7 skull and mandibular measurements. I quantified skull and mandibular measurements and found that specimens collected in the west Atlantic demonstrated significantly greater median values for total skull length, proximal beak width, total mandibular length, and mandibular symphysis to distal end length. Quadratic discriminant analysis of skull and mandibular measurements successfully assigned specimens to their collection location in 78.6% of specimens, suggesting this species may be comprised of ≥2 distinct populations. For 178 specimens I collected a bone or soft tissue sample for stable isotope analysis. It was not possible to sample the same location in each specimen, which necessitated a means to account for variation in isotope values among tissues, and especially across multiple skeletal elements. To quantify carbon and nitrogen intraskeletal variation I sampled the same eight skeletal elements from 72 cetacean skeletons from 14 cetacean species. Isotope variation across skeletons was greater than anticipated based on previous studies. Carbon intraskeletal ranges varied from 0.4 to 7.6‰, with 84.7% (n = 61) of skeletons having a range >1‰, and 55.5% (n = 40) exhibiting a range >2‰. Similarly, nitrogen intraskeletal ranges varied from 0.4 to 5.2‰, with 59.7% (n = 43) of skeletons exhibiting a range >1‰, and 15.3% (n = 11) with a range >2‰. For Sowerby’s beaked whale, I identified a median carbon intraskeletal variation of ~4.1‰ and nitrogen variation of ~1.3‰. For soft tissue samples I needed to verify that the current lipid extraction methods were appropriate for the samples I had collected. Some cetacean tissues, such as skin and muscle, are depleted in 13C compared to synthesized proteins, so that the presence of lipids within protein samples tends to reduce bulk tissue δ13C values and influence stable isotope analysis. However, extraction methods can also alter stable isotope ratios, especially nitrogen. Thus, I trialed two extraction methods, chloroform-methanol and cyclohexane, and identified the appropriate extraction for each soft tissue type. For low or moderate lipid proportion tissues, such as kidney and muscle, cyclohexane should be used, and chloroform:methanol for higher lipid proportion tissues such as skin. Finally, I conducted stable isotope analysis for δ13C and δ15N on Sowerby’s beaked whale bone, muscle, and skin tissues. I found consistent trends in isotope values across all three tissue types. Specimens collected in the east Atlantic had less enriched δ13C and δ15N than west Atlantic specimens, and median isotope values were significantly different between regions. Quadratic discriminant analysis considering δ13C and δ15N simultaneously correctly assigned 92.0, 90.0, and 80.3% of skin, muscle, and bone samples, respectively, to their collection location. These results indicate the Sowerby’s beaked whale specimens in this study exhibited short- and long-term regional site fidelity to the region from which they were collected. These stable isotope data from bone, muscle, and skin samples, combined with the significant differences in median skull measurements between regions, strongly suggests the Sowerby’s beaked whale exhibits a metapopulation structure. This information lays the groundwork for future studies in this species and provides critical knowledge regional and international conservationist scientists need.
Smith, Kerri Jean, "Specimens of Opportunity Reveal Novel Information on the Sowerby's Beaked Whale (Mesoplodon bidens)" (2020). ETD Collection for University of Texas, El Paso. AAI27998837.