Improving the Precision of Our Ecosystem Calipers: A Modified Morphometric Technique for Estimating Marine Mammal Mass and Body Composition

Abstract

Mass and body composition are indices of overall animal health and energetic balance and are often used as indicators of resource availability in the environment. This study used morphometric models and isotopic dilution techniques, two commonly used methods in the marine mammal field, to assess body composition of Weddell seals (Leptonychotes weddellii, N = 111). Findings indicated that traditional morphometric models that use a series of circular, truncated cones to calculate marine mammal blubber volume and mass overestimated the animal’s measured body mass by 26.9±1.5% SE. However, we developed a new morphometric model that uses elliptical truncated cones, and estimates mass with only −2.8±1.7% error (N = 10). Because this elliptical truncated cone model can estimate body mass without the need for additional correction factors, it has the potential to be a broadly applicable method in marine mammal species. While using elliptical truncated cones yielded significantly smaller blubber mass estimates than circular cones (10.2±0.8% difference; or 3.5±0.3% total body mass), both truncated cone models significantly underestimated total body lipid content as compared to isotopic dilution results, suggesting that animals have substantial internal lipid stores (N = 76). Multiple linear regressions were used to determine the minimum number of morphometric measurements needed to reliably estimate animal mass and body composition so that future animal handling times could be reduced. Reduced models estimated body mass and lipid mass with reasonable accuracy using fewer than five morphometric measurements (root-mean-square-error: 4.91% for body mass, 10.90% for lipid mass, and 10.43% for % lipid). This indicates that when test datasets are available to create calibration coefficients, regression models also offer a way to improve body mass and condition estimates in situations where animal handling times must be short and efficient.

Weighing our measures: approach-appropriate modeling of body composition in juvenile Steller sea lions (Eumetopias jubatus)

“…In this instance, they utilized a length to girth index with great success for estimating mass, but was not tested for estimating elements of body composition, as was the case with others (Shero et al 2014). Other studies have used this dual-cone model to estimate fat and core volumes in phocids (harbor seal, Phoca vitulina L., 1758; Rosen and Renouf 1997).…”

“…Unfortunately, limited sampling ability prevented the current analysis from following that path. Most recently, Shero et al (2014) further refined the cone model approach for phocids, demonstrating that the incorporation of blubber depth can improve this indirect estimation of body composition as well as incorporating the elliptical nature of a seal body in mass estimation. Prior to the current study, however, this combined morphometric and ultrasound index approach had not yet been performed and validated for living otariids.…”

“…A second model included transformed morphometric terms for comparison with later models (e.g., Tierney et al 2001). Subsequent models used AXGI and STLE to generate two equal-length cones, attached at their bases, representing the whole body volume (WV; Table 1) of each individual, which is a popular method for estimating condition in phocids (e.g., Shero et al 2014). Three blubber indices were used to estimate the core radius of each cone-modeled individual and derive an inner lean core volume (LV) and an outer blubber volume (BV; Table 1).…”

Scaling matters: incorporating body composition into Weddell seal seasonal oxygen store comparisons reveals maintenance of aerobic capacities

“…A straight length was taken and total body mass (TBM) was measured using a sling with a hand winch and scale (MSI-7200-IT DynaLink digital dynamometer, capacity 1000 ± 1.0 kg), while animals were suspended from a tripod. Body composition (%lipid) and lean body mass (LBM) were determined for each animal using tritiated water dilution as described in Shero et al (2014Shero et al ( , 2015 (Table 1).…”

“…Mean corpuscular Table 1 Mean ± SE Weddell seal straight length, total body mass, lean mass, and lipid (as %TBM) among reproductive groups Sample sizes are shown in parentheses, and different letters indicate significant differences between reproductive groups. Note that animal straight length was included as a covariate for total body and lean mass comparisons, and that there were yearly variations in animal mass and lipid stores (see Shero et al 2014Shero et al , 2015 Body condition parameter Fall Spring Evan's Blue dye into the extradural vein. Syringes used to distribute the dye were pre-weighed and flushed with blood to accurately determine the amount of dye injected for each animal.…”