Fig 1.
The photogrammetry procedure requires the photographer to slowly circle the seal, taking 8–12 photographs from all possible perspectives (i.e. kneeling, standing, portrait photographs, and landscape photographs).
Photos are imported into PhotoModeler and a 3D shape is created by referencing scaled photographs to one another.
Fig 2.
For the “silhouette slice” method, the portion of the seal below the ground plane is identified in PhotoModeler (top panel) and is removed to reduce error (bottom panel).
Table 1.
Comparison of actual mass, parameter estimations, and percent error across estimation methods given as mean ± standard deviation for 56 animals.
Mass was estimated using three methods: elliptical “cones”, “correction equation”, and “silhouette slice”. Percent error was calculated as 100*the difference between the estimated mass and the actual mass divided by the actual mass. Superscript letters denote a significant difference in parameters across estimation methods. The sedation, equipment, and time requirements for all methods are noted.
Fig 3.
Estimated body density for adult, female Weddell seals calculated from actual body mass and estimated volume.
The elliptical “cones” method estimated a higher density than both “correction equation” and “silhouette slice” methods. For reference, vertical lines show the density of seawater (black dotted line, 1.027 g cm-3 [36]), blubber (black dashed line, 0.920 g cm-3, this paper), and lean tissue (black dashed-dotted line, 1.1 g cm-3 [35]).
Fig 4.
Regressions between actual and estimated mass for the three methods discussed: The equations calculated from de Bruyn et al.
[32] (“correction equation”, left panel, p<0.0001), the above-ground estimation (“silhouette slice”, middle panel, p<0.0001), and the truncated cones method (“elliptical cones”, right panel, p<0.0001). Black dashed lines show the 1:1 relationship between estimated and actual mass, whereas colored solid lines show the regression for each method. Horizontal lines show the offsets between data points and the 1:1 line.
Fig 5.
Boxplots of the percentage error for each estimation method with frequency distributions overlaid as dashed lines.
Mass estimates from each method were not significantly different than actual mass in the Weddell seals.