title: Comparisons and Uncertainty in Fat and Adipose Tissue Estimation Techniques: The Northern Elephant Seal as a Case Study.
abstract: Fat mass and body condition are important metrics in bioenergetics and physiological studies. They can also link foraging success with demographic rates, making them key components of models that predict population-level outcomes of environmental change. Therefore, it is important to incorporate uncertainty in physiological indicators if results will lead to species management decisions. Maternal fat mass in elephant seals (Mirounga spp) can predict reproductive rate and pup survival, but no one has quantified or identified the sources of uncertainty for the two fat mass estimation techniques (labeled-water and truncated cones). The current cones method can provide estimates of proportion adipose tissue in adult females and proportion fat of juveniles in northern elephant seals (M. angustirostris) comparable to labeled-water methods, but it does not work for all cases or species. We reviewed components and assumptions of the technique via measurements of seven early-molt and seven late-molt adult females. We show that seals are elliptical on land, rather than the assumed circular shape, and skin may account for a high proportion of what is often defined as blubber. Also, blubber extends past the neck-to-pelvis region, and comparisons of new and old ultrasound instrumentation indicate previous measurements of sculp thickness may be biased low. Accounting for such differences, and incorporating new measurements of blubber density and proportion of fat in blubber, we propose a modified cones method that can isolate blubber from non-blubber adipose tissue and separate fat into skin, blubber, and core compartments. Lastly, we found that adipose tissue and fat estimates using tritiated water may be biased high during the early molt. Both the tritiated water and modified cones methods had high, but reducible, uncertainty. The improved cones method for estimating body condition allows for more accurate quantification of the various tissue masses and may also be transferrable to other species.
authors.affiliation: Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Schwarz
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Stella
authors.initials: S
authors.lastname: Villegas-Amtmann
authors.affiliation: Department of Biological Sciences, University of Alaska, Anchorage, Alaska, United States of America.
authors.forename: Roxanne S
authors.initials: RS
authors.lastname: Beltran
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Costa
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Chandra
authors.initials: C
authors.lastname: Goetsch
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Luis
authors.initials: L
authors.lastname: Hückstädt
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Maresh
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California, United States of America.
authors.forename: Sarah H
authors.initials: SH
authors.lastname: Peterson
mesh_terms: D000273:Adipose Tissue / Q000502:physiology*; D050154:Adiposity; D000818:Animals; D005260:Female; D046048:Seals, Earless / Q000033:anatomy & histology / Q000502:physiology*; D014316:Tritium / Q000378:metabolism; D014465:Ultrasonics / Q000379:methods*; D035501:Uncertainty
chemical_list: D014316:Tritium