Comparisons and Uncertainty in Fat and Adipose Tissue Estimation Techniques: The Northern Elephant Seal as a Case Study

<div><p>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 (<i>Mirounga</i> 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 (<i>M</i>. <i>angustirostris</i>) 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.</p></div

Cold birds under pressure: Can thermal substitution ease heat loss in diving penguins?

Thermoregulation could represent a significant fraction of the total energy budget of endotherms under unfavourable environmental conditions. This cost affects several traits of the ecology of an organism such as its behaviour, distribution, or life history. Heat produced by muscle contraction during activity can be used to pay for heat loss or thermoregulation in many species (known as “thermal substitution”). This study seeks to unite the effects of temperature, depth, and activity on the energetic costs of endotherm divers using the Magellanic penguin as model species and to evaluate whether penguins may benefit from thermal substitution. This species operates under highly variable temperature and depth conditions along its breeding range and provides an ideal natural experiment. A developed thermodynamic model describing foraging activity predicted that the major element affecting heat loss was depth, exacerbated by temperature. Birds living in colder waters are predicted to be able to minimize costs by executing shallower dives and benefit from thermal substitution by swimming faster, particularly during deeper dives. The model was evaluated in two contrasting scenarios: (1) when birds swim near the surface commuting to the foraging areas and (2) when birds dive to depth to forage. Activity data from tags on free-living penguins indicated two of these predictions were apparent; penguins generally travelled faster while commuting at the surface in colder waters, while birds from warmer water colonies dived deeper while foraging. Contrary to predictions, however, penguins swam slower at deeper depths during both descent and ascent phases of foraging dives. These results suggest that penguins may benefit from thermal substitution by swimming faster when birds perform shallow dives commuting to and back from foraging areas, but they provide no evidence of behavioural response (via swimming faster) for thermoregulation when diving to depth to forage. Reasons for this are discussed and include the relevance of prey abundance in 3-d space and maximizing dive duration by conserving oxygen reserves. The way the bird operates will have profound consequences for the energy needed and therefore necessary energy acquisition rates. Expansion of our findings to other diving endotherms might help explain both global activity patterns and energy flow in ecosystems.Fil: Ciancio Blanc, Javier Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto de Biología de Organismos Marinos; ArgentinaFil: Quintana, Flavio Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto de Biología de Organismos Marinos; ArgentinaFil: Sala, Juan Emilio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto de Biología de Organismos Marinos; ArgentinaFil: Wilson, Rory P. Swansea University. College Of Sciences. Departament Of Biosciences; Reino Unid