The blubber adipocyte index: A nondestructive biomarker of adiposity in humpback whales (Megaptera novaeangliae)

Abstract

The ability to accurately evaluate the energetic health of wildlife is of critical importance, particularly under conditions of environmental change. Despite the relevance of this issue, currently there are no reliable, standardized, nonlethal measures to assess the energetic reserves of large, free‐roaming marine mammals such as baleen whales. This study investigated the potential of adipocyte area analysis and further, a standardized adipocyte index (AI), to yield reliable information regarding humpback whale (Megaptera novaeangliae) adiposity. Adipocyte area and AI, as ascertained by image analysis, showed a direct correlation with each other but only a weak correlation with the commonly used, but error prone, blubber lipid‐percent measure. The relative power of the three respective measures was further evaluated by comparing humpback whale cohorts at different stages of migration and fasting. Adipocyte area, AI, and blubber lipid‐percent were assessed by binary logistic regression revealing that adipocyte area had the greatest probability to predict the migration cohort with a high level of redundancy attributed to the AI given their strong linear relationship (r = −.784). When only AI and lipid‐percent were assessed, the performance of both predictor variables was significant but the power of AI far exceeded lipid‐percent. The sensitivity of adipocyte metrics and the rapid, nonlethal, and inexpensive nature of the methodology and AI calculation validate the inclusion of the AI in long‐term monitoring of humpback whale population health, and further raises its potential for broader wildlife applications.

Assessing cetacean body condition: Is total lipid content in blubber biopsies a useful monitoring tool?

“…For example, blubber cellularity, specifically adipocyte size and number, shows some potential as a biomarker of adiposity (Castrillon, Huston, & Bengtson Nash, 2017). For example, blubber cellularity, specifically adipocyte size and number, shows some potential as a biomarker of adiposity (Castrillon, Huston, & Bengtson Nash, 2017).…”

“…Further work should prioritize measuring other components of blubber biopsies as potential markers of condition. For example, blubber cellularity, specifically adipocyte size and number, shows some potential as a biomarker of adiposity (Castrillon, Huston, & Bengtson Nash, 2017). Using the proportions of different saturated/unsaturated fatty acids or the proportion of triaclyglycerols to wax esters, for example, could also be investigated as a more reliable marker.…”

Signals from the south; humpback whales carry messages of Antarctic sea‐ice ecosystem variability

“…This monitoring also included the sentinel parameter of "diet," evaluated by traditional carbon and nitrogen Bulk Stable Isotope (BSI) measures. The first adiposity marker, the adipocyte index (AI; Castrillon, Huston, & Bengtson Nash, 2017), is based upon the principle that adipocyte number is set early in mammalian development and remains constant throughout adulthood and fluctuating weight (Faust, Johnson, Stern, & Hirsch, 1978). Any change in lipid stores is therefore a result of a change in the volume of existing adipocytes, rather than adipogenesis.…”

“…The Adipocyte Index was derived as described elsewhere (Castrillon et al, 2017). Five to twenty-seven animals per sampling period were used for AI metrics.…”

Not just fat: investigating the proteome of cetacean blubber tissue

“…However, this approach has problems associated with the loss of lipid from the sample upon collection ( Ryan et al , 2012 ), and the stratification of lipid through the blubber depth ( Koopman et al , 1996 ; Olsen and Grahl-Nielsen, 2003 ; Smith and Worthy, 2006 ). Another potential approach to infer condition is to measure the size of the adipocytes themselves which may be more promising ( Castrillon et al , 2017 ).…”