# Assessing the Relationship Between Blubber Thickness and Adipocyte Size in Beluga Whales

## TL;DR

The study found that blubber thickness and fat cell size are linked in male beluga whales but not in females, suggesting sex-specific differences in energy storage.

## Contribution

The paper reveals a sex-specific relationship between blubber thickness and adipocyte size in beluga whales, offering new insights into health indicators.

## Key findings

- Male beluga whales with thicker blubber have larger fat cells, indicating a link between blubber thickness and energy storage.
- Female beluga whales do not show a relationship between blubber thickness and fat cell size, possibly due to maintaining blubber structure during pregnancy and nursing.
- Sex-specific differences in fat storage suggest the need for tailored health assessment methods in beluga whales.

## Abstract

Beluga whales store energy from food as fat in their blubber, making blubber thickness a common measure for beluga whale health. However, blubber also functions as insulation, and helps with swimming efficiency, meaning blubber thickness may not always be reflective of energy stores. To further assess blubber thickness as a health indicator in beluga whales, we tested whether a relationship existed between blubber thickness and blubber fat cell size, which is a more direct measurement of energy storage. We examined beluga whale fat cells from different blubber depths in both sexes to determine their influence on the relationship between blubber thickness and fat cell size. We found that male beluga whales with thicker blubber had larger fat cells, indicating that blubber thickness and fat cell size can be used interchangeably to assess male beluga whale health. However, this relationship was not observed for females. Female beluga whales may require stable blubber structure even when using energy reserves, such as during pregnancy and nursing, to maintain insulation and buoyancy when supporting swimming calves. Continuing to develop methods to measure beluga whale health will be important for understanding the impact of current and future environmental changes on beluga whale populations.

Body condition, the relative amount of energy reserves in an individual, reflects nutritional status and overall health in marine mammals and can indicate the influence of stressors on individuals. Energy in marine mammals is primarily stored as lipids within adipocytes in blubber tissue, making blubber thickness a common proxy for body condition. However, blubber also serves structural roles, complicating its use as a body condition indicator. Our objective was to assess the relationship between adipocyte size, a common measure of cetacean adiposity, and blubber thickness in beluga whales (Delphinapterus leucas). We used mixed-effect generalized linear models to test how sex and blubber layer influenced this relationship. We found a significant positive relationship between adipocyte size and blubber thickness in male but not female beluga whales, suggesting sex-specific differences in fat storage or mobilization. Blubber thickness may be maintained in female beluga whales during periods with low energy reserves, for example during gestation and lactation, to preserve buoyancy, insulation, and hydrodynamism, which may be especially important when supporting swimming calves. Continuing to develop methods to assess beluga whale health will further our understanding of the impact of current and future stressors on beluga whale populations.