Beluga Whales
Blubber
Blubber accounts for about 30 to 50% of the beluga's entire body weight (Fig. 2). What is blubber and why is it so important?
Blubber is a specialized type of adipose tissue that serves a multitude of essential functions in belugas. This subcutaneous layer of fat that is found exclusively in marine mammals covers almost the entire body, and may occur in varying depths, lipid composition, and amount depending on the location on the body (these factors also differ from species to species). These differences influence the blubber's function across the body, such as thermoregulation in one area of the body, while serving as structural support in another.
Blubber Structure
Blubber is a spongy material made up of adipocytes, or fat cells, that can vary in size due to the ability to take in and let go of lipids. These fat cells are kept in place with special elastic and collagen fibers not found in other adipose tissue (Iverson, 2008). Blubber is also vascularized with several blood vessels and specialized shunts known as arterio-venous anastomoses (AVAs). Beluga blubber can measure around 10 centimeters thick, which is comparatively less compared to other marine mammals. The beluga's small body size may be responsible for the limited depth in blubber.
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A majority of the lipids are stored as triacylglycerols that can be broken down into unsaturated fatty acids, which later are taken up through the blood and biosynthesized into blubber. Belugas derive many of these polyunsaturated fatty acids from the lipids of the fish and other prey they eat (Iverson, 2008). Aside from unsaturated fatty acids, belugas also contain a very high portion of an atypical fatty acid known as isovaleric acid. Found in only a handful of other odontocetes, the isovaleric acid concentration is highest in the outer layer of blubber near the skin.
Mean blubber depth also seems to depend a lot on age and season (Fig. 1). Measuring blubber depth with ultrasound seems to be relatively accurate, and Cornick et al. (2016) suggests that according to the ultrasound measurements they made, adults have almost triple the mean body mass of juvenile belugas. Juvenile belugas, however, do seem to take on larger blubber stores than adults. Regardless of age, belugas carry larger stores of blubber in the fall than in the spring (Fig. 2). This increase in blubber stores in the fall is probably in preparation for the colder winter months, especially since belugas live in Arctic and sub-Arctic habitats.
Figure 1: (a) This is a 10cm x 10cm cut of skin and blubber. (b) This is the ultrasound image measuring the depth of the blubber.
Figure 2: This is a bar graph with 95% confidence intervals indicating the range of blubber stores that juvenile and adults take on in different seasons. The adults show a significant increase in blubber stores once it is fall!
How Blubber Fulfills So Many Critical Functions
Insulation/Thermoregulation
Unsaturated fatty acids also have lower melting points, which allow for the outer layer of blubber and skin to stay close to the ambient temperature of the water. Additionally, lipid conductance is one-third of water conductance (Castellini, 2008), which means a high lipid composition in the blubber makes heat loss to the ambient water very difficult. Moreover, the high concentration of the unusual isovaleric acid in belugas has an extremely low melting point of -37.6 degrees Celsius (Iverson, 2008), which might further aid the beluga to survive in extremely cold, sub-Arctic environments. Simply put, the more lipid content in the blubber, the higher the insulative ability.
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Besides the fatty acid properties, the adipose tissue itself is metabolically less active (Iverson, 2008), which means less blood can travel to this tissue to adequately meet its needs. Due to this characteristic, the beluga AVAs of the blubber can shunt, or restrict, the blood flow to the blubber surface, allowing the blubber to serve as a powerful insulator.
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Energy Conservation/Storage
The adipose tissue of the blubber requires less energy than other tissues, which favorably allows for resource reallocation. The beluga's body can stay close to the ambient temperature, the beluga gains a metabolic advantage and can expend less energy to produce heat to maintain a constant body core temperature.
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During molting and breeding seasons, male belugas depend strongly on their blubber as an energy source, as they search for food for less at these times. This also applies to all belugas when undergoing fasting.
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In female belugas, the demanding lactation process requires increased food consumption. Although belugas are smaller marine mammals, female belugas can still fast for a few periods during lactation, allowing for periods where the female does not need to find food. The newborn beluga also heavily relies on the blubber to survive while learning how to find hunt for food (Iverson, 2008).
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Depending on the degree of blubber deposition, the amount of energy used to keep oneself afloat increases or decreases. As the fatty acid content is rather high in belugas, they typically do not need to expend energy keeping themselves afloat, thus allowing energy conservation.
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Water Balance
While blubber is a great source of energy, oxidizing this fat layer can produce around one kilogram of metabolic water (Iverson, 2008). This usually is a sufficient amount to the point that belugas do not need to obtain more water, even for lactating female belugas.
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Movement in Water
Among the many forces a beluga must counteract as a marine mammal, the inconsistent blubber thicknesses across the body form a contour that supports a streamlined, spindle-shaped body (Iverson, 2008). Thicker areas of blubber, such as around the tail and caudal peduncle, allow those body areas to serve more as a spring to release elastic energy while swimming forward in the water. In addition, the degree of blubber deposition can affect their buoyancy, as high lipid content allows for a greater buoyancy effect.
Picture Credits and Licenses - Title Image (scaled to fit into column): Javier Yaya Tur (CAC, S. A.), CC BY 2.0, via Wikimedia Commons