Belugas are no strangers to the obstacles that diving poses, especially in habitats where breathing holes amongst endless ice are far and few. Aside from maintaining a stable body temperature, belugas must move with limited oxygen supplies and counteract different forces to stay afloat and swim forward in the water. Although there is limited research on diving physiology and behavior of belugas and marine mammals in general, recent studies focused on myoglobin and oxygen affinity are a step in the right direction. Generally, belugas will not engage in long dives if not necessary, averaging around 13.1 minutes per dive across mature female and male belugas (Noren & Suydam, 2016). Belugas can dive around 500 to 1000 feet deep in search of food (Joling, 2016).

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Diving

Belugas are no strangers to the obstacles that diving poses, especially in habitats where breathing holes amongst endless ice are far and few. Aside from maintaining a stable body temperature, belugas must move with limited oxygen supplies and counteract different forces to stay afloat and swim forward in the water. Generally, belugas will not engage in long dives if not necessary, averaging around 13.1 minutes per dive across mature female and male belugas (Noren & Suydam, 2016). Belugas can dive around 500 to 1000 feet deep in search of food (Joling, 2016).

Physiological Hurdles and Adaptations

Hypoxia

Belugas take in air through a single water-tight blowhole on its dorsal side (Physical Characteristics, n.d.). Just as several other diving mammals, belugas will void the highest amount of carbon dioxide possible, and like dolphins, collapse their lungs just before diving (Dan, n.d.). There are three areas in which belugas store oxygen: the lungs, blood, and muscles (Kooyman, 2008). Respiratory oxygen intake is dependent on lung volume and the amount of oxygen taken in at the beginning of a breath hold. For blood oxygen concentration, the blood volume and concentration of hemoglobin in the blood are key factors, while muscle oxygen stores are dependent on muscle mass and the concentration of myoglobin. Belugas, like many marine mammals, have evolved to increase their blood volume and hemoglobin concentrations in addition to increased myoglobin concentration. These adaptations allow for belugas to increase the overall oxygen they intake, thus allowing for longer time underwater, such as for foraging.

While belugas have adapted in ways to increase oxygen intake, they have also adapted to decrease/minimize oxygen consumption. While the beluga must maintain constant blood flow for essential purposes like providing constant oxygen and transporting waste around the body, the beluga reduces the blood flow amount. For example, when slowing down one's heart rate(bradycardia), the rate at which blood flows to the heart is lowered because of the decreased need for oxygen, which in turn reduces the overall oxygen consumption for pumping the heart (Kooyman, 2008).

The aforementioned adaptations allow the beluga to maintain a longer dive under aerobic metabolic pathways before resorting to expensive, anaerobic metabolism. On an additional note, creatine phosphate concentration in mammals is 15-20mmol/kg (Kooyman, 2008), which is sufficient to produce energy without oxygen before significant amounts of lactate are created. This extra creatine phosphate concentration can help belugas in a pinch for a little bit without costing them a lot of time afterward (voiding lactate).

Pressure

Gas exchanges in airspaces within the body can be troublesome when diving, especially if under water for too long, or moving to different pressure levels too quickly. These problems can range from high pressure nervous syndrome (HPNS) to oxygen toxicity to nitrogen narcosis. Fortunately, like many other marine mammals, belugas have adapted their body to avoid such problems. One such adaptation is reducing the importance of the lungs as an oxygen store. By taking in more oxygen via increased concentrations of myoglobin and hemoglobin, gas exchange between the lungs and blood declined, resulting in unequal amounts of nitrogen in the lungs and blood. Elimination of gas exchange between the lungs and blood prevents nitrogen narcosis and oxygen toxicity (Kooyman, 2008). Additionally, belugas and other odontocetes have dramatically evolved their lungs with development of supporting structures for peripheral pathways and replacing respiratory bronchioles (tiny branches that are connected to respiratory sacs) with bronchial sphincters (muscles that can open and close passages within the lungs). In addition, belugas have minimized other pressure issues by evolving heads lacking facial sinuses and reducing the airspace within the middle ear that is needed for hydraulic compression (Kooyman, 2008).

Beluga Breath Hold Develops with Age

Figure 1: Above is a comparison of the total oxygen stores of belugas at different ages. Notice the strikingly similar lung- muscle-blood oxygen stores amongst the 2-year-old beluga with the adult female and male!

Interesting video of a beluga near the water surface in San Diego (very far from normal habitat)!

Newborn belugas have low levels of myoglobin and little buffering capacity within their muscles compared to adults. However, they do undergo rapid maturation that enable them to double their aerobic diving limits within their first year of life. Namely, muscle biochemistry (primarily, myoglobin concentration and muscle buffering capacity) grows, with overall development occurring simultaneously. The beluga is able to increase the biochemistry of its major locomotor muscles as its body lengthens, which mostly occurs before the age of weaning (getting used to food that does not come from the mother). This linear trend seems to continue until the beluga reaches a straight body length of around 228 centimeters, signaling that this length may be a general point of reference for beluga muscle maturation (Noren & Suydam, 2016).

Interestingly, belugas around one to three years old display comparable muscle biochemistry to that of juveniles and adults of the same gender, which include myoglobin content and buffering capacity (Noren & Suydam, 2016). This swift maturation within the first year of life seems to be a logical adaptation, as this allows for young belugas to swim longer lengths and depths with their mothers and pod. As cetaceans appear heavily dependent on working skeletal (striated) muscle that may compose up to 50% of their entire oxygen store, this rapid maturation finding supports this theory. Additionally, young belugas need to acquire and conserve a sufficient amount of oxygen to survive the sea ice habitats in which they live, as there are few breathing spots. This abiotic pressure is a reasonable motivation for rapid muscle biochemistry maturation, albeit breath-hold restrictions due to small body size. After all, the physiological capabilities for diving determine how the beluga pod can forage and use their environment.

According to Noren and Suydam (2016), the breath-hold capacities of two-year-old belugas only measure up to 74% of adult female and 69% of adult male counterparts. This is surprising due to the fact that by this time, a two-year-old would have attained a similar amount of myoglobin content and hemoglobin content (Fig. 1). As mentioned earlier, body size imposes a significant constraint on the amount of oxygen one can store at a time.

Picture Credits and Licenses - Title Image (scaled to fit into column): Elyzhium, CC BY-SA 3.0, via Wikimedia Commons