Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Suborder: Mysticeti
Family: Balaenidae
Genus: Balaena
Species: Mysticetus

I first became interested in the bowhead whale (Balaena mysticetus) after finishing a 1000 mile, 40 day row across the Arctic Ocean. In addition to rowing, our four man crew collaborated with scientists at the University of Alaska at Fairbanks to collect plankton samples important in determining migratory patterns of whales. We were also tasked with documenting sightings of every marine mammal we encountered. As we travelled for weeks on end through the ice floes of the Arctic, pods of curious belugas followed, orcas breeched in the distance and bowheads surfaced – sometimes too close for comfort.

Bowheads have a body length of 30-60 feet and powerful flukes up to 27-feet wide. One slap on the water could have created a wave large enough to swamp our tiny rowboat. Ultimately, I thought it might be safer to study cetaceans once we got back to shore. But I was curious how the bowhead’s anatomical and physiological adaptations allowed them to dive so deep, survive so long without breath, and live hundreds of years in freezing oceanic temperatures. In this paper I will address these topics and share my findings.

Anatomically, the bowhead has the largest mouth (10 feet wide x 20 feet deep) and thickest blubber (over 1-foot thick) of any mammal. Their reinforced skull gives rise to the name “bow head.” According to Inuit hunters like the ones we met in Point Hope, bowheads use their heavy skull bones to break through 1-2 feet of arctic ice when they need a breath. Though they are reported to dive underwater for up to an hour, the average is closer to 15 minutes with depths down to 500 feet. Their underwater ability is impressive and made me question the physiological difference between our mammalian lungs and theirs. Furthermore, I wondered if it was a slower metabolism that allowed them to maintain tissue perfusion without a breath? Finally, how did they equalize hollow organs and sinus cavities to survive pressure differences in the deep?

To answer these questions we must turn not only to anatomy and physiology but also to chemistry. Boyle’s law and Henry’s law (among others) are important in understanding how a bowhead whale prevents barotrauma to the lungs, maintains oxygen perfusion in the tissues and avoids complications such as barosinusitis – a condition that causes pain in the sinuses under variable atmospheric pressures.

To refresh, Boyle’s law states that the pressure exerted by a gas at a constant temperature varies inversely with the volume of the gas. This phenomenon explains how human lungs can burst on a rapid ascent from the deep and why intracranial cavities need to equalize to prevent damage and pain. Boyle’s law helps us understand how atmospheric pressure changes encountered on a deep dive have real world implications on the volume and function of anatomical structures.

Next, Henry’s law states the amount of a given gas that dissolves in a given type and volume of liquid (i.e. blood) is directly proportional to the partial pressure of that gas in equilibrium with that liquid. For human scuba divers, Henry’s law influences the amount of oxygen diffused in the blood and explains how nitrogen narcosis and decompression sickness can occur when nitrogen molecules enter the bloodstream at depths greater than 100 feet.

Since laws of  chemistry are empirically the same for bowhead whales, it must be anatomical and physiological differences that allow them to avoid barotrauma, barosinusitis and nitrogen related illnesses.

Let’s deal with barotrauma first. Anatomically, a bowhead whale can dive deep because peripheral airways are reinforced and this allows their lungs to collapse during travel to great depths (Scientific American, 2006). Collapsible lungs that maintain integrity reduce the risk of barotrauma. However, collapsible lungs have another crucial physiological function; they help prevent nitrogen narcosis and decompression sickness. As Paul J. Ponganis and Gerald L. Kooyman of the Center for Marine Biotechnology and Biomedicine at Scripps Institution of Oceanography state:

Collapse of the lungs forces air away from the alveoli, where gas exchange between the lungs and blood occurs. This blunting of gas exchange is important because it prevents the absorption of nitrogen into the blood and the subsequent development of high blood nitrogen levels.

Because their lungs collapse and gas exchange is thwarted the bowhead avoids barotrauma, nitrogen narcosis, and decompression sickness that is common with human scuba divers. Furthermore, any free gases they do carry to the bottom are absorbed under pressure into rigid, thick-walled parts of the respiratory system and into the blubber. But what about the barosinusitis then? How do bowhead whales equalize their internal air-filled cavities?

First and foremost, bowheads do not have cranial sinuses like those present in terrestrial mammals. This anatomical difference is helpful because there are fewer structures that require equalization. Secondly, their air cavities have more extensive vasculature. With a more extensive pressure-related trauma. However, none of this explains the bowhead’s ability to survive so long without a breath. The more you dive into it, the more interesting it becomes.

If bowhead lungs collapse on a deep dive then the alveoli are compromised and cannot operate as they do under normal surface conditions. Thus, the oxygen attached to the hemoglobin already in the bloodstream must be utilized at a greater return than that of humans. Since bowheads have about 4 times the amount of blood per kilogram as humans do, they have more hemoglobin. But they also have enormous lungs, hearts, and organs that require a greater amount of oxygen. Therefore, blood volume alone cannot explain increased function because myocardial demand is also increased due to the bowhead’s incredible mass.

A slower metabolism explains another part of the bowhead’s ability to reduce oxygen requirements. When a whale dives, their metabolism and heart rate decrease so that they use oxygen stores more slowly. At the same time, arterial networks shunt blood away from the extremities to the brain. These adaptations allow them to breathe less frequently than land mammals. However, humans also have this ability – albeit less pronounced. The response is known as the mammalian diving reflex. When we (and many other mammals) dive into cold water, we experience bradycardia, peripheral vasoconstriction and a blood shift that causes organ and circulatory walls to allow plasma and water to pass freely throughout the thoracic cavity so pressure remains constant and organs are not crushed. Still, no human being can routinely hold their breath underwater for an hour. So there must be other anatomical and physiological variants that give the bowhead whale its unique breath-holding talent.

Here we must note that the concentration of hemoglobin in marine mammals is twice that of those found in humans. Additionally, the concentration of myoglobin is 10 times that found in human muscle. Hemoglobin is the oxygen-transport protein in blood and myoglobin is the oxygen storage protein in muscle. Therefore, it is not only the greater amount of blood volume per kilogram that assists the bowhead but also the increased oxygen carrying capacity within the blood and muscles. In fact, increased concentrations of hemoglobin and myoglobin are the primary adaptation that allows bowheads to survive such long intervals without a breath.

Understood as a whole, the unique anatomy and physiology of the bowhead whale allows them to dive to great depths, equalize air cavities and maintain long intervals without oxygen. A 2002 article in Scientific American summarizes these findings as follows:

Air cavities, when present, are lined with venous plexuses, which are thought to fill at depth, obliterate the air space, and prevent “the squeeze.” The lungs collapse, which prevents lung rupture and (important with regard to physiology) blocks gas exchange in the lung. Lack of nitrogen absorption at depth prevents the development of nitrogen narcosis and decompression sickness. In addition, because the lungs do not serve as a source of oxygen at depth, deep divers (like the bowhead) rely on enhanced oxygen stores in their blood and muscle.

Fascinating as these facts may be, not everything is known about bowhead whales. An intriguing discovery of late is their estimated lifespan. As of 2001, scientists are fairly certain the bowhead whale can live over 200 years. If correct, this would make it the oldest living mammal on the planet. This theory came about in 1993, after Inupiaq whaler Ben Ahmaogak Sr. from Wainwright found stone harpoon points inside the neck of a bowhead whale. These hunting tools were out of use by the turn of the 20th century. Scientists at the Scripps Institute examined changes in levels of the amino aspartic acid found in the whales’ eye lens and teeth. Their scientific results supported the physical evidence of the ancient hunting blade and the whale was determined to be 211 years old. As author Ned Rozell states in an Alaska Science Forum article, “That whale, alive during the term of President Clinton, was also gliding slowly and gracefully through the Bering, Chukchi and Beaufort seas when Thomas Jefferson was president.” (Rozell, Article #1529, 2001).

This is an incredible discovery. But what makes the bowhead so special? Why does it live longer than any other mammal? Is it the cold water of the Arctic? The plethora of food without the need for extensive energy expenditure? The fact that they have the longest baleen compared to any other whale? On these questions, the science is still unclear.

Studying the anatomy and physiology of an animal like the bowhead makes me curious about my own. What makes humans different? What makes us the same? The world is a beautiful place filled with fascinating facts. Our expedition rowing a boat across the Arctic ocean was both a physical and an academic journey. And though we eventually stopped rowing, the adventure of learning and the joy of discovery will always continue.