Humans and Whales: An Intimate Connection
by Alex Brylske
Photos by Marty Snyderman
There's no group of animals for which humans have more affinity than whales and dolphins. The name Flipper, for example, is about as well-known as Mick Jagger. And Keiko is probably the only nonhuman on Earth who could inspire tens of thousands of school children to empty their piggy banks to save him.
Whales and dolphins, along with their other marine mammal cousins, have also been the subject of the most successful campaigns in the history of marine conservation. Whether it's the successful "dolphin safe" tuna campaign of the 1970s, or the continued public pressure to ban international whaling, it's clear where humans stand when it comes to our long-distant aquatic relatives. Furthermore, it's not surprising that divers often share a special bond with whales and dolphins, for we are some of the lucky few who may visit them in their natural environment. And because of our common marine experience, we more than any others can appreciate their marvelous diving ability, and envy their immunity to decompression sickness.
Scientists have a term for the intense affinity animals engender in humans. They call such creatures "charismatic megafauna." But why is it that we humans are such fans of creatures that most will never see outside a tank in an aquarium? It also seems odd that humans would have such affection for animals who seemingly share so little with us. Perhaps the reason for this almost maniacal passion for whales and dolphins is that we share deeper roots with them than we even realize.
A Whale of a Tale
Whales, dolphins and porpoises belong to a group (order) of animals scientists term cetaceans (from the Latin word for whale, cetus). Actually, the original root comes from the Greek ketos, "sea monster", hardly our modern conception of a whale.
Cetaceans are, of course, mammals and not fish. They're warm-blooded, breathe using lungs, and give birth to young that are nursed by milk produced by the mother. But unlike the pinnipeds (the seals, walruses and sea lions) who occasionally haul up on dry land, cetaceans are what scientists call "obligate marine mammals", they cannot survive on land. (In fact, the internal organs of whales would be crushed by their own weight if brought on to land for an extended period.)
Cetacean evolution is continually being examined and altered according to new findings. In the 1990s, fossils from a region of Pakistan helped to clarify the origin of cetaceans. From that evidence, most taxonomists now agree that cetaceans evolved from the same ancestral animal that gave rise to even-toed ungulates (deer, camels, pigs, cows and hippos). Now the scientific debate is over which particular type of ungulate ancestor is their closest relative. Based on DNA analysis, it seems that the winner is the modern hippo.
Finding more food in the sea than on land, cetacean ancestors grew large. And freed from the constraints of land-based animals, being held up by legs, there was no natural limit on their size. This is why the largest animal ever to exist on Earth isn't a dinosaur; it's the blue whale (Balaenoptera musculus) which can grow up to 100 feet (30 m) in length, and weigh between 90 and 120 tons. The heaviest ever recorded was a female weighing 190 tons, and the record for the largest was another female measured at 110 feet (33 m). Blue whales eat more than 4 tons of shrimplike krill each day. That's the equivalent of one adult African elephant. The large size of most whales also provides an adaptive advantage when living in the cold sea. The favorable surface-to-volume ratio of a large body enables them to retain warmth much better than if they were smaller.
Cetaceans are the mammals most fully adapted to aquatic life. Over millions of years of evolution, their bodies became more tapered and streamlined, and their tail replaced by a pair of horizontal, propellerlike flukes. As part of this streamlining process, the bones in the cetacean's front limbs fused, and in time became a solid mass of bone, blubber and tissue, making very effective flippers that balance their tremendous bulk. Their hind limbs disappeared (although they retain tiny vestigial hind limbs that are no longer attached to the backbone).
Although cetaceans still retain the mammalian characteristic of fur bearing, it's confined to tiny hairs mainly around their jaw region. As it created resistance to swimming, cetaceans lost the bulk of their fur in exchange for an even better insulator, a thick layer of blubber. This can also act as an emergency source of energy; and in some cetaceans the blubber layer can be more than a foot (30 cm) thick. And whales have even more adaptations to aid in heat retention. A recent discovery is that a network of blood vessels in the tongue reduces heat loss by transferring it from the warm blood into vessels that carry it back to the body core.
Take a Deep Breath
Understanding just how marine mammals accomplish these incredible diving feats requires a little insight into their unique physiology. But what's even more intriguing than the science is that other nonmarine mammals, including humans, have retained some of these basic adaptations. One such adaptation is called the "mammalian diving reflex." In marine mammals, it enables prolonged diving by restricting circulation, and thus oxygen, to less sensitive organs, reserving it for the more vital organs (heart and brain). An associated feature of the reflex is bradycardia (slowing of the heat rate).
While the diving reflex has been shown to occur in a wide range of nonmarine mammals, from humans to dogs to pigs, it's not nearly as pronounced as in marine mammals. In addition, there's dispute over whether in humans the oxygen conservation part of the mechanism functions at all; and the consensus has been that it probably doesn't. But new research is beginning to point in a somewhat different direction. Perhaps, when it comes to this diving adaptation, humans may be closer to whales than most researchers have realized.
The first of many amazing facts about how most marine mammals breathe involves their lung volume. Proportionally the lung volume of whales is only about half of that found in terrestrial mammals. But what they lack in size they make up for in efficiency. Unlike humans who can exchange only about 15 percent of the air we breathe in one breath, something called "tidal volume", whales can exchange up to 85-90 percent of their air. Whales can also fill their lungs in less than two seconds, half the time it takes humans, even though they inhale more than 3,000 times more air.
But the remarkable physiological adaptations don't stop there. In addition to the high exchange rate of air, the red blood cells of whales are larger than in humans, and there are proportionally far more of them. This makes for a greater rate of exchange of oxygen from the lungs to hemoglobin. The net result is that whales can distribute oxygen to their tissues far more efficiently than humans.
And there's still more. Along with the more commonly known substance, hemoglobin, oxygen is also stored directly in the muscles by a protein called myoglobin. There's nothing unique about myoglobin; human muscles have it, too (though not nearly as much as whales). It binds with oxygen and stores it in the muscle for later use. It's also responsible for the dark coloration seen in some muscle tissue of many animals. (It's why, for example, dark meat is dark.) In fact, sperm whale (Physeter catodon) muscle contains so much myoglobin that its flesh appears almost black. It's no surprise, then, that these are the deep diving champions of the world, capable of diving to 10,500 feet (3,181 m). What's different about myoglobin in whales vs. us is how efficiently it's used.
During a dive, the limited oxygen supply is shunted by alterations in circulation, as mentioned earlier, primarily to the heart and brain. But because of their prodigious myoglobin supply, whales' muscles continue working unimpeded. Eventually, though, even their supply runs low. As in humans, when oxygen stores are exhausted, muscles must work anaerobically (without oxygen). And as in humans, this results in high levels of lactic acid building up in the muscle tissues (this is the same substance responsible for the "burn" we feel after heavy exercise). However, unlike humans, whales have a very high tolerance for lactic acid buildup, so their muscles can function anaerobically for extended periods. Whales can also tolerate a far higher level of carbon dioxide in their blood than we can; and due to this tolerance, they can remain underwater while their oxygen supply is used up almost entirely. For instance, while the oxygen content of the air exhaled by a human is about 16-18 percent, it's a mere 1.5 percent for a whale.
Humans and the Diving Reflex
A diving reflex, involving both a reduced heart rate and peripheral blood flow, has been shown to occur in humans and marine mammals. Until recently, however, physiologists thought that in humans, the purpose of the reflex was to reduce heat loss by restricting blood vessels in the limbs rather than oxygen conservation. In fact, instead of conserving oxygen, it was held that cold-water immersion actually has the opposite effect in humans. It increases rather than decreases oxygen demand because a faster metabolism is needed to maintain warmth.
As further evidence against oxygen conservation, physiologists point to several other dissimilarities between humans and marine mammals. First, they've found that cold immersion has little if any positive effect on humans' ability to hold their breath; and that the reduction in peripheral blood flow is also significantly less in humans than in marine mammals. Other differences involve the changes in heart rate. In marine mammals, the diving reflex reduces their heart rate by 80-85 percent. In humans, this reduction is about 40 percent, at best. (Although this varies widely among individuals.)
In humans, the reflex mechanism appears to have two aspects. First, cold receptors in the face send a direct signal to the brain, lowering the heart rate. Second, vasoconstriction in the limbs shunts blood to the thorax, providing more blood to the heart. More blood in the heart, in turn, increases what's termed its "stroke volume," allowing the heart to slow down and still maintain consistent blood flow.
The reflex is often described as a "cold water" response, but this can be misleading. For humans, immersion in very cold water, lower than 41 degrees Fahrenheit (5 degrees Celsius), can actually increase heart rate due to a pain response. Warm-water immersion appears to have no effect on heart rate, and may even cause an increase.
The diving reflex has also been assumed to be the protective mechanism explaining numerous cases where humans, especially children, have been resuscitated after submergence for periods approaching an hour. But until recently most physiologists disagreed, insisting that it was the cooling effect alone that reduced metabolic oxygen demand. Interestingly, initiating the reflex does not require immersion of the entire body. Immersing the face alone, or even just wetting it, is enough.
Are Humans and Whales That Far Apart?
Several years ago, researchers at Sweden's Lund University furthered our understanding of the diving reflex in humans; and much of what they found pointed to humans being more "whalelike" than we realized. That year the university set up a laboratory to study the human diving reflex. Their facilities enabled simultaneous and continuous recording of cardiovascular and respiratory parameters, including fast transient changes in heart rate, blood pressure, arterial and capillary blood flow, temperatures, and thoracic movements. The aim of the research was to understand the control of the diving reflex in humans, with emphasis on the effects of temperature and what effect training may have on the reflex.
One of the first questions the researchers asked was, exactly where are the receptors, which initiate the reflex? Based on previous studies, it was known that the reflex is triggered by apnea (breath-holding) in combination with face immersion in cold water; and that the combination of cold water and apnea is twice as effective as apnea alone. Most investigators assumed that the trigger receptors are nerves around the mouth and nostrils. However, the Lund researchers found that the receptors responsible for the reflex are instead in the forehead and eye regions. These locations have an important implication for diving, as they are most often covered by a face mask. Therefore, according to the researchers, a suitable breath-hold diving mask should cover as little of the forehead as possible.
A second area of investigation was what effect water temperature might have on the response. Several studies have established that the temperature of the water is, as mentioned, an important determinant of the magnitude of the reflex. For example, the water temperature that seems to maximize the reflex is about 50 degrees Fahrenheit (10 degrees Celsius). This has led to the conclusion that the diving reflex will not be efficiently triggered where much diving activity among humans is centered.
What the Lund research team found in this regard was quite surprising. They showed that subjects acclimated to different ambient temperatures, and that both ambient water temperature and air temperature have significant, but opposite, effects on the magnitude of bradycardia. In other words, in the range of temperatures most likely to be encountered by breath-hold divers, the warmer the ambient air and the colder the water, the stronger the diving response will be. Thus, the difference in temperature between air and water seemed to be more important than the actual water temperature. Based on this, researchers advise that divers try to stay warm between dives to maintain high peripheral blood flow, which would allow for fast recovery of the face skin temperature.
Perhaps the most controversial aspect of the Lund research involves determining whether the diving reflex in humans, as it does in marine mammals, conserves oxygen. Their research, based on studies of nine different groups of divers and nondivers, showed a positive correlation between the diving response and breath-hold time. Furthermore, trained divers showed the most pronounced reflex and the longest breath-hold time; and trained divers with a powerful diving reflex were found to prolong their breath-holding ability only when the reflex was most efficiently triggered.
Next, researchers compared breath holding of a given duration, with and without face immersion, with respect to arterial hemoglobin oxygen saturation. The results were that the arterial hemoglobin was more saturated after breath holding with face immersion, and the diving reflex was more pronounced. Breath holding without face immersion resulted in the subjects using more oxygen. Thus, the researchers concluded, "Our data favor the view that an efficient, oxygen-conserving diving response is present in man, as in diving mammals."
An especially intriguing aspect of the Lund research involved the effect of training to increase one's breath-holding ability. Their results showed that repeated apneic episodes with intervals of less than 10 minutes can prolong breath-hold time, although, they admit, what causes this "short-term training effect" isn't well-understood. Previous research suggested that either an increase in the diving reflex, progressive hyperventilation or an increased inspired lung volume throughout the series causes the increased breath-hold time. The Lund researchers have found that the reasons probably aren't all physiological; there's also a powerful psychological factor contributing to breath-hold time. Moreover, the magnitude of the slowed heart rate aspect of the diving reflex is also affected by training. In untrained subjects, for example, heart rate reduction varied between 15 percent and 30 percent, while a 30-50 percent reduction was seen in trained divers (a figure comparable to the response found in many semiaquatic mammals like beavers). As the researchers conclude, "After apneic training, the dive becomes not only longer, but also easier."
No matter how refined our technology becomes, humans always confront the unavoidable problem associated with diving, decompression sickness. But in whales, Mother Nature has given them a pass. For an animal that spends so much time at depth, susceptibility to decompression sickness would definitely be a fly in the ointment. So what explains this enviable immunity to the bends?
The reason cetaceans don't get decompression sickness is related to what we've already discussed, their breath-holding adaptations. It also is because they hold their breath rather than, as a scuba diver, breathe a continuous supply of air.
As any trained diver knows, decompression sickness is a problem that only plagues those using scuba. The reason breath-hold divers do not get the bends is that they don't dive deep enough nor stay submerged long enough. But another reason is that they just don't absorb enough excess nitrogen in a single breath to make any difference. Scuba divers are different; they breathe constantly and are therefore continually absorbing nitrogen. Thus, in scuba divers the time, depth and quantity of nitrogen is sufficient, under certain circumstances, for the formation of debilitating nitrogen bubbles we call the bends. (While in some rare cases, like with highly trained, world-class free divers or special populations like the Ama pearl divers, breath-hold divers do succumb to decompression sickness, it's beyond the scope and relevance of this discussion.)
To understand why bends doesn't happen in whales, we must keep in mind that they are breath-hold divers. Proportionally to their body mass, a whale simply does not absorb as much nitrogen as a continuously breathing scuba diver. So there's less nitrogen in a whale, proportional to its body size mass, than in a scuba diver. However, it's even more complicated than that.
Some of the diving adaptations discussed earlier that make cetaceans efficient breath-hold divers also help protect them from decompression sickness. When a whale dives, its chest compresses and lungs collapse. In particular, the alveolar (gas exchanging) portion of the lungs collapse, pushing air back into regions where there is no gas exchange (so-called "dead air" spaces). This alveolar collapse explains the protective mechanism in cetaceans that dive below 230 feet (70 m), but scientists still don't completely understand what protects those that don't dive as deep.
Renowned diving physiologist Dr. Fred Bove offers what might be another piece of the puzzle. In an article published several years ago, he showed that, at least theoretically, alveolar collapse not only slowed gas absorption, it altered the circulatory dynamics in other ways. In essence, the blood of whales, rather than being a "fast" tissue (absorbing and eliminating nitrogen quickly) as it is in humans, has a much slower rate of nitrogen uptake and elimination. Some marine mammalogists have even speculated that the sperm whale (P. catodon), may somehow use its mysterious spermaceti organ, which gives the whale its characteristic bulbous head, as a nitrogen reservoir. Such speculation is interesting when one considers that sperm whales are also the deepest-diving cetaceans on Earth.
The idea that the profound change in circulatory dynamics seen in whales accompanying the diving reflex may also aid in avoiding decompression sickness is also compelling. There may also be an evolutionary adaptation that somehow protects whales from gas phase separation (bubble formation). However, this is speculation as, to my knowledge, no one has studied this phenomenon directly.
The love affair between humans and cetaceans is an abiding one that's not likely to end anytime soon. But unlike our affinity for other animals, our feelings may come from more than the "cute and cuddly" factor. The appeal of these gentle leviathans is perhaps recognition of something shared by all distant relatives.