Evolution Events!

As a combined #EvolutionEvents for both last week and this week, we take a look under the waves and wonder how cetaceans (whales, dolphins, porpoises) evolved to become so efficient and capable of honing their underwater environment.

Every species is different, and some cetaceans have greater diving or breath-holding capacities than others. Though, all of them will follow similar physiological adaptations that allow them to thrive in an aquatic realm. 

Some cetaceans are fast swimmers, others are extreme deep divers, some are acrobats and others are great listeners. We’ll discuss how they have evolved to dive, hold their breath, stay warm and use sound. Though we have a great deal in common with cetaceans (e.g. we are both social mammals), the two images above point out the immense amount of equipment a human diver would need to accomplish just some of the daily tasks of a whale (as well as having proper training and legal diving certifications). 

We may use the term “whales” interchangeably with cetaceans; all cetaceans are in fact whales, in a truly scientific sense. Read some of our earlier articles to help you understand this classification system! 

Whales initially evolved from land mammals that entered the sea, thus linking them to other mammals despite their fully-aquatic lives. As mammals, whales definitely need to breathe air and maintain warm body temperatures. When diving, these two requirements become increasingly difficult. When breathing only air, of course whales would have needed to develop efficient systems to hold their breath and keep themselves oxygenated while under the surface, and especially while diving further and further from the surface.

 If you witness a cetacean breathe at the surface, its blowhole(s) (one for toothed whales, two for baleen whales) is/are located on the top of their head for maximum efficiency and quick breaths. The animal does not have to roll to force its belly side up, or lift its entire head above the water as a human would. Once at the surface, the cetacean takes one or multiple quick breaths, which allows gas exchange to occur efficiently (the speed is also aided by the large surface area of their lungs). For example, humans only “turn-over” about 10-15% of oxygen in our lungs with a breath while whales can master a feat of 80-90%. Cetaceans are conscious breathers, which means that they have to think directly about each breath that they are taking- unlike us as humans, who have an automatic response. 

These are great systems to utilize as a whale, but they don’t solve the problem of keeping oxygen in the body while diving and/or swimming underwater. Both humans and whales have blood of course, and blood contains oxygen-carrying molecules called hemoglobin. With a greater relative blood volume as well as greater hemoglobin concentration, the potential to carry oxygen becomes greater. These adaptations are exactly what whales have in comparison to humans. In fact, they are designed with double the amount of hemoglobin in their blood than humans! Myoglobin, which stores oxygen in muscles, is also much greater in whales. Whales can therefore store oxygen in a way that we cannot. 

For reference, the average person can hold their breath for between 30-90 seconds on land, depending on numerous factors (e.g. health conditions, breath-training etc.). This is not considering energy exertion and extra stressors that would be present while diving. In terms of free-diving (human divers who reply on their own breath while diving and not an apparatus), the record stands at 9 minutes for women and 11 minutes for men; note that these divers have extensive training to stay safe. In contrast it is not unusual for a beaked whale, for example, to dive for up to an hour.

The ability to breathe efficiently at the surface, replace oxygen quickly and store way more oxygen helps cetaceans to maintain the complex balance that is being a mammal spending most of its life underwater! All of these things help whales to be able to control, maximize, use and hold each breath with efficiency that far outperforms land mammals. 

The ability to hold a breath while diving is an extra complication; as water depth increases, so do other challenges, namely pressure and energy exertion. How do whales deal with the pressure on their lungs and bodies, which limits where human divers can go? It is true that some specially trained divers, like Navy divers, may reach 2,000m but only with certain atmospheric pressure suits. 

Cetaceans can dive tens, hundreds, a thousand, some over 2,000m meters deep (of course diving patterns and depths vary amongst species). The common theme remains; all cetaceans need a way to deal with pressure that comes with deeper water depths. With increasing pressure, the stress on the ribcage and lungs becomes a concern (due to barotrauma); yet cetaceans have found an adaptation to help avoid this risk, something that humans have not evolved. Their lungs and jointed rib cage can actually collapse when diving, preventing injury. Some seals can also collapse their lungs. Cetaceans and some marine mammals (like seals) have adaptations in their sinuses or middle ear cavities, and even lack of sinuses in certain areas, that help to prevent more pressure problems between the animal and its outside surroundings. 

In addition to these adaptations that help prevent barotrauma, marine mammals also have evolved a system for helping to prevent the “bends” (decompression sickness) and nitrogen narcosis, when and where other land mammals would be at risk. Scuba divers must also be aware of these injuries and follow safe procedures- for example, divers must take precautions to prevent the bends, and will ascend from their dives in a properly timed and staged approach, including remaining at certain depths for a scheduled amount of time.

To help prevent nitrogen problems while diving, cetaceans carry with them a way of limiting the transfer of nitrogen when at deep depths. However, these animals are not immune to “the bends”; gas bubbles have been found in dolphin gill-net victims that drowned and cetaceans exposed to navy sonar (who may “flee” and not regulate their diving systems properly). 

With all of these evolution processes listed above, some of you might assume that this is enough! But we have to consider when a whale is diving, how fast is it using oxygen? Plus, it’s using a ton of energy to swim into deep water. Now you might have more questions. So, doesn’t this extra oxygen get used up anyway, with the extra energy and time spent underwater? 

If you are asking these questions, your intuition is correct. Whales need something else to protect them when they’re diving. The answer lies in how they regulate their heart rate…deliberately! Whales have a great deal of control over the rate that their hearts beat, which is necessary for being able to conserve the great deal of oxygen their bodies have evolved to carry when diving. Cetaceans wouldn’t want to expend their oxygen stores too quickly, this would defeat the entire purpose of being able to hold so much oxygen in the first place!

Thus, when diving, cetaceans can reduce their heart rate by over half; reducing the oxygen consumption around the body and directing it to the most important organs such as the brain. As well, other pathways to certain organs like the stomach, may be completely halted during this time. Several organs, when not receiving a great deal of oxygen, will switch over to “anaerobic metabolism” (aerobic metabolism involves the use of oxygen). 

The streamlined nature of cetaceans can also promote a “gliding down effect” to greater depths with less of their energy used. Other adaptations may also exist; for example, the sperm whale (a notorious deep diver) can utilize a compartment of whale oil/fat at the tip of its head to help it whale sink and float, by altering its density. These circulatory and physiological adaptations are just another way that the cetacean/some cetaceans has evolved for some of Earth’s most difficult environments. 

We have discussed the method in which whales have evolved to hold their breath and undertake new dives without any oxygen. But how can whales stay warm in the cool ocean, especially at cold deeper depths? Humans require wet-suits or dry-suits to stay warm when diving.

Some of you may know this already, but all cetaceans have some form of blubber; a thick layer of fat that acts as heat insulation and promotes the maintenance of an even, warm body temperature that a mammal needs to survive. Other marine mammals, namely seals, walruses and sea lions, also have blubber. Different species of cetaceans and marine mammals will have varying thicknesses of blubber. 

A more complicated concept is that of volume to surface area; we will avoid delving into this scientific phenomena too much in detail, however we will bring the main concept forwards. Many cetaceans are large, with some like the humpback whale reaching between 40-60 feet long and up to 40 tons. This size helps with heat loss, believe it or not! For a more detailed explanation, visit here.

Cetaceans aim to keep the heat they produce on the inside of their body, rather than on their outer surface of skin (blubber is underneath the skin). They don’t put energy into keeping their skin as warm as their bodies, and their skin tends to follow the same temperature of the water. This helps prevent heat loss by preventing heat conduction and keeps their internal bodies and organs functioning properly. 

Despite these great adaptations, whales have another hidden tool, if we look even closer. The way that their blood flows around their body and to their heart is specialized. Whales can warm cool blood that is flowing from exposed regions of the body (like the flippers); this helps reduce the chance that heat can be lost. 

We have covered a lot of information thus far, and hopefully we have provided you a foundational knowledge into the many ways whales have evolved for a tough marine lifestyle. We have yet to cover one very important aspect of being a mammal, communication! How do cetaceans, as intelligent mammals, communicate and hear under the surface? 

The structure of human ears and brains are not evolved to decipher clear sounds underwater, or easily able to tell where sounds may be coming from; this is much different than cetaceans who rely on sound heavily. Cetaceans have specially developed ears that allow them to perform the above much better than humans. Sound is very important underwater, and sound waves can travel across oceans and over greater distances than in air. 

Sounds help toothed cetaceans navigate and find food using a “natural sonar” process called echolocation- a process also used by bats. Baleen cetaceans cannot utilize echolocation physiologically. Toothed whales produce sounds from systems underneath their blowhole (membranes, sinuses and canals) that then travel through the melon and reverberate into the water. The result is usually a single “click” or “pulse” sound. These sounds bounce off objects and return to the animal; picked up by its lower jaw and sent to the brain via the inner ear. These sounds form a 3-D image for the toothed whale and allow it to identify or “see” its environment. When clicks are used in multiple sequence (instead of a single click at a time), it is likely for communication purposes rather than echolocation.

In addition to clicks, toothed whales can produce a variety of other sounds such as whistles, groans and chirps, among others, that form a communication network and a fascinating “chatter’, that is still very much a mystery to humans. 

Scuba divers, for example, cannot speak to each other underwater without special transducers that use ultrasound signals. Other than these systems, humans underwater can use hand signals or underwater writing boards as our voices can’t carry much past the air-water interface (remember that air is being expelled from our mouths to speak). All in all, we need a lot more equipment than a toothed whale needs to chat with friends! 

Baleen whales, in contrast to toothed whales, have a very different system and do not have the same adaptations as toothed whales do. They cannot echolocate, but they do produce sound and can communicate over very long distances with each other. Perhaps most famous for their “whale songs”, baleen whales produce very low-frequency vocalisations that can even carry across different oceans, provided the conditions are optimal. These sounds come across as squeals, groans, roars and sighs. 

Some whale songs are famously used by males to attract females to mate (see humpback whales), but it is also believed that baleen whale sounds are used for identifying each other and other communication such as warning each other of threats and dangers. It is possible that despite baleen whales not having a known form of echolocation, low-frequency sounds may be able to assist in identifying at least very large structures (like reefs). 

We have covered a great deal of cetacean physiology, and we hope you have enjoyed these fascinating topics. These are some of the evolutionary achievements that cetaceans have gained to not only survive, but thrive in an aquatic environment. These are traits that make a whale a whale, and completely unique to other mammals. 

For further reading on some of the topics mentioned here, check out some of our previous blog posts! 

Article By: Alexa D., B.Sc./ Naturalist with Five Star Whale Watching

RESOURCES:

Natural History Museum, Whales online(1) and Whales online(2), WDC(1) and WDC(2) and WDC(3), United States Naval Undersea Museum, SCUBA Diving, Scientific American(1) and Scientific American(2) and Scientific American(3), Nat Geo WILD Video YouTube, Carnegie Museum of Natural History, Aquamobile, AU Department of Agriculture, Water and the Environment, Jervis Bay Wild, Kimberley Kamaral thesis: How does a dolphin echolocate? , The University of Rhode Island, Minke Whale Project, Dive Buddies 4 Life, Medical News Today, The Conversation, University of Michigan Health, Mpora.