Evolution Events!

Episode 8 of #EvolutionEvents bears similarities to our last article and may answer some questions you might have had! 

Now, many of us probably know that whales, dolphins and porpoises are mammals. Marine mammals! This means they originated, a long time ago, from the same ancestors as other mammals, like us! To discover why cetaceans are mammals, and to learn how much we still have in common with them, continue reading below.  

At first glance, it may appear that cetaceans have lost quite a few traits that link them to their land-dwelling mammal ancestors, especially now that they live a similar life to a fish, lest breathing air of course. But in this article we will delve into why this is not true on a more detailed level.

Breathing air is exactly one of the clues that tells us cetaceans are still mammals. This is a clue that can be obtained from watching and listening to a cetacean at the surface. All mammals breathe air through a respiratory system that includes lungs…as do whales!

For most mammals, it is impossible to breathe while swallowing, but not for whales. It is impossible for most mammals due to the mechanics of our intertwined digestive and respiratory tracts. But for whales, these systems are separate and can function at the same time! Wow! 

The next fact may surprise you even more! Did you know that cetaceans still have hair? All mammals have hair at one point in their life. Usually, follicles and/or any tiny corresponding hairs are invisible with most species. In terms of the humpback whale, for example, the large bumps on the head (tubercles) give rise to one whisker-like hair each. Amazon River dolphins (botos) also retain hairs for sensory purposes through adulthood. Though hair does not provide a warming feature to cetaceans anymore, the presence of follicles and/or hair adaptations is remnant of cetacean’s evolution from land mammals. Some scientists estimate that even follicles without hair, like those on most adult toothed whales, may still provide sensory information. 

One of the biggest problems with living in the ocean full-time, is, you guessed it….warmth. You might wonder just how a mammal can survive in the cool and frigid oceans without hair or a fur coat, something that many land mammals depend on. It is true that just like land mammals, cetaceans are warm-blooded. To stay warm and regulate body temperature, cetaceans employ a thick fat/oil-based layer called “blubber” to insulate their bodies; this is located underneath their skin. There is even visible evidence of cetaceans’ high body temperatures- seen by the condensation cloud that appears when they take a breath (especially larger whales with larger lung capacities). This is due to the physical effect of warmer air (from a warm–blooded animal) meeting the cooler air of the Sea. It is not the animal expelling plumes of water from their body (despite what Finding Nemo may have portrayed)!

The animal kingdom can provide exceptions to classifying characteristics, such as giving birth to live young. Animals that are not mammals may produce live young, and not eggs (e.g. some fish, sharks and reptiles). Some mammals may alternatively lay eggs, like the platypus and echidna. So how then do we classify mammalian birth strategy? Well, we can say that most mammals have live birth, including all cetaceans. However, a more descriptive explanation includes the use of mammary glands and nursing young. Female cetaceans nurse their young with milk, like land mammals. Their milk is much thicker than that of land mammals, in order to supply the fast-growing young with enough fat and nutrition for a demanding life in the Sea. 

One of the more vague traits of cetaceans that links them to mammals is within their ear structures. Cetaceans, like other mammals, have three inner ear bones. Cetaceans hear sound very differently than we do underwater (sound is much more clear); this is due to a lack of attachment of these bones. Some marine animals, like cetaceans and seals, do not have ear flaps as many other mammals do. Sound is crucial to both baleen whales and toothed whales, however baleen whales do not echolocate as toothed whales do; but they can produce impressive, low-frequency vocalizations. Echolocation in toothed whales can be thought of as a natural SONAR; this is the same system bats use to navigate. 

The above helps to describe the many clues that describe cetaceans as mammals today. There are other interesting features that tie cetaceans to land ancestors, traits which may be also seen in other groups aside from mammals. 

Cetaceans’ pectoral flippers (fins on either side of their body) have evolved from forelimbs (complete with an immobile elbow joint). This reminds us that whales are tetrapods (animals with backbones and limbs). The Tetrapod group includes mammals and non-mammals in the reptile, bird and amphibian groups.

Cetaceans also carry with them remnants of a pelvis- just take a look at a complete whale skeleton and look where the hips would have been.

There is clearly much evidence to the well-known fact that whales’ ancestors once dwelled on land before trading their land adaptations with ones designed for the ocean. Despite cetaceans’ long journeys to becoming some of the largest, smartest and most fascinating creatures in the world, their physical traits still link them strongly to ourselves and other mammals that we know and love.

Article by: Alexa D. B.Sc. Naturalist with Five Star Whale Watching

Sources:

The Smithsonian, Blue Ocean Society, WDC, Whales online, Encyclopaedia Britannica, USC News.