Episode 41 – The Evolution of Whales

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Listen to Episode 41 on PodBean, Spotify, YouTube, or your preferred podcast place!

This episode – the second most requested topic to date – deals with some of the largest and cleverest animals our planet has ever seen. We discuss how one group of mammals developed and moved from land to the sea as we explore the Evolution of Whales.

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Whales

Whales (Cetaceans) include the largest living animals in Earth history and one of the few groups of mammals that have evolved a fully aquatic lifestyle. They have adapted so fully to life away from land that they no longer possess functional back limbs, instead using their tails to propel themselves.

Modern whales are categorized into two main groups; the toothed whales and baleen whales. The toothed whales (Odontoceti) include dolphins, porpoises, orcas, narwhals, and the sperm whales. These tend to be active hunters, catching their prey with pointed teeth. They also are well known for their use of echolocation to navigate and hunt for prey.

White and Black Killer Whale on Blue Pool
Pictured is an orca (Orcinus orca) displaying the sharp teeth it uses to eat everything from fish and seals to sharks and other whales. Image from Pixabay.
The male beaked whale, much like the male narwhal, displays specialized teeth that have formed tusks. Strangely enough, these are actually teeth from the lower jaw that have protruded above the head to make these “horns”. Blainville’s beaked whale (Mesoplodon densirostris) pictured here. Image by Dawn Pedersen, CC BY 2.0.

The baleen whales (Mysticeti) included the larger members such as blue whales, right whales, humpbacks, and gray whales. These whales have lost their true teeth and replaced them with the hair-like, keratin structure known a baleen that they use to filter small animals from the water. Filter-feeding at this scale is known as bulk feeding. Though they are famous for their whale songs, they do not use echolocation.

sea animal biology mammal humpback whale feeding vertebrate whale dolphin whales humpback mouth open marine mammals big fish marine mammal killer whale marine biology spinner dolphin whales dolphins and porpoises stenella common bottlenose dolphin short beaked common dolphin rough toothed dolphin striped dolphin tucuxi
A humpback whale (Megaptera novaeangliae) can engulf huge amounts of water, trapping small fish or krill. Their powerful tongue then pushes the water through the screen of baleen to isolate the food. Here you can see the baleen hanging from the upper jaw. Image by DSLR-A230 form Pxhere.

Whales are mammals; they give live birth and feed their young milk even in the open ocean. But where they fit on the mammal family tree was the subject of much debate for a long time.

Relatives on Land

Morphology (eg. fossils and skeleton structures) and genetics both suggest whales evolved from land-based ancestors, but in a classic case of morphology vs. molecules, the two approaches pointed to different relatives. The bones and teeth of whales led researchers to suspect they were closest to an extinct group of Eocene hoofed predators called mesonychids.

File:Synoplotherium112DB.jpg
Mesonychids had hoof-like toes and relatively large heads compared to most mammal carnivores today. Here is Synoplotherium, an Eocene mesonychid from Wyoming. Image by Dmitry Bogdanov from Wikimedia Commons.

DNA, on the other hand, traditionally placed whales within artiodactyls, the even-toed ungulates including pigs, deer, cows, antelope, and more. As our data has improved, the genetic hypothesis has been strongly supported. In fact, the group Whippomorpha was established to link whales to their apparent closest artiodactyl cousins, the hippos, and you’ll commonly now see the taxonomic term Cetartiodactyla referring to the whole group – cetaceans plus artiodactyls.

File:Hippopotamus - 04.jpg
The fact that both whales and hippopotamus are large aquatic mammals is a convergent feature. Their aquatic lifestyles were likely acquired separately. Image by Kabacchi from Wikimedia Commons.

Land Whales

The fossil record of whale evolution is one of the most beautiful transitional sequences known, along with equally-famous cases like Bird and Human evolution.

Many fossil whales fall within a group known as the Archaeoceti (ancient whales). Most of the members of this taxon do not resembles whales on the surface; most still had hind limbs, a more typically mammal-shaped skull, and likely spent plenty of time on land. These early whales lived from the Eocene through the Oligocene (55-23 mya).

File:Rodhocetus.jpg
This early whale was known as Rodhocetus and lived around 46-47 mya in what is now Pakistan. At 2.5 m (8 ft) long it is among the larger of the semi-aquatic Eocene whales. Image by Pavel Riha from Wikimedia Commons.

Though many of the earliest archaeocetes from Pakistan wouldn’t have looked much like whales on the outside, the first clues to their whale-relation came from the shape of their ankle and ear bones. Whales hear through their lower jaw instead of having external ear openings, and their middle ear is actually covered by a layer of bone called the involucrum. This feature was known only from true aquatic cetaceans until it was found in those Eocene land-dwelling whale ancestors.

File:The Childrens Museum of Indianapolis - Killer whale skull cast.jpg
The whale mode of hearing through the lower jaw actually gives them directional hearing underwater just like we have in the air. Image of an orca skull by Michelle Pemberton form Wikimedia Commons.

The ankle bone was actually the secret to linking whales and artiodactyls, specifically the bone known as the astragalus. These early whales had charactersitic artiodactyl astragalus bones.

March to the Sea

As whales began to adapt to a life in the ocean, their ancestors and relatives took on many different forms and body shapes. What follows is just a series of examples of archaeocetes that display differing amounts of aquatic and terrestrial features.

Indohyus – Eocene 49–48 mya, India. This is a very early relative of whales that is actually not within Archaeoceti. This cat-sized animals likely grazed on terrestrial plants and may have used the water as a means of escape instead of as a home. Image by Nobu Tamura, CC BY 3.0.

Archaeoceti contains five families that contain the majority of ancient whales.

Pakicetidae

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Pakicetus, the “first whale” –  Eocene 49–48, Pakistan. One of the earliest cetaceans discovered to date. About the size of a modern wolf, this predator likely fed heavily on fish. This is a reconstruction of Pakicetus inachus. Image by Federigo Federighi form Wikimedia Commons.

Ambulocetidae

File:AmbulocetusNatansPisa.JPG
Ambulocetus natans, the “walking whale” –  Eocene 49–48 mya, Pakistan.  A larger predator, measuring 3.5 m (11 ft). long, it likely hunted in brackish and coastal waters, and may even have behaved similar to a crocodile, taking prey from the shore. Image by Notafly from Wikimedia Commons.

Remingtonocetidae

File:Kutchicetus BW.jpg
Kutchicetus – Eocene 46-43 mya, Pakistan and India.  Roughly similar in size and shape to otters, it made its home in tropical seas likely around barrier islands. The hind legs are small and likely were not important for swimming. Its tail may have been used for propulsion but was probably not finned. Image by Nobu Tamura from Wikimedia Commons.

Protocetidae

File:Maiacetus inuus.jpg
Maiacetus inuus, the “mother whale” – Eocene 47 mya, Pakistan. The first three specimens of this species actually included what appeared to be an adult male and pregnant female with a fetal skeleton still within the rib cage. This amazing find gave hints about the lifestyle and biology of this early whale. Rodhocetus is also a member of Protocetidae. Image by Ghedoghedo from Wikimedia Commons.

Basilosauridae

File:Basilosaurus cropped.png
Basilosaurus, the “king lizard” – Eocene 40-35 mya, global distribution. This notably large whale, 15-18 m (50-60 ft) long, was also an apex predator that fed on prey up to the size of smaller whales. The ‘saurus’ in its name is due to the fact that when it was first described it was interpreted as a reptile due to its long serpentine body. Image by Dmitry Bogdanov from Wikimedia Commons.

More Whales…

As usual, this is but a glimpse into an incredibly diverse and bizarre group of animals. If you’re interested in reading further, please take a look at the links below.

How did baleen filter feeding evolve in whales?
How did whale echolocation evolve?
This Smithsonian animation shows the physical traits of some of the species mentioned in this episode.
The recent hybrid “wholphin” making the news, is actually just a dolphin.

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ALSO, Will had a guest appearance on The Cruise Geeks podcast a little while back to discuss fossil sharks with Matt Allen from The Florida Aquarium. If you’re interested in tips and reviews about going on cruises give this podcast a listen. Matt is also full of cool animal facts.

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