Back to the Deep, Part 1

Common Name: Whales, Dolphins, and Porpoises

A.K.A.:  Order Cetacea of Class Mammalia

Vital Stats:

• Consists of 88 living species
• Order is divided into Odontoceti, the toothed whales (73 sp.), and Mysticeti, the baleen whales (15 sp.)
• Odontoceti includes both dolphins and porpoises
• The largest whale, a blue whale, can grow up to 30m (98’) in length and weigh as much as 20 elephants

Found: Throughout the world’s oceans, save the very northernmost regions

It Does What?!

Picture it: the time is just over 50 million years before the present – the early Eocene – the climate is much warmer than today, undergoing a period of rapid global warming… it is the Age of Mammals. On the shores of the tropical Tethys Sea, in what would eventually become India, a small, deer-like animal, not much larger than a housecat, wades into the water and dives briefly to retrieve a fish before returning to dry land. This has become a successful strategy for its species, avoiding competition from other mammals by eating marine life. Well-fed, the creatures reproduce rapidly, creating competition amongst themselves. Those individuals with greater lung capacity and better swimming ability catch more food, outcompeting those who don’t. Over great stretches of time, characteristics enabling speed and skill under water become more important than those enabling life on land, and selection tilts in favour of a longer, more lithe body, smaller hindlimbs, stronger forelimbs for paddling, and less fur.

Millions of years pass as our small hunter’s descendants eventually lose the ability to ever return to land. They have no fur now… it isn’t useful for retaining heat beneath the waves. Fat is, though, and this begins to accumulate in thicker layers under their bare skin. Their front legs are nearly inflexible at the joints, trading range of movement for strength and widening into precision rudders to control direction as they swim. In concert, the tail becomes more muscular and widens into flukes at the tip, propelling them forward powerfully with each stroke. Their back legs – unneeded – atrophy, gradually losing both size and bone structure, until the foot is completely gone. A small stub lingers for a time before the last vestigial bones simply remain inside the smooth body wall, evidence of a distant terrestrial past. The nasal opening has migrated to the top of the head for ease in surface breathing. Ten million years have passed since the scene on the shore, and we now have our first fully aquatic whale.

Of course, much still had to happen before we arrived at the whales of today. In the time since aquatic mammals first arose, a major division took place within the Cetacea. One group, the toothed whales, or Odontoceti, continued to hunt and eat fish and large marine fauna, including squid and even other whales. To aid in finding their prey, these whales developed echolocation, the use of projected sound to create an image of the surrounding area, thereby becoming the loudest mammal, with vocalisations of more than 180 decibels (a jackhammer tops out at about 120dB). The large bulge we see on the forehead of dolphins and other toothed cetaceans is an organ called a ‘melon’ (because they couldn’t think of anything more science-y sounding just then), which is thought to help direct and focus these sounds.

Being a top-level predator isn’t very energetically efficient, though, and there isn’t always enough prey to go around. So at some point, one group of whales began to move toward a different strategy. The origins of the Mysticeti, the baleen whales, are still a bit unclear, but these animals switched from hunting large fauna to eating colossal numbers of tiny sea creatures such as krill. In order to do this, the whales lost their teeth and developed baleen in their place. Baleen is essentially a fine-toothed comb that filters small animals from the water as it passes. The whale takes a giant mouthful of water and pushes it out against the combs until only food remains. While this may seem less efficient than just grabbing a big fish and eating it, filter feeding is what allowed the largest whales to evolve to their present size. The blue whale, Balaenoptera musculus, is believed to be the largest animal which has ever existed on Earth, and it got that way eating mostly shrimp the size of your thumbnail. Amazing, isn’t it?

Now that we’ve covered how they got that way, tune in next time for part two, where we’ll explore the many weird and wonderful aspects of life as a modern whale.

Fun Facts:

• Baleen whales still have teeth during the embryonic stage of their development, much as human fetuses briefly develop tails.
• Toothed whales do not chew their food; it is eaten whole or torn into large pieces and swallowed. This may be related to the fact that, unlike most mammals, they have only one set of teeth.

Says Who?

• Gatesy & O’Leary (2001) Trends in Ecology and Evolution 16(10): 562-570
• Gatesy et al. (2013) Molecular Phylogenetics and Evolution 66: 479-506
• Lyamin et al. (2008) Neuroscience and Behavioral Reviews 32: 1451-1484
• Uhen (2010) Annual Review of Earth and Planetary Sciences 38: 189-219