Biology Index
By Richard Bruce BA, MA, and PhC in Economics

Whales Grew Rapidly In Size as Megalodon Went Extinct

Whales, particularly baleen whales, most specifically blue whales, are huge. Blue whales are generally believed to be the largest animals that ever lived. A recent discovery has shown that these huge sizes only evolved in the last three or four million years, particularly the last two and a half million years that the scientists call the Pleistocene and the rest of us call the ice ages. For most of this period vast glaciers covered much of the land.

It is argued that the glaciers depsosited minerals in the oceans and thereby fertilized them and encouraged algae blooms. Krill and other zooplankton feed on the algae or phytoplankton and were in turn feed upon by the baleen whales. This abundant food source allowed baleen whales to evolve to huge sizes.

Megalodon Went Extinct

I would suggest an alternative or perhaps additional explanation. Three million years ago, the beginning of the ice ages is approximately when Megalodon, the giant shark that fed on whales, went extinct. This may be the factor that allowed the whales to grow so large. The absence of this large predator took the lid off of whale size.

The ice ages perhaps had something to do with the extinction of Megalodon. Many whales, particularly the larger ones, thrive in colder water. Sharks, on the other hand, are more limited to warmer water. The Greenland shark and the sleeper sharks do live in very cold water, just above freezing, but as the name sleeper shark suggests they move very slowly and can not chase down a seal let alone a whale. So the colder waters of the ice ages probably provided a larger area where the whales could feed almost completely free from shark predation.

While great white sharks, which currently prey on marine mammals, are limited to tropical and temperate oceans, killer whales do hunt in the arctic and antarctic. The ice ages probably gave the killer whales large areas of the oceans where they were the top predator, free or almost free of competition from Megalodon and predation by Megalodon.

Furthermore, the baleen whales, which may have been Megalodon's main source of food may have disappeared from warmer waters for much of the year. Baleen whales commonly give birth in warmer waters but feed in colder Arctic and Antarctic waters. Perhaps, Megalodon could not survive the disappearance of their main food source for much of the year.

Disease might have been another difficulty that lead to Megaladon's extinction. If the whales and other marine mammals were absent for months then Megaladon may have been looking for other large prey, the other large animals in the sea were sharks, including whale sharks, basking sharks, and great whites. As these animals are sharks, like the Megaladon they would carry shark specific diseases. So for Megaladon the danger of catching a deadly plague would be greater if Megaladon was eating its fellow sharks as opposed to marine mammals.

Finally, communicable diseases make a large population size more important for survial. If a deadly plague infects a large population there is greater probablity that one member of the population will have a mutation that allows it to survive the plague. If it can survive the plague and pass this mutation on the species may survive and enjoy the protection of the mutation. A small population is less likely to have an idividual with the key mutation.

So if the iceage greatly decreased the population of Megaladon, the small remaining population might have been unable to generate the mutations necessary to protect itself from comunicable diseases.

Dinosaurs and Whales

The blue whale is currently the largest animal we know of that has ever lived. The dinosaurs were the largest animals that ever lived on land. Both groups grew to extraordinary size. But both groups have more than size in common.

The largest dinosaurs and the largest whales were both preyed upon by their relatives. Other than the relatively recent human whaling industry, the most and perhaps only significant predator of the blue whale is the killer whale, which is a whale, and therefore fairly closely related to the blue whale.

The largest dinosaurs were sauropods which were related to their theropod predators. Both sauropods and theropods were saurischian dinosaurs, commonly called lizard hipped dinosaurs. The other herbivorous dinosaurs were bird hipped dinosaurs, scientifically called ornithischian dinosaurs. So the largest herbivorous dinosaurs were more closely related to the predatory dinosaurs that preyed on them.

The large predatory dinosaurs that preyed on the sauropods particularly in the Jurassic would have been particularly close to their sauropod prey because both the large predatory theropods and the larger herbivore sauropods would have been relatively rare compared to smaller animals and have longer life cycles than smaller animals.

Long Life Cycles and Small Numbers

The long life cycles are important because for evolution what counts is generations not simply time. Large animals with long life cycles will evolve away from one another more slowly. If the last common ancestor between a predator and prey was ten million years in the past, then there will be fewer generations between both the predator and the prey and that last common ancestor if they are large rather than small.

The number of individuals living at any one moment in time is also important, because the more individual animals the more opportunities for mutations, including beneficial mutations. So two groups of small animals will tend to accumulate useful mutations and evolve away from one another more rapidly than two groups of large animals.

Close Relatives, Disease, and Cannibalism

This is important because the more closely related two animals are to each other the more communicable diseases they will tend to share. For example, chimps and humans share every communicable disease that either species suffers from, and the other great apes are subject to most of our communicable diseases. As more communicable diseases evolve to exploit the huge population of the human species the survival of our close relatives the apes is very much in question.

The case of deer in North America also illustrates the issue. The Virginia white-tailed deer carries several diseases that do not kill it, most famously a brain worm. But these diseases are deadly for the larger deer, for example, elk and moose. (Note to Europeans, what we Americans call moose are what you call elk, and what we call elk are red deer, or very closely related to red deer.) As the white-tailed deer's range expands the range of the larger deer retreats. For the most part the larger deer are extinct within the range of the white-tailed deer.

So it is not safe for predators, particularly large predators to eat their close relatives because it is likely make the predator sick, particularly if they do not thoroughly cook their food, which most species other than humans are very lax about. Avoiding cannibalism and near cannibalism is frequently a good survival strategy.

So it maybe more than coincidence that the largest animals have predators that are close relatives. This may reduce the pressure of predation.

Thus the extinction of megalodon, a predator that was not closely related to whales, may have been very important to evolution of very large whales.

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