Evolving Giants and Miniatures
Animals of gigantic size fascinate us. So do animals that are incredibly tiny. But how do they get that way? Why do some animals evolve into giants, while others downsize?
Scientists have several hypotheses, all with evidence but also exceptions
Island evolution. Suppose a population gets trapped on an island, with finite real estate and other resources. Those who miniaturize (also called dwarfing) need less “stuff” to grow more offspring, and to reach reproductive maturity sooner; thus, natural selection. Examples include Shetland ponies, mini mammoths, even dwarf dinosaurs. And there’s the famous maybe-human “hobbit” of Flores, Indonesia. Other resource-limited environments–such as the interior of a pitcher plant–can select for the tiniest frog.
But…islands also evolve the giant rat, and the Galapagos tortoise. Hm, back to the drawing board.
Temperature. In colder climates, according to Bergmann’s rule, larger animals do better, because they have larger volume-to-surface ratio. Since heat loss is proportional to surface area, larger animals lose less heat per calorie than smaller ones. That explains why the ice age saw large mammals such as the woolly mammoth and rhino. Once the ice thawed, smaller forages outgrazed and outreproduced them. Similarly, polar indigenous people tend to be heavier than those of the tropics.
But some birds (chickadees) are smaller in colder climates. For that matter, how do teensy robins and cardinals ever make it through our Ohio winter, while I huddle in three sweaters–never mind.
Predation. To avoid predation, it helps to get large. This is especially true of marine fish. The open ocean, as we all know from Jaws, has no place to hide. Fish and other marine denizens, such as squid and whales, evolve to tremendous sizes.
But–the alternative is to be small and make a gazillion offspring, some of whom always evade capture. The ocean also contains the largest populations of medium small fish such as menhaden, critical to support the larger ones; along with teensy plankton. And the teensiest microbes of all, such as Prochlorococcus. Barely big enough to contain working ribosomes, yet one of nature’s most important producers of oxygen.
A British research team concludes, “The course of size evolution is dependent on a complex interplay of many other factors.”
What do you think?