Exploring the Scale of Life: From Minuscule Gobies to Massive Whale Sharks
Paragraph 1
A goby of the genus Eviota is a minute fish, one of the smallest vertebrates in existence, only about a centimetre long and less than 1/10th of a gramme light. It’s about a million times lighter than a man, with the same basic vertebrate body: a spinal cord, a bony skull, a brain, kidneys and a liver. A whale shark, Rhincodon typus, is a truly gigantic fish. It is the largest living fish and swims the same oceanic region as the goby inhabits. Large adults among whale sharks weigh up to 34 tonnes more than 300 times a man’s weight. The difference in weight between the tiny goby and the whale shark is a startling eight orders of magnitude. Life on Earth is truly diverse…
Paragraph 2
These massive disparities in animal sizes have fascinated biologists for more than a century. Of course, there are enormous advantages that come with being large. Big animals have an easier time avoiding predators. In large warm-blooded animals, maintaining a constant body temperature is easier due to their better surface-to-volume ratio. And in large herbivores, the larger volume of the intestines leads to more effective fermentation processes, which are needed to break down plant material. It pays to be big.
Paragraph 3
Indeed, many lineages of animals have vastly increased in size during the course of their evolution. This trend is called Cope’s rule, named after the 19th-century American palaeontologist Edward Drinker Cope. Prominent examples of lineages following Cope’s rule are dinosaurs, which originated from an already sizeable two-metre-long reptile alive in the mid-Triassic (231 million years ago). During the following 165 million years, dinosaurs evolved into the largest land animals ever, the Titanosaurs (up to 37 metres long), and the largest land predator ever, the mighty Tyrannosaurus rex.
Paragraph 4
Given all these advantages of a larger body size, an obvious question to ask is: why are not all animal species big? One reason is that species of small animals give rise to new species more rapidly. In a recent theoretical study at the University of the Philippines, we connected the well-established fact that small animals are more numerous (there are more gobies than whale sharks in the ocean) to the insight that larger populations give rise to new species – a process called speciation – at a faster rate. Hence, some animal species will evolve towards larger body sizes (following Cope’s rule), but the remaining small species will multiply much more rapidly into new small species, and hence keep the majority of animal species small.
It is also worth noting that the advantages conferred by large body sizes are dependent on ecological or anatomical conditions. For example, the early lineages of birds in the Mesozoic did not increase in size; flying is notoriously harder with a bigger body. North American freshwater fishes even decreased in size over the course of evolution, perhaps to make it easier for them to invade smaller bodies of water.
Paragraph 5
Another ecological situation that favours smaller body sizes is mass extinction. The mass extinction at the end of the Cretaceous, for instance, is thought to have been caused by a meteorite impact 66 million years ago, which darkened the skies, cooled the atmosphere, and upset the ecological balance on Earth. The event eliminated dinosaurs and, with the exception of a few cold-blooded crocodiles and turtles, no land animal heavier than 25 kilograms survived.
