Why Aren't All Animal Species Big?

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It is Better to Be Big

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 .

Another striking example are the cetaceans, the whales and dolphins. These secondary marine mammals descended from a cat-sized amphibious omnivore roaming around India 48 million years ago called Indohyus . When becoming fully aquatic, the cetaceans’ size increased, with the ancient Basilosaurid whales 41 million years ago already up to 25 metres long. The size increase of baleen whales further accelerated during the past 10 million years, and today’s blue whale is the largest animal to ever live, with adults attaining lengths of up to 30 metres and weighing close to 200 tonnes.

But Small Animals Are Innovators

Given all these advantages of large body sizes, 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 together with Timothy Quimpo 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 giver 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.

size of animals surely has nothing to do with its health.
in fact all creatures easily get bigger as a sign of health. Except those who muist be small to be healthy.
dinosaurs, which is a mythical group i say, were not bigger then other creatures. In fact a fossil rhino was found in India that was as hugh as some of the largest “dinos” .
in fact all creatures in the old days were bigger then the shrimps of today.