Why aren't all animal species big?

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 living on land and, with the exception of a few cold-blooded crocodiles and turtles, no land animal larger than 25 kilogrammes survived.

An earlier mass-extinction occurred at the end of the Permian epoch 250 million years ago, and purged a record percentage of animal species on Earth – an estimated 95 per cent of marine species vanished after massive volcanic eruptions radically changed the planet’s atmosphere. The early Triassic, immediately following this mass extinction, was an oddly boring time when it came to biodiversity. Gone were the land-reptiles of the Permian, which had evolved to cow-size, and the continents were mainly populated by Lystrosaurs – dog-sized beaked reptiles. This elimination of large animal species and the survival of small to mid-sized animals is called the ‘Lilliput effect’.

Unfortunately, the study of mass extinctions is of more than academic interest these days – we are living in the age of a Homo sapiens-made mass extinction.