This is because the proportion of the power of a muscle is relative to its cross-sectional area – or the area of the muscle when sliced in half. No matter how big a flea could grow, there would be a limit to their jumping ability.
While there are some fundamental reasons that animals are the size they are, there is still some flexibility. One animal might end up growing much larger or smaller than another animal of the same species just a few kilometers away. This is sometimes described as the “island rule”, where large animals become smaller on islands and smaller ones larger.
It might be the case that smaller animals – which tend to be lower on the food chain – are liberated on islands in the absence of their usual predators and grow larger than their mainland counterparts. While larger animals, limited by the lack of things to feed on, shrink.
Gargano peninsula, which forms the spur on the back of Italy’s boot, provides several examples in its fossil record. From the late Miocene to the early Pliocene era (around 5.3 million years ago), when the Mediterranean sea level was higher, Gargano was separated from mainland Italy, and the island was overrun with giant critters.
Hairy hedgehogs (also known as moonrats) dwarfed their mainland counterparts. one species, Deinogalerix koenigswaldi, had a 20cm-long skull. There were also giant hamsters (Hattomys gargantua), massive otters (Paralutra garganensis) and gigantic owls (Tyto giganteabigger than the largest species found today).
Elsewhere, a species of human – Homo floresiensis – is a possible case of island dwarfism. Dubbed “the hobbit” because of its diminutive size, the first example of a H. floresiensis was found on the Indonesian island of Flores and is thought to be 95,000 years old. One theory is that H. floresiensis is a pygmy descendant of H. sapiensor perhaps a dwarfed H. erectus. While its origins are unclear, these small humans seemed to only thrive on this island for thousands of years.
But, the evidence for the “island rule” is patchy (the biologist who first proposed it only suggested there are more examples that fit the rule than exceptions, but did not say it always applies). While it is a neat idea, it might just be the case that there is more flexibility in the size of animals growing than we realise.
Just look at humans. In the tallest nation, the Netherlands, men reach on average 184cm (6ft) and women 170cm (5ft 7in), while in the shortest nation, Timor Leste, men stand at 160cm (5 ft 3in) and women 153cm (5ft). Diet and environment play a large part in why the Dutch have added 20cm (7.9in) to their average height in the past two centuries, but a big part is also played by sexual selection. Being taller in the Netherlands is more attractive, and so the Dutch keep getting taller. (Learn more about why the Dutch are the tallest people in the world on BBC Reel.)
our H. floresiensis cousins, who were a tiny 106cm (3ft 5in), might be the result of the island rule – short because there was less around for them to eat. Or, they might be so small because being short was attractive to them. Or both, or neither.
While there is some flexibility in the size that humans can grow, let’s be grateful that biology and physics keep things mostly in check. The topsy-turvy world that Lyons and Cockerill showed us might be a little too chaotic for me.
* This article was updated on 25 January 2023 to correct an error that wrongly stated whales were among the giant herbivores. They are, of course, carnivorous – preying upon zooplankton such as krill or larger animals.
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