If you’ve seen movies or documentaries about Earth’s past and its fauna, you’ve probably noticed that not only were reptiles huge in other eras, but we also had mammals, arthropods, and even giant insects living on our planet. Why don’t they exist anymore? What influences the size of an animal? The answer, as always, is complex, but we can boil it down to 3 factors: predators, environment and available food.
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In the Permian period, before the emergence of dinosaurs, between 299 million and 252 million years ago, the world was infested with insects such as Meganeuropsis permiana, a dragonfly with a 71 cm wingspan, the size of a modern crow. Earlier, in the Carboniferous period (358 million to 298 million years ago), there were invertebrates such as Arthropleura, ancestor of today’s centipedes, 2 meters long and 50 kilos, dying at the beginning of the Permian.
What allowed huge insects?
One of the factors that made this possible was the level of oxygen in the air, at 30% of its composition, against the current 21%. Insects breathe through spiracles, openings on the sides that lead air to fluid tubes, from where oxygen is spread and absorbed by the muscles. It’s far more inefficient than the way we breathe, limiting the size of modern insects — with the current climate situation, they wouldn’t be able to breathe if they were as big as they once were.
Another important factor was the absence of birds, ridding the environment of so many predators and allowing the invertebrates to develop more fully. In the sea, large arthropods also appeared, such as the Jaekelopterus rhenaniae, nicknamed the sea scorpion for its appearance, measuring 2.5 meters in length, from the Devonian period (100 million years before the Permian). Currently, the largest arthropods in the world also live in water, such as the Japanese giant crab (Macrocheira kaempferi), with a wingspan of 3 meters.
This is because the aquatic environment allows the body to sustain itself, as the buoyancy that water offers helps support the body’s weight. A very large shell would be deformed by gravity, and on land, a large crab’s exoskeleton shedding would take hours or days, leaving it vulnerable to predators.
Vertebrates: food and weight
For vertebrates, there are also physical forces involved, such as the need to generate enough blood pressure to circulate the blood. Breathing can be made too difficult by the effort of filling the lungs enough to feed a very large body. That’s why, in the sea, blue whales can weigh the equivalent of 40 African elephants: without water, the organs would be crushed by the body’s own weight.
By the way, from whales to elephants and sauropod dinosaurs (those with long necks), the planet’s giants tend to be herbivores, due to metabolism and body temperature. Having more volume than body surface, their bodies are very hot, and they need, for example, to take mud baths to cool off. A vegetarian diet makes them release a lot of energy in the form of heat, fermenting the food.
The skeletons of diplodocus, large, long-necked dinosaurs, for example, had lots of air sacs, helping to dissipate heat and lightening the bones, but still making them strong enough to support the weight of the body. They only managed to get huge because they were efficient in releasing the heat generated by digestion. If it were the size of a mouse, however, the animal would not survive, as well as a gigantic mouse would not have enough energy to stay alive.
The physical limitation does not stop there. Fleas are famous for jumping to incredible heights compared to their body size. And a giant flea? Well, she wouldn’t jump so much. A large muscle is less strong, as the strength ratio is relative to the cross-sectional area, that is, the area of the muscle when cut in half. With limited strength and, consequently, movement, an animal like that would be easily subdued.
Last but not least, there is sexual selection—some animals prefer large mates, and this creates differences even within species like humans. In the Netherlands, the population is the highest in the world on average, which is considered attractive. This makes the country keep getting taller, while other countries, such as East Timor, with the shortest people in the world, continue with small humans.
Having said all that, don’t worry — the chance of having giant cockroaches roaming around is slim to none, at least as long as we have little oxygen, lots of birds, and a distaste for tall arthropod partners well established on our planet.
Source: BBC Future