Why have many species after dinosaurs not reached such huge sizes?
In: Biology
If I remember correctly, the amount of oxygen in the environment was higher back then, so they could breathe less and get the same amount of oxygen. It’s really not feasible to be that big in most environments. Takes a lot of bone density and requires massive amounts of food. It’s more efficient to be smaller.
Most dinosaurs weren’t that big either, it’s just that the really huge ones speak to our imagination.
The most important thing that’s needed is for the benefits of their size to outweigh the drawbacks. The biggest animal that ever lived is alive today, the blue whale. So animals can still get that big.
It’s just that size isn’t always an advantage. Large animals usually need enormous amounts of food. And that’s something dinosaurs had readily available. But that’s usually not enough, being big needs to be an advantage. And there are lots of ways that can work out.
Arctic animals are often much bigger than similar animals elsewhere. Because large bodies are better at maintaining body heat and carrying around fat reserves.
Blue whales are so big because they have a very unique lifestyle. They need enormous amounts of food, so they travel around the world to feed during krill blooms. The mating season when krill populations explode. Since they only eat during those krill blooms, blue whales need to be big enough to carry enormous energy reserves while they literally cross the oceans chasing seasonal events.
Island gigantism is a phenomenon where animals on isolated islands don’t have any predators and plenty of food. And as a result, there’s no selective pressure for staying small so they need less food and hide more easily. The giant turtles on the Galapagos are a great example.
It’s no different for dinosaurs really. Being large is a great defence strategy. Today we can see how elephants, giraffes, rhino’s and so on are fairly safe from predation because even large predators like lions would prefer to hunt easier prey. Which means that if dinosaurs had plenty of food and opportunity, being large was a viable strategy.
We have plenty of very large animals today. But being large needs to make sense. Under most conditions, there are more advantages to staying smaller.
If you look at the size of all organisms throughout Earth’s history, even a human is already a very, very large animal going by the average size for an organism.
A prey species and predator species will grow larger in an arms race, though this makes both species more vulnerable to extinction events, because large animals reproduce more slowly(takes more time to adapt to a changing environment), have much lower populations(so they lose a lot more genetic diversity when the same proportion of their population dies), and require a lot more food.
There was a lot of food. A *lot* of food. This allowed the herbivores to grow bigger and bigger, and so the carnivores grew bigger to prey on them.
Alligators grow to the size that is sustainable by food and territory available.
If an alligator is 3 feet long for 10 years, then is moved to an area that will sustain a 6 ft alligator, it will begin to grow again, until it again reaches the limits of the available resources.
Dinosaurs may have been similar.
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