eli5: why are spiders and bugs so big in Australia?
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There is something called “Island Gigantism” which is a trend in which organisms isolated on an island will grow in size compared to their original “continental” ancestors. I’m not exactly sure the “Why” here, I’m not a biologist. But I was learning about this somewhat recently and they scientists explained it had to with gaps in the food in the food chain.
If you look at places like Australia, it was populated with a random sampling of animals that made their way there from other places that had complete food chains. But if you consider something like an apex predator, like a tiger, it’s much less likely a population of tigers would make it to Australia than smaller creatures like mice and rats, so you have a food chain “hole” there. Eventually *some animal* is going to occupy that hole and that means something like a random mouse will eventually evolve into growing in size and developing “Apex Predator” features like massive sharp teeth and claws. That’s why in Australia specifically you had animals that technically related to mice and small marsupials that *look* like tigers and lions. It’s not that those animals are descended from lions and tigers, it’s just that “tigerness” are the traits any apex predator needs and so the mouse evolved to have them.
Similarly animals like Monitor Lizards which look like huge, pissed off iguanas are actually more closely related to snakes than to other large lizards. The Komodo Dragon would be the quintessential example of Island Gigantism.
They’re not particularly big for that climate.
Arthropods get big in warm climates, they can survive for many years, have lots of prey, and don’t have to worry about weathering long, cold winters.
You see freakish giant bugs all throughout the world’s jungles and hot deserts.
Most of Australia is considered hot desert or savanna, good conditions for arthropods to get huge.
These other posters are right about relative size, but it’s also important to note that it’s not really the size of Australian fauna that’s often discussed. They have a reputation for having deadly varieties of things like snakes and spiders, as well as a large selection of violent animals. Even so, they don’t really exceed other similar countries. Mexico and Brazil have similar levels of deadly animals.
They are no bigger than anywhere else in that type of environment.
The United States for instance has: The Texas Brown Tarantula, which can span up to 10cm (4in), the Tan Tarantula (5in), and several 8 inch centipedes with some serious bites. Not to mention several large scorpions.
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