Why do south america rain forest animals evolve to be small?

Surgo · Post by **Surgo** » Mon Jul 20, 2009 5:49 am

I was watching Planet Earth yesterday, and it struck me that in most of the world's rain forests (they listed Borneo as a notable exception -- you'll find elephants there), animals evolved to be really small. I was kind of left wondering why that happened, exactly -- unless there's something I'm missing like every plant evolving poison, there would seem to be enough vegetation to support large leaf eaters. I understand that competition is incredibly intense and evolving a small size is advantageous, but still, there's a lot of leaf canopy all the time. What gives, or are there some obvious exceptions that I'm missing?

Koumei · Post by **Koumei** » Mon Jul 20, 2009 6:13 am

Because they evolved by use of the Plant Stone instead of by just levelling up. If they had levelled up normally and evolved that way, they'd grow larger.

CatharzGodfoot · Post by **CatharzGodfoot** » Mon Jul 20, 2009 6:43 am

This is just a guess, but getting between trees is probably important in dense rain forest.

Grek · Post by **Grek** » Mon Jul 20, 2009 6:44 am

My guess is that large herbivores have trouble running away from predators in the jungle. Also, trying to stay cool in the heat.

TavishArtair · Post by **TavishArtair** » Mon Jul 20, 2009 11:16 am

Large individuals often actually do fairly well for themselves.

As species, however, they usually tank, especially if they don't have unrestricted access to massive amounts of resources in order to support themselves and their expansion. So while there's plenty of resources in the jungle, there's also lots of competition. My estimation is that any species got out-competed for resources even though individually huge dudes really do lay the smack down.

Parthenon · Post by **Parthenon** » Mon Jul 20, 2009 12:03 pm

Aren't tigers and capybaras pretty damn huge? (well, relatively huge for capybaras) Capybaras live in the south american rainforest and are the largest rodents in the world at about 130cm long, and bengal tigers live in rainforests among other places and are about as large as lions (longer but shorter). You also have tapirs which are a bit bigger than capybaras at about 2m long.

I'm not sure whether gorillas can be counted as being in rainforests but if they are then thats another very large mammal in rainforests.

So, not all animals evolved to be small. You could make the argument that animals in South America specifically are smaller than elsewhere (e.g. jaguar smaller than tiger) but it would need a few more examples to back up the argument.

PhoneLobster · Post by **PhoneLobster** » Mon Jul 20, 2009 12:10 pm

Rain forests often have histories of being populated by tribes of human hunters.

And unless the society morphs into a more of a symbiotic nomadic herder life style those sorts of humans wipe out all the large game.

Same thing happened in Australia, when the Aborigines moved in and shrank all our wombats.

angelfromanotherpin · Mon Jul 20, 2009 12:23 pm

South America used to have a really non-competitive ecology compared to most of the world. Kind of like Australia these days. And when South America and North America became conjoined, a whole bunch of predators, most notably big cats, migrated south past where Panama is today and found an extremely welcoming environment.

To give you an idea of how bad it was to be a South American species at that point, the fossil record shows that their apex predator at the time was not only completely outcompeted for prey by the cats, it actually became a prey species for the cats.

So yeah, cats ate most everything that was big and mostly left the smaller critters alone, because why run down a non-filling rat when there's a big ol' sloth right there, not perceptibly fleeing at all?

PhoneLobster · Post by **PhoneLobster** » Mon Jul 20, 2009 1:43 pm

Well if you want to talk about South America. There is the whole thing where large swathes of the Amazon are actually regrowth over the farmlands of a civilization, or group of civilizations, that collapsed shortly after being first glimpsed by western explorers.

CatharzGodfoot · Post by **CatharzGodfoot** » Mon Jul 20, 2009 4:59 pm

PhoneLobster wrote:Well if you want to talk about South America. There is the whole thing where large swathes of the Amazon are actually regrowth over the farmlands of a civilization, or group of civilizations, that collapsed shortly after being first glimpsed by western explorers.

An regrowth over older civilizations that collapsed after massive deforestation.

Crissa · Post by **Crissa** » Mon Jul 20, 2009 7:11 pm

Forests, while dense in plant matter, are incredibly difficult to traverse and eat. It's been a few million years since plant eaters were able to grow to the size that they could actually topple jungle canopy or eat it standing. These animals had to also be big enough to handle the apex predators... And ten thousand years ago, the last of these died out, replaced by... Humans, mostly.

While tigers and capybara are large, they don't primarily live in jungles. Tigers hunt on the edge of forests, and capybara live in open swamps.

Small things can pick fruits and use trees as cover instead of barriers to their movement.

Forest Gorillas don't eat trees, and they don't really live in a jungle, and they don't have access to much of the continent because they don't travel like elephants.

Forest elephants are smaller. And elephants over all probably still exist due to large brain size and ability to get along with humans.

-Crissa

JonSetanta · Post by **JonSetanta** » Tue Jul 21, 2009 5:26 am

They are small because they are delicious, and they know it.
They seem less tasty when resembling a White Castle burger than a Whopper.