Viewing a single comment thread. View all comments

senorali t1_jduc191 wrote

Bipedal animals that primarily move with their legs are efficient long-distance runners but less nimble than comparable four-legged animals. They do well on open plains, typically. Humans, ostriches, and kangaroos all fall into this category.

These types of open environments can't support as much biomass or biodiversity as, say, forests. So even if there was an equal distribution of forests and plains across the world, the plains could support far fewer species overall and thus there would be fewer species optimized for this type of long-distance running.

And honestly, it's not a terribly effective body plan. A lot of flightless birds go extinct when they come in contact with quadrapedal mammals, from the terror birds of old to modern species that are currently being wiped out by invasive rats and cats in isolated island habitats. Kangaroos survive because they live on the only continent without mammalian megafauna. The only things big enough to regularly threaten them are slow-moving reptiles like monitors and crocs.

Humans are kind of a fluke. We developed tool use before we were fully bipedal, and even with that advantage, our ancestors were preyed upon by big cats and other quadrapedal mammals. The loss of two functional limbs for locomotion is a huge risk, even with tools.

13

Peter_deT t1_jduykza wrote

There are other factors involved. The dinosauria had very efficient lungs and light bones, and seems to have dominated over most types of terrain. Kangaroos have evolved connective tissues between the diaphragm and the legs, so use their leaps to power their breathing, and are very efficient over long distances. They do fine against wild dogs (dingos).

8

senorali t1_jdv0ctn wrote

Oh, definitely. All the extant bipedal animals are pretty decent at what they do in various ways (the ostrich uses its wings as rudders to steer at high speeds, which is neat). It just seems that large cats are their greatest enemy, and probably a much bigger obstacle to their success than other carnivorans like dogs and bears. Big cats have successfully hunted both apes and flightless birds for millions of years, and would probably give kangaroos a lot of trouble as ambush predators if they ever found their way to Australia.

5

hal2k1 t1_jdv1ck0 wrote

Well they are not native but there are over a million Australian feral camels. These animals are much bigger than kangaroos. Yet there are roughly 50 million kangaroos in Australia. In Australia kangaroos fare better than camels. Effective body plan?

1

senorali t1_jdv2bqm wrote

I should've been more specific, I was thinking more along the lines of carnivorous megafauna that would directly antagonize kangaroos. The camels are an interesting case, but haven't been there long enough to seriously disrupt the ecosystem. Given a few million years, they likely will push the kangaroos out of some prime real estate if left unchecked. Every other part of the world has large hoofed mammals and large cats, and nothing quite like a kangaroo.

4

Rolldal t1_jdvpgmo wrote

Australia did have some megafauna. Thylacoleo was a masupial lion (101- 130 kg) roughly comparable to the weight of a lioness. They died out during the pleistocene and were Australia's largest known carnivorous mammal. There were also grazers such as Diprotodon (a kind of giant wombat), Palorchestes, plus a few others nearly all of which died out in the pleistocene

5