My mom was telling me about a nature show she saw that claimed Koalas’ brains are smaller than their skulls. It went on to say that this was because of their vegetarian diet, which couldn’t support a large brain. On the face of it, it seems preposterous to me. Why can’t a vegetarian diet support a large brain? If that part’s true, why wouldn’t they just evolve smaller heads?
So, do Koalas really have brains that rattle around? If so, why?
Having a brain bigger than your skull can be problematic too. 
As for the whole vegetarian thing, I dunno…Opal? Has your brain gotten smaller?
Opal switched to an all Eucalyptus diet?
Say it ain’t so!
I’m working off old memory here, so bear with me.
If I recall correctly, only placental mammals and birds have brains that completely fill their craniums. Possibly monotremes do as well, but as there are only three species of them still extant it is very easy to ignore them. If this is true, the question should not be “Why do koala’s brains rattle around” but “Why don’t placental mammals have a normal amount of cushioning between their brains and their skulls?”
Koalas are slow and stupid. They eat the leaves of trees and nothing preys on them, so they don’t have to be fast and smart. In terms of ecological niche they are marsupial sloths. A vegetarian diet probably could support a large brain (horses are pretty bright) but a species will only develop intelligence if there is an environmental advantage to it. Brains are expensive, and almost every animal in the world has gotten along fine without them since the Cambrian.
Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
“You cannot reason a man out of a position that he did not reach through reason.”
I’ve always been taught, quite convincingly, that eating protien and fat (ergo, animals) is critical to the development of large brain size and capacity. It is credited with the major increase in the techincological advance of man, shifting from wholy foraging on veggies to being hunters and subsequently domesticating animals. So there is some validity to the idea that their diet wouldn’t support a large brain. Darwin would add that there are mechanisms going on that make this overly simplistic, and is likely not the cause of their small brain size, but their lifestyle has enabled them to survive with a small brain, and therefore they have no evolutionary advantage to eat “brain” food.
The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…
Omniscient; BAG
Maybe the fact that koalas evolved into a species with no predators and no need to move anywhere quickly is the “smartest” evolutionary step of all.
Did somebody say “Bear with me…”?
Other than humans, the most intelligent species are primates, most of which are either vegetarian or close to vegetarian, eating meat only occasionally and when the opportunity arises. Some primates hunt, but Gorillas, for instance are vegetarian, as are many others. All are highly intelligent as animals go.
Next to primates, the most intelligent order is Cetacean. There are two types: Mysticetes and Odontocetes. As the name implies, Odontocetes have teeth and are carnivorous, eating fish and squid. This group includes the dolphins, sperm whales, and killer whales, among others. The Mysticetes are also highly intelligent, but they have baleen and are “vegetarian” in that they eat only plankton. Plankton, of course has some animals as well as “plants” (photosynthetic organisms, anyway). So it’s hard to say, exactly, but Mysticetes are generally considered in diet to be the aquatic equivalent of cows.
Of the land animals, elephants are among the most intelligent, and they are vegetarian.
It ain’t the meat, it’s the motion - er, I meant not the food, but what’s needed to get it. As Dr Fidelius pointed out, the size of the brain is proportional to evolutionary advantage. Hence, koalas, with little need [or difficulty] to search & discriminate between friend & foe, or tasty eucalyptus & icky non-eucalyptus, have little brains. Fruit bats (see a previous thread) have bigger brains than their insectivorous cousins. Cetaceans tend to stay half awake, in order not to drown -hence big brains with redundancy in the system, to borrow computer systems lingo.
The food doesn’t make the brain - what’s needed to get it and [enjoy ? make use ?] of it does.
“Proverbs for Paranoids, 2: The innocence of the creatures is in inverse proportion to the immortality of the Master.”
Thomas Pynchon, Gravity’s Rainbow.
I’m not debating that koalas have small brains. I would have accepted that from any reputable nature show without question. It’s the idea that their brains have shrunk during the evolutionary process, but their skulls have stayed the same size that I’m finding hard to swallow. Is DrFidelius correct that many animals are like this?
Righto, Greg, we seemed to be responding to each others’ responses, not your question. I find zero in my texts (so far), but still working on it, being curious myself.
Was thinking about water or fat retention, cushioning brain against heat in times of Aussie drought, a a rationale. No mention of the cranial-space-problem [?] in New Guinea mammal text I skimmed through, where drought wouldn’t be much of a problem. Any kangaroo rat specialists out there ? Crack their skull (figuratively, pse): that might point to an answer as an adaptation, not placental vrs. marsupial family tree.