Koala Brains

Some species of animals, like lions and tigers, are called charismatic. In their case, it probably means they are majestic, frightening, and fascinating, all at the same time. But some other animals, like pandas and koalas, are also called charismatic. In their case, I suspect the word means that they are mysteriously charming in some universal, inexplicable way. If you ask someone to define why they like pandas, the answer is likely to be something circular like, “Because they’re so cute.”

Okay, so we like them because we like them. Circular, but very human. I know people who work with pandas and they are just about as smitten as the general public. Granted, they see them as more truly bear-like than we do, but there’s just something about them.

On the other hand, consider the koala. Slow-moving, kind of roly-poly, with a face a bit like a teddy bear. For some reason, we human beings find them charming. (That’s not as universal as with pandas, by the way. I’ve got a friend who works with pandas. For some reason she’s color-blind where koalas are concerned. To her they are un-charming and anything but cute. But I digress.)

If you ever have a chance to really get to know koalas and you will discover they are smelly, loud, and have no particular fondness for human beings.

But, exceeding all of those, koalas are simply unbelievably stupid.

Of all of the mammals, koalas have the smallest brains in proportion to their body mass. About .2% of their weight is taken up by brains. And if you were to examine a koala’s brain, you would discover that its surface is surprisingly smooth, with few convolutions. Hardly redolent of genius IQ.

But, happily, that’s not the whole story. The koala brain itself is the living remnant of a remarkable story of adaptation and survival.

Once upon a time, when Antarctica and Australia were neighboring parts of Pangaea, Australia was a lush, wet land, with huge rain forests. It was a perfect place for herbivores. However, as Pangaea began to break up, Australia drifted north and the climate became drier and drier. The great rainforests first retreated and then disappeared. The eucalyptus began to dominate.

The koalas of Australia faced the typical evolutionary choice: change or die.

Two species of koalas failed the test. They simply died out, to be commemorated only as fossils in museums. The surviving species of koala, ancestors of our current animals, found themselves in a world with less and less vegetation. And what there was was less and less edible. They had to change.

But change how?

All of the good food was disappearing, replaced by that lousy eucalyptus. And eucalyptus packed a double whammy that made it a wretched diet. First of all, its food value was miserably low. To deal with this, koalas were faced with two options: Either radically increase their intake or radically slow down their lifestyle. Option one had problems. Eucalyptus leaves are extremely tough. But even if your jaws lasted long enough to chomp down a whole bunch of it, the simple energy of digestion would take up a lot of the food value, not to mention having to lug around a giant digestive tract to process all those leaves. That left slowing down.

But the second problem compounded the first. Eucalyptus leaves are mildly toxic. Not enough to really bother anybody…unless you had to live on the stuff. But the poisons compounded the dietary problem. The more you ate, the more poisons you collected.

So the surviving koala species had to change — radically. The ones who adapted quickly enough survived the climate change, at least long enough to breed. The one who didn’t, didn’t leave any joeys to pass their errors along.

Gradually, koalas developed heavy jaw muscles and teeth to process the eucalyptus. They developed a talented liver that filtered out the toxins. Finally, they dramatically slowed down their metabolism to require as little energy as possible.

But it wasn’t enough. Australia kept becoming drier and drier.

Slow movement and a slow digestion cut down on energy usage till it was the bare minimum required…when they were awake. So they started sleeping 16 to 20 hours a day to further reduce their energy consumption. Heart, muscles, lungs, guts: they’d reduced them all to a bare minimum.

It still wasn’t enough.

One part of the body, the part that consumed the most energy, was still ticking along, sucking up nutrients: the brain.

To solve this final problem, the koalas did something unheard of in evolution. They reversed the evolutionary process. Having, over uncounted millennia, developed a reasonably bright brain in its halcyon days, they began to undo the operation. In natural selection terms, their changing environment imposed such a stringent ecology that it began to favor those with smaller, less energy-hungry brains. Interestingly, evolution didn’t operate on skull size. The skull stayed the same size, but the brain inside it shrank.

How do we know this? That unchanged skull tells the story. Today, a koala’s skull is only about 60%-61% percent full of brain. The rest of the skull is just filled with cerebral-spinal fluid. It’s like a living fossil, neatly tracing out the koala’s strange evolutionary history.

The result, what we see today, is a triumphant animal refined to the limit to match its stringent environment. In its own way, the koala is a miraculous success story. And for those of us who find them irresistible, they’re also ridiculously cute.

But, don’t bother to look deeply into their eyes. There’s nobody home.

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