Has an animal ever evolved to be less intelligent in order to survive?
I was once asked this question and the best example I’m aware of is the koala. This cute little fuzzy guy has evolved to be a complete idiot. An adorable idiot, but an idiot nonetheless and there is a reason for it.
Early koalas, and we’re talking very early (like 25–30 million years ago), would have had a more varied diet. As the Australian climate became drier, koalas had to transition to harder-to-digest food sources or die out. Species that didn’t adapt disappeared. Thus, the koala eventually came to eat only eucalyptus.
Fossil evidence suggests that koalas were already pretty sluggish early on (probably due to a lack of predators) and that helped them with their new diet since eucalyptus sucks as a food source. It seriously sucks. For one, the stuff is poisonous. They must chew the leaves thoroughly, sometimes several times as they vomit the leaves back into their mouth (regurgitation sounds so much nicer, but let’s be real about it). Babies can’t digest the leaves at all and must eat the mother’s poop. The leaves have to be fermented in their digestive tract. The whole process of digesting a mouthful of leaves can take 100–200 hours. They have to sleep 20 hours a day for digestion and to save energy. Their entire body is nearly taken up by digestive organs.
If a koala does not die violently (accident/predator/fire), of disease (Chlamydia… yes, really), or of thirst during a drought, then it is guaranteed to die of starvation because koala teeth are worn down to nothing by eucalyptus before the natural span of their lives is reached and every koala looks forward to eventually losing the ability to eat and slowly starving to death.
Son of a bitch, right? So why share all of this in a question about intelligence? Well, sitting stationary in a tree without predators all day chewing on leaves doesn’t require a whole lot of intelligence. And with their diet so goddamned awful, koalas can’t spare a single friggin’ calorie. As such, their brains have wasted away to practically nothing.
Koalas have the smallest brain-to-body-size ratio of any mammal. It is 60% smaller than other marsupials. The brain is smooth, lacking the wrinkles that allow us to cram more intelligence into tiny spaces. They don’t even come close to using the space they have: only 61% of their cranial cavity is used by the brain; the rest is filled with space-filling fluid. A koala’s brain weighs 19.2 grams. A beagle, at about 2/3 the weight of a full grown koala, has a brain weighing 72 grams. Big difference.
That dramatic reduction in brain is reflected in their ability to learn. Forget about being trained or adaptable or being able to learn complex new tasks. Your graphing calculator probably has more processing power than a koala brain. A koala cannot identify plucked leaves as food. If you give it a plate full of eucalyptus leaves, it will starve before eating them. Leaves must be served to koalas in zoos on branches or they will die of hunger.
The loss of their brain capacity and corresponding loss of intelligence seems tragic, but it has clearly had an evolutionary advantage. They’ve survived whereas other ancient koala species did not.
As I completed this answer, I found a fun little video about our adorable idiots of oz. Enjoy:
