The New York Times, May 6, 2008
“Why are humans so smart?” is a question that fascinates scientists. Tadeusz Kawecki, an evolutionary biologist at the University of Fribourg, likes to turn around the question.
“If it’s so great to be smart,” Dr. Kawecki asks, “why have most animals remained dumb?”
Dr. Kawecki and like-minded scientists are trying to figure out why animals learn and why some have evolved to be better at learning than others. One reason for the difference, their research finds, is that being smart can be bad for an animal’s health.
Learning is remarkably widespread in the animal kingdom. Even the microscopic vinegar worm, Caenorhabditis elegans, can learn, despite having just 302 neurons. It feeds on bacteria. But if it eats a disease-causing strain, it can become sick.
The worms are not born with an innate aversion to the dangerous bacteria. They need time to learn to tell the difference and avoid becoming sick.
Many insects are also good at learning. “People thought insects were little robots doing everything by instinct,” said Reuven Dukas, a biologist at McMaster University.
Research by Dr. Dukas and others has shown that insects deserve more respect. Dr. Dukas has found that the larvae of one of the all-time favorite lab animals, the fruit fly, Drosophila melanogaster, could learn to associate certain odors with food and other odors with predators.
In another set of experiments, Dr. Dukas discovered that young male flies wasted a lot of time trying to court unreceptive females. It takes time to learn the signs of a receptive fly.
Dr. Dukas hypothesizes that any animal with a nervous system can learn. Even in cases where scientists have failed to document learning in a species, he thinks they should not be too quick to rule it out. “Is it because I’m not a good teacher or because the animal doesn’t learn?” Dr. Dukas asked.
Although learning may be widespread among animals, Dr. Dukas wonders why they bothered to evolve it in the first place. “You cannot just say that learning is an adaptation to a changing environment,” he said.
It is possible to adapt to a changing environment without using a nervous system to learn. Bacteria can alter behavior to help their survival. If a microbe senses a toxin, it can swim away. If it senses a new food, it can switch genes on and off to alter its metabolism.
“A genetic network like the one in E. coli is amazingly good in changing environments,” Dr. Dukas said.
Learning also turns out to have dangerous side effects that make its evolution even more puzzling. Dr. Kawecki and his colleagues have produced striking evidence for these side effects by studying flies as they evolve into better learners in the lab.
To produce smarter flies, the researchers present the insects with a choice of orange or pineapple jelly to eat. Both smell delicious to the insect. But the flies that land on the orange jelly discover that it is spiked with bitter-tasting quinine. The flies have three hours to learn that the nice odor of oranges is followed by a nasty taste.
To test the flies, the scientists then present them with two plates of jelly, one orange and one pineapple. This time, neither has quinine. The flies settle on both plates of jelly, feed, and the females lay their eggs.
“The flies that remember they had a bad experience with orange should continue to avoid orange and go to the pineapple,” Dr. Kawecki said.
Dr. Kawecki and his colleagues collect the eggs from the quinine-free pineapple jelly and use them to produce the next generation of flies. The scientists repeat the procedure on the new flies, except that the pineapple jelly is spiked with quinine instead of the orange.
It takes just 15 generations under these conditions for the flies to become genetically programmed to learn better. At the beginning of the experiment, the flies take many hours to learn the difference between the normal and quinine-spiked jellies. The fast-learning strain of flies needs less than an hour.
But the flies pay a price for fast learning. Dr. Kawecki and his colleagues pitted smart fly larvae against a different strain of flies, mixing the insects and giving them a meager supply of yeast to see who would survive. The scientists then ran the same experiment, but with the ordinary relatives of the smart flies competing against the new strain. About half the smart flies survived; 80 percent of the ordinary flies did.
Reversing the experiment showed that being smart does not ensure survival. “We took some population of flies and kept them over 30 generations on really poor food so they adapted so they could develop better on it,” Dr. Kawecki said. “And then we asked what happened to the learning ability. It went down.”
The ability to learn does not just harm the flies in their youth, though. In a paper to be published in the journal Evolution, Dr. Kawecki and his colleagues report that their fast-learning flies live on average 15 percent shorter lives than flies that had not experienced selection on the quinine-spiked jelly. Flies that have undergone selection for long life were up to 40 percent worse at learning than ordinary flies.
“We don’t know what the mechanism of this is,” Dr. Kawecki said.
One clue comes from another experiment, in which he and his colleagues found that the very act of learning takes a toll. The scientists trained some fast-learning flies to associate an odor with powerful vibrations. “These flies died about 20 percent faster than flies with the same genes, but which were not forced to learn,” he said.
Forming neuron connections may cause harmful side effects. It is also possible that genes that allow learning to develop faster and last longer may cause other changes.
“We use computers with memory that’s almost free, but biological information is costly,” Dr. Dukas said. He added that the costs Dr. Kawecki documented were not smart animals’ only penalties. “It means you start out in life being inexperienced,” Dr. Dukas said.
When birds leave the nest, they need time to learn to find food and avoid predators. As a result, they are more likely to starve or be killed.
Dr. Dukas argues that learning evolves to higher levels only when it is a better way to respond to the environment than relying on automatic responses.
“It’s good when you want to rely on information that’s unique to a time and place,” Dr. Dukas said. Some bee species, for example, feed on a single flower species. They can find plenty of nectar using automatic cues. Other bees are adapted to many different flowers, each with a different shape and a different flowering time. Learning may be a better strategy in such cases.
Scientists have carried out few studies to test this idea. One study, published this year by scientists at the University of London, showed that fast-learning colonies of bumblebees collected up to 40 percent more nectar than slower colonies.
Dr. Kawecki suspects that each species evolves until it reaches an equilibrium between the costs and benefits of learning. His experiments demonstrate that flies have the genetic potential to become significantly smarter in the wild. But only under his lab conditions does evolution actually move in that direction. In nature, any improvement in learning would cost too much.
Dr. Kawecki and Dr. Dukas agree that scientists need to pinpoint the tradeoffs, and they will have to gauge the role of learning in the lives of many species. As their own knowledge increases, they will understand more about humans’ gift for learning.
“Humans have gone to the extreme,” said Dr. Dukas, both in the ability of our species to learn and in the cost for that ability.
Humans’ oversize brains require 20 percent of all the calories burned at rest. A newborn’s brain is so big that it can create serious risks for mother and child at birth. Yet newborns know so little that they are entirely helpless. It takes many years for humans to learn enough to live on their own.
Dr. Kawecki says it is worth investigating whether humans also pay hidden costs for extreme learning. “We could speculate that some diseases are a byproduct of intelligence,” he said.
The benefits of learning must have been enormous for evolution to have overcome those costs, Dr. Kawecki argues. For many animals, learning mainly offers a benefit in finding food or a mate. But humans also live in complex societies where learning has benefits, as well.
“If you’re using your intelligence to outsmart your group, then there’s an arms race,” Dr. Kawecki said. “So there’s no absolute optimal level. You just have to be smarter than the others.”
Copyright 2008 The New York Times Company. Reprinted with permission.