Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [17]
Such trade-offs are common among living things, as I discuss in the chapter on personality. Animals that have many young also tend to have smaller babies, whereas species like us that give birth to one or a few offspring at a time generally produce relatively large ones. Here the trade-off seems to be that when natural selection gives a good learner, it takes away a long life. This could happen at two time scales. Within the lifetime of the fly, the energy a fly acquires could go either to helping it survive longer, or to nervous system machinery, but not both. It may be cheap to upgrade the memory in your laptop, but doing so in the brain is going to cost you.
Over many generations, a different process may be at work. Say that a gene makes a fly smart, but because most genes have more than one effect, it also makes the fly vulnerable to starvation, or maybe more susceptible to infections. If being smart is advantageous enough—in Kawecki's lab, it made the difference between reproducing or not—then the gene conferring it will persist in the population, even if it also has some downsides.
Of course, it's not as if all animals get to go to some primordial retail smorgasbord and shop for a certain number of abilities, with some picking learning, long legs, and a mean tennis serve, while others choose curly eyelashes and a talent for languages but end up dimwitted. Exactly which abilities end up having to trade off against which others is still a mystery. But Kawecki's work suggests that the ability to learn, and hence perhaps intelligence, exacts a high price. And that in turn could shed some light on our own evolution. Humans may well have given up some other abilities when we evolved our large brains. What's more, having to learn everything from infancy, rather than being born with our skills, makes our childhoods vulnerable to everything from hot stoves to saber-toothed tigers or their modern-day equivalents. The trade-off in our case must have been substantial, but scientists are still wondering about exactly what it was that we humans had to pay for our intellect.
Better Learning through Chemistry
ONE OF the wonderful things about using animals such as fruit flies and other insects to study learning is that they present a window into the brain. Exactly what happens in the body when you learn the capital of Mongolia, or how to get to the theater? We all have some vague idea that nerve cells send messages somewhere, that electrical impulses in the brain do ... something. And we can use complicated brain scans with colorful images of different centers of nerve activity, or detailed dissections, to try and figure out what that might be. But insects, unlike humans, let us alter a chemical here, or breed up offspring with a special mutation there, which means it is sometimes possible to pinpoint precisely what makes an individual able to perform a certain task. If one bug has gene variant A, and another bug's genes are exactly like it except for having gene variant B, and if the two differ in the time it takes them to find a food reward in a maze, then presto, we have a gene linked to learning.
In most cases, those carefully bred and engineered insects are fruit flies. In the chapter on personality I mention the "rover" and "sitter" flies, which exhibit genetically programmed differences in behavior. Kawecki and his colleagues, most notably Frederic Mery, examined these tendencies in light of their studies on learning. Each behavior is associated with a form of a single gene, and flies with the rover variant of the rover-sitter gene have better short-term, but worse long-term memory, something of a reversed Alzheimer's, where it is easier for sufferers to recall events of decades past than what they had for lunch. Sitters show the opposite pattern and can remember associations