American Scientist

What can we learn about the brain by getting up close and personal with the sense of smell of the giant sphinx moth? More than you’d think, says John G. Hildebrand, a neuroscience professor at the University of Arizona and one of Sigma Xi’s Distinguished Lecturers. Hildebrand studies insect nervous systems, particularly the neurobiology of the olfactory system, its roles in behavior, and related research areas of chemical ecology and the biology of disease vectors. He spoke with contributing editor Sandra Ackerman about his research.

You have spent a lot of time studying the hawk moth, Manduca sexta. What does it look like, and how common is it?

Probably many people have seen them without realizing it. Anybody in the United States who has tried to grow tomato plants in their garden and has had a great big caterpillar eat the plants, in all likelihood has had the Manduca sexta caterpillar working against them. It’s a green creature the size of a pretty good-sized cigar, and it’s voracious. A single one can eat all the leaves off a single tomato plant. As an adult, they’re a big moth. They have a wingspan of about 4 inches, and they behave a lot like hummingbirds. So people seeing them at dusk, or even later in the evening during the warm season, may think they’re seeing a hummingbird—but in fact they may well be seeing Manduca. They hover over flowers to feed on nectar, and the females hover over plants to lay eggs.

Why is Manduca sexta such a good model for the study of the neurobiology of olfaction?

Insects like Manduca depend on olfaction for practically everything they do. If one wants to study the neurobiological basis of olfactory-dependent behavior, it’s good to use an animal that does everything on the basis of olfaction. Finding mates, finding food, finding places to lay eggs: For Manduca, all of that depends on olfaction. We know a lot about the behaviors that depend on olfaction, and it’s most of their natural behaviors.

The advantage of using an animal that depends heavily on olfaction is that we know a lot about the behaviors that are served by the olfactory system. I am a neurobiologist whose point of view is what we call neuroethological. I’m interested in nervous systems because of what they do for the animal. I’m not interested in them because of disease and so forth. To understand what a sensory system like an olfactory system does, I want to understand how it serves the natural behavior of the animal in its real world, going about the business of living. Finding an animal like Manduca that really heavily depends on olfaction offers a rich array of opportunities to study the olfactory basis of behavior. It also informs us about what that olfactory system is meant to be detecting, and what it’s meant to be responding to.

What makes a good animal model for a particular area of study?

The layout and organization of the olfactory system in an insect like Manduca is very similar to that of a mammal like us. That’s what we mean by a model system: It’s a system that bears many resemblances to the systems in creatures like us.

Moreover, the Manduca’s olfactory system is much simpler, in the sense of having far fewer nerve cells. The system is more comprehensible. We can actually analyze the circuitry and analyze what happens to information detected by the receptors of the periphery as we trace the information through the brain, in a way that’s much more facile—and, I would say, powerful—than we can in a more elaborate vertebrate animal.

One of the main reasons I chose to work on Manduca about 45 years ago is that it is a big insect, and its nervous system, its brain, is big too—much bigger than that of many other insects. That’s good, because the nerve cells in the system are also bigger than those of other insects. Having big nerve cells in relatively smaller numbers is a big advantage.

You also study insects that are significant vectors of disease?

Mosquitoes are responsible for more human morbidity and death than any other creature on the planet, if you add up all the viral, bacterial, and parasitic diseases that are spread by mosquito vectors in humans. The total toll on human life is enormous. The most obvious primary example is malaria, but there are many other diseases, such as West Nile virus. Dengue is now in parts of the United States. Chikungunya is a new disease that’s just started to show up in the Caribbean. I could go on and on. There are just so many diseases that are vectored by mosquitoes.

Do you think these diseases are spreading more because of climate change? Or perhaps we’re just hearing more about them? Or is there some other factor?

I think that the scientific community is quite confident that the changing pattern of distribution of diseases of this kind really is due to global climate change. Certainly, just take the examples that I mentioned, which are in South America. They are now coming right up to the border of the United States. That’s because of global climate change. Warming trends, the fact that the winters are less harsh, and deep freezes don’t happen in many places the way they used to. The same could be said for mosquitoes and other vector insects, and even for things that aren’t insects, such as ticks, that are vectors. These creatures now have a much wider distribution and therefore ability to spread disease than they did in the days when we had really hard freezes all over the country.

Yes, I think the biggest factor is global climate change. Another is travel. Humans now travel everywhere by air. They carry with them parasites and viral and bacterial pathogens and the airplanes can also carry the vector creatures, so there’s a very efficient distribution system now that we didn’t expect. Most of the Chagas disease that we see in the United States is brought in by people who were infected in other countries.