Picking and choosing

Our sense of smell is one of the most important methods by which we interact with the world, but the most poorly understood. It is hard to study due to the difficulty of establishing efficient controls, and its inherently subjective nature: the same odorants may smell very different to different people.

A recent study, however, published in Science (Vol. 339, pp. 200-204) by researchers at Arizona and Washington Universities, aims to shed light on how we learn to respond to odorant molecules. In the study, scientists were able to identify a single class of aromatic-ester compounds present in the aroma of flowers selected by the hawk moth (Manduca sexta) as sources of nectar. This adaptation allows the moths to save considerable time and energy by prioritising flowers known to provide the preferred nectar. The scientists were able to show that this preference is innate, i.e. is present at birth, rather than learned by the moths after hatching.

To see if the moths could learn to feed from plants which lacked the crucial identifying compounds, the researchers laboratory-reared moths to feed from Agave palmeri, a plant lacking the distinguishing odorant of the moths’ usual food sources and therefore not usually attractive to them. The moths learned to associate the odour profile of this new plant with food, and so were able to feed on A. palmeri as normal. Crucially, when these moths were introduced to their usual food source, their preference for these flowers was unaffected, leading the researchers to conclude that innate and learned odorant processing may occur through separate brain processes.

This research has demonstrated that the hawk moth has an innate sensitivity to a certain narrow range of compounds, which is unaffected by laboratory conditioning. Furthermore, it has raised the possibility that there are different neural mechanisms involved in processing innate and learned odorants, with interesting implications for evolutionary biology. And, of course, it raises questions about  how our own preferences for different foods and smells is treated by our brains.