BEACON Researchers at Work: Of Moths and Math

Octavio CamposThis week’s BEACON Researchers at Work blog post is by University of Washington graduate student Octavio Campos.

We can all appreciate the beauty and diversity of flowers. After all, they come in so many different shapes and sizes – not to mention colors – that there’s bound to be something that appeals to everyone’s taste. Even for the dark and broody among us there is the Black bat flower! But although flowers hold strong symbolic meanings for people all over the world, their vast variation in appearance is thought to have been greatly influenced by their need to attract particular species of animals for the purpose of efficient pollination, and therefore efficient reproduction. Moths, bats, bees, and hummingbirds (and even some small rodents), for example, benefit from consuming the energy-rich nectar that many flowers produce while inadvertently transferring pollen between flowers as it sticks to their fuzzy bodies. 

Manduca sexta, a crepuscular hovering hawkmoth with long proboscisIf you really stop to think about it, drinking the nectar from a flower isn’t exactly the easiest way to making a living out there. Some animals can only feed from flowers by hovering in front of them. That by itself is a hugely complicated process that demands an impressive amount of coordination and control. On top of that, some hovering animals such as hawkmoths must be able to insert their proboscis, which is a hollow drinking straw-like appendage that serves as their mouthparts, into a very narrow opening in the flower in order to reach the nectar reservoir. Bear in mind, the proboscis can be from one and a half to three times the length of the animal itself! Imagine holding a rubbery, flexible pole that was 15 feet long and trying to precisely touch the bullseye of a dartboard, and you might begin to appreciate the scope of this challenge. And to make things even worse, many (but not all) hawkmoths do their foraging at night or dusk and dawn, when light levels are extremely low.

Some of the variation in flower shape in nature.  Adapted from http://theseedsite.co.uk/flowershapes.html

Some of the variation in flower shape in nature. Adapted from http://theseedsite.co.uk/flowershapes.html

Given the challenge that hovering animals must face when attempting to feed from flowers, I was interested by whether certain shapes of flower might be able to help the moth find the nectar source. For example, some plant species have flowers whose petals more or less resemble a flat disk, while others have petals that form the shape of a trumpet, funnel, or bowl, and anywhere in between. Intuitively, it might make sense flowers that are more trumpet and funnel-shaped might be able to better guide the long proboscis of hovering hawkmoths toward the nectar reservoir of a flower. After all, the military seems to have come to a similar solution in the context of mid-air refueling of military aircraft. During this process, a fuel hose with a cone-shaped tip is presented by a fuel tanker to another aircraft trailing behind it. The trailing aircraft, seeking to be refueled, will attempt to dock with the fuel hose via a long-thin probe. Having a cone at the tip of the fuel hose effectively makes the tip of the fuel hose bigger, thus providing a larger target for the refueling aircraft to aim for.

Flat disk flowers should be the most difficult to exploit while more "trumpet-shaped" flowers should be easier for moths to exploit

Flat disk flowers should be the most difficult to exploit while more “trumpet-shaped” flowers should be easier for moths to exploit

A trumpet-shaped 3D-printed flower (ABS plastic), with shape parameters specified by a mathematical equation

A trumpet-shaped 3D-printed flower (ABS plastic), with shape parameters specified by a mathematical equation

The trouble is that, as a scientist, I would like a quantitative way in which to investigate flower shape and its supposed affect on pollinator foraging ability. In other words, how can I describe flower shape using numbers instead of phrases such as, “funnel-like” and “disk-like?” The solution that my collaborators and I settle upon was to reduce the vast complexity of floral 3-dimensional shape into as few key “traits” as possible and then describe those traits using a mathematical equation. If you can do that, then you essentially have an equation for numerically specifying any imaginable flower shape that you can think of. And the beauty of such an equation is that 3D printers can be used to make a real-life sculpture of any particular combination of shape “traits” specified by the equation. For example, my flower shape equation can specify four aspects of floral shape: flower length, width at the outer edge of the petals, width of the central nectar reservoir, and, most crucially, the degree of curvature of the petals. If you give me any four numbers, one for each of the four flower traits, then I can use my shape equation and a 3D printer to make a plastic prototype of that hypothetical flower!

Now we’re getting somewhere… 

Image from infrared video of a hawkmoth (lower left) foraging on one of the 16 artificial flowers in this 16-flower array.  There are two distinct flower shapes in this array, 8 of each.

Image from infrared video of a hawkmoth (lower left) foraging on one of the 16 artificial flowers in this 16-flower array. There are two distinct flower shapes in this array, 8 of each.

What all of this allows me to do is to design artificial flowers of varying but precisely defined shapes and then make them using a 3D printer. I then take these artificial flowers and attach tubes filled with sugar water (which is all that flower nectar actually is), and then I expose these flowers to visitation by real pollinators, hawkmoths in this case. By carefully documenting the order in which these flowers are visited and for how long, I can gain insights into how floral shape influences pollinator foraging ability! Ultimately, I hope that my data can be used down the line to investigate how (if at all) animal visitation has influenced the evolution of flower shape diversity throughout the millions of years of flowering plant history, bringing my research back full-circle. Flowers have captivated human senses for m
illennia. We are quite capable of altering many aspects of floral appearance through careful selective breeding. But we aren’t the only “choosy” ones out there. Thanks to my flower shape equation, we can begin to take some initial steps in figuring out how influential the “birds and the bees” are at determining the evolution of floral shape… Time will tell!

For more information about Octavio’s work, you can contact him at eocampos at uw dot edu.

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