Ant garden in a tree: smells help explain rainforest relationship between ants and plants
(NC&T/NCSU) In a paper published this week in Proceedings of the National Academy of Sciences, Elsa Youngsteadt, a doctoral student in entomology, describes research designed to help understand the relationship between certain rainforest ants and plants. Youngsteadt is the lead author of the paper. Her co-authors are Dr. Satoshi Nojima, senior researcher in entomology at NC State; Dr. Coby Schal, Blanton J. Whitmire Distinguished Professor of Entomology at NC State; and Christopher Häberlein and Dr. Stefan Schulz, both from the Institut fur Organische Chemie, Technische Universitat Braunschweig in Braunschweig, Germany. Youngsteadt traveled to Peru in 2004, 2005 and 2006 to study what are known as ant gardens and the ants and plants that live in them. The ant gardens Youngsteadt studied are made by an ant known as Camponotus femoratus. C. femoratus ants are found throughout the Amazon, Youngsteadt says, and these relatives of the carpenter ant are known for the nest gardens they build in trees. In these gardens, which Youngsteadt says range from the size of a fist to as much as 3 feet across, grows a plant called Peperomia macrostachya. C. femoratus ants are gardeners, Youngsteadt explains. They collect P. macrostachya seeds and carry them to their nests, where the seeds grow and form ant gardens.
|C. femoratus ants build nests in trees and turn their homes into gardens by planting seeds of specific plants in the nest walls. (Photo: PNAS)|
What Youngsteadt went to the Amazon to find out was: How do ants know which seeds among the millions in a tropical rainforest to pick up and take back to their nests?
She started with an extract. She soaked P. macrostachya seeds in a solvent to pull the seed chemistry out of the seeds. She put the extract on seeds from other plants, and ants carried the treated seeds, so she knew something in the extract attracted the ants and cued them to carry the seeds. But, Youngsteadt says, the extract is a complicated mix of more than 100 different chemical components. So she broke down the extract into fractions, which she tested by exposing each to ants. She identified one fraction ants seemed to like the best, and working with Nojima, the senior researcher in entomology at NC State, was able to identify five chemical compounds that attract ants.
Nojima pinpointed the compounds by dissecting ant antenna, then putting the antenna in a saline solution and hooking it to electrodes and an amplifier. When exposed to certain compounds, the antenna responded by registering electrical activity through the electrodes. In this way, Nojima found out which compounds were of potential interest to ants.
Working with Häberlein and Schulz, the German chemists, the research group identified the chemical structures of the five compounds and put them together in a seed-like blend, which they tested to see if ants were attracted by the odor. Using an olfactometer, Youngsteadt exposed ants to the chemical blend and found that it attracted them. She says there are no other examples of which she is aware of ants being attracted to seeds by odor. And indeed, the ant gardens of the Amazon are aromatic. Youngsteadt describes the odor as "a pleasant, citrus-vanilla-pine smell."
Yet while Youngsteadt learned much about the interaction of ants and plants in the Amazon, her research did not fully explore the chemicals on P. macrostachya that cause C. femoratus ants to carry seeds.
Youngsteadt found that while the chemical cocktail she mixed attracted ants, the ants would not pick up and carry off seeds soaked in the chemicals. Yet when Youngsteadt put the extract she made from P. macrostachya seeds on seeds from other plants or even other objects such as twigs, ants would pick up the objects and carry them back to their nests.
So, says Youngsteadt, while the chemical compounds she identified certainly attract ants, there's another set of chemical cues, yet to be discovered, that plays a role in telling an Amazonian ant which seeds it can use to plant and grow an ant garden.
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