Wednesday, January 18, 2006

Ants with antibiotics and herbicides

One of the fun biology facts I love to introduce in my biology classes is that humans weren't the first species to develop farming; ants began farming millions of years before our ancestors were even walking bipedally. For example, leafcutter ants (genera Atta and Acromyrmex) bring back pieces of leaves to their nest, which they then use as a medium for growing fungus; the fungus digests the cellulose in the leaves (which the ants can't do alone), and then the ants eat the fungus. The ants even bring fungal spores with them when they start a new nest.

I've known for a while that leafcutter ants weed their fungus farms, but Scientific American's online news has a few neat stories showing that tropical ants use both antibiotics and herbicides to modify their environment.

Some ants rear a type of bacteria that kills other bacteria:
Entomologist Cameron Currie of the University of Wisconsin and his colleagues discovered the antibiotic bacteria in crescent-shaped pits on the exoskeletons of two species of Panamanian ants, Cyphomyrex longiscapus and C. muelleri, after scanning them with an electron microscope. The bacteria--of the Pseudonocarida genus--bloom on the individual face plates and other exterior parts of the ant, allowing it to rub the antiparasitic agent on its fungi crop. The ant also nurtures the microbe by secreting nutrients from special exocrine glands connected to the shallow pits.
And some ants use formic acid as an herbicide:
Sections of the rainforest made-up almost entirely of the tree species Duroia hirsuta, are called "Devil's gardens." Local legend holds that they were produced by an evil forest spirit, report Megan E. Frederickson of Stanford University and her colleagues. But the results of a four-year field study reveal that ants that make their nests in D. hirsuta are the driving force between the homogeneous plots of vegetation. The team introduced saplings of another common Amazonia tree to "Devil's gardens," using a barrier to protect some of them from the ants. Those that were unprotected, however, were quickly attacked by worker ants, which injected poisonous formic acid into the leaves. Trees began to show signs of withering within 24 hours and most of the leaves were lost after five days.

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