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Ants versus fungi

Ants that grow fungi for food have to control other fungi that attack their gardens, but what about fungi that attack the ants themselves? Two papers published recently reveal surprising sophistication in both ants and fungi.

Sandra Anderson and colleagues discuss "The life of a dead ant: the expression of an adaptive extended phenotype" in American Naturalist. Richard Dawkins coined the term "extended phenotype" to refer to a consistent effect of a gene inside an individual on something outside that individual. For example, it might be possible to link differences in the shape of webs made by different spiders to genetic differences among those spiders. This week's paper shows that ants infected by certain fungi show complex behavior that benefits the fungi. Ants infected by fungi with different genes would probably not show this behavior, but the genes involved have not yet been identified.

Before the fungus-infected ants die, they attach themselves (by biting) to the underside of leaves that are ideally located for fungal reproduction: on the cooler and moister north side of trees, near (but not on) the ground. The researchers showed that these locations were favorable for fungal reproduction by moving infected ants higher in the canopy or down to the ground. Ants on the ground mostly disappeared, but fungi grew abnormally in those that remained. Fungi were unable to compete their life-cycle on ants moved higher in the canopy.

I can imagine a fungus producing an ant hormone (or perhaps destroying a particular neuron) to make its ant host bite a leaf, but getting ants to bite leaves in a particular humidity and temperature range and then hold on until dying seems pretty sophisticated. It would be easier if the ants spent most of their time in that zone anyway, but the one ant colony they found was much higher, about 15 meters.

The second paper shows greater sophistication on the part of the ants. "Adaptive social immunity in leaf-cutting ants" was published by Tom Walker and William Hughes in Biology Letters. The paper is freely available on-line.

These social ants protect each other from fungal infection by grooming each other, much like meerkats or baboons. Ants exposed to the fungus got groomed about twice as long as ants exposed to a control solution without the fungus, or about three times as long if their nest had been exposed to the same fungus two days before. (Another example of learning in insects.) Ants placed in nests that were previously exposed to the fungus were twice as likely to survive for two weeks after they were inoculated.

Comments

Thank you for this. I am so excited to talk about the "Farmer" ants and the Fungi with my class for biology. I think that is the neatest thing ever.

I'm not sure if the increased grooming time after fungal exposure is an example of learning. It would clearly be learning if there were negative effects within the two days after exposure to fungal spores that the ants could associate with the fungus. But if the incubation time is greater than two days, then this suggests that increased grooming time is an instinctive behavior governed by natural selection and not cognition. Do you have any clarifying data?

[If learning is "a change in behavior based on experience" and "experience" involves evaluating results of past behavior(s), you are probably right. -- Ford]

Thank you for this post. It will help my in my a biology class.

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