"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 11, 2014

Apocephalus attophilus

This is the third post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2014. This particular post was written by Jon Schlenert on a paper published in 1990 about a way that leaf-cutter ants defend themselves against parasitoid flies (you can read the previous post about a tardigrade-killing fungus here).

Photo by Norbert Potensky
Leaf cutter ants are well known for their important ecological role as herbivores of tropical forests. The ants harvest large quantities of leaf material which they use as garden beds to grow a highly specialised, symbiotic fungus which is eaten by the ants. Less well known is that leaf cutter ants exhibit a peculiar behaviour  whereby smaller ants of the same species called minims, hitchhike on the leaves being carried back to the colony by the larger foraging ants.

Observations of this hitchhiking behaviour provoked much speculation about its purpose, and two alternative hypotheses arose to explain it. The first hypothesis, dubbed the energy conservation hypothesis, suggested that the smaller ants undertook important roles at the foraging site, and would then hitchhike back to the colony on the leaves to reduce energy costs. The second hypothesis, the ant protection hypothesis, posited that hitchhiking behaviour was a defensive response to pressures from parasitic flies.

The flies belong to the family Phoridae and mostly parasitise hymenopterans: ants, bees wasps. The reproductive strategy of parasitic phorids involves the female laying eggs inside the bodies of living insects. The parasitised insects are kept alive whilst the larvae hatch from the egg and begin to consume the host’s tissue. Eventually the larva is ready to leave the host and become a free living adult, when it emerges the host is left mortally wounded or in some cases is already dead beforehand.

Leaf cutter ants have their share of parasitic flies. Foragers are particularly vulnerable to attacks from the flies as they are unable to defend themselves whilst carrying leaves back to the colony. A female fly will land on the leaf fragment being carried, and make its way down towards the joint between the cephalon (head) and cephalothorax (first thoracic segment) of the ant. The fly, using it’s long ovipositor, injects an egg between the armoured plates and quickly absconds.

In the paper, researchers set out to quantitatively assess whether the hitchhiking behaviour in leaf cutting ants is a response to attack from parasitic flies. Research was carried out on Barro Colarado Island in Panama; an important research location for studying tropical ecosystems. The two species looked at were the ant species Atta colombica and its parasitic fly Apocephalus attophilus. The study found strong evidence in favour of the ant protection hypothesis and concluded that hitchhiking behaviour is driven by parasitism rather than energy conservation. The study also found that flies require leaf fragments to stand on whilst they inject their eggs, thus only leaf carriers were susceptible to parasite attack. The presence of hitchhikers significantly reduced the probability of attack from the flies and represents a major investment into parasite defence. Furthermore, the researchers observed that the ants were able to adjust the level of hitchhiking behaviour displayed in response to daily and seasonal changes in parasite abundance.

The highly specialised defensive response of leaf cutter ants represents a significant cost to the colony, as ants are diverted from undertaking other important tasks. Consequently, there is a trade-off between investing in parasite defence or other areas such as caring for offspring. This demonstrates that parasitism from phorids has shaped the evolutionary responses of leaf cutter ants, and is a strong influence on their ecology and behaviour.

Feener Jr, D. H., & Moss, K. A. (1990). Defence against parasites by hitchhikers in leaf-cutting ants: a quantitative assessment. Behavioral Ecology and Sociobiology 26:17-29.

This post was written by Jon Schlenert

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