‘Zombie Ants‘, ‘Graveyards on the Move‘ … fungi can make the headlines of newspapers, such as The Guardian, and blog posts (Ancient Zombie Ants, Fungal Parasites & Zombie Ants, Genetics and Behavior, Cordyceps and Nature’s parasitic relationship with it). They referred to the parasitoid Cordyceps fungi (Ascomycota, Pezizomycotina, Sordariomycete, Hypocreales) — a genus that includes about >600 described species. All Cordyceps species are endoparasitoids, mainly on insects and other arthropods.
Jason’s recent tweet ‘A fun blog post & paper review on Cordyceps entitled “fungal parasites & zombie ants“‘ reminded me one of the more stunning sections of the 2006 BBC’s documentary series Planet Earth showing an ant being preyed on by Cordyceps — you can check out this segment narrated by Sir David Attenborough on YouTube — . Fruiting body of the infecting cordyceps erupting from the ant head and the mushrooming bodies of dead insects are frightening, but so beautiful.
The fungus Ophiocordyceps unilateralis, which is pan-tropical in distribution, causes infected worker ants to leave their nest and die under leaves in the understory of tropical rainforests. Cordyceps spores glue to foraging Carpenter ants (Camponotus leonardi), germinate on the insect cuticule and the fungal hyphae then grows inside the ant body where it releases chemicals that affect host behaviour. Some ants leave the colony and wander off to find fresh leaves on their own, while others fall from their tree-top havens on to leaves nearer the ground. The final stage of the parasitic infection (which may last 3 to 6 days) is the most macabre. In their last hours, infected ants — the zombies — move towards the underside of the leaf they are on and clamp their mandibles in a “death grip” around the central vein, immobilising themselves and locking the fungus in position. Very high densities of dead ants can occur underside of leaves leading to patches. Spores are too large to be wind dispersed and instead fall directly to the ground where they produce secondary spores that infect foraging Carpenter ants as they walk over them.
In a study published last year in PLoS One, Maj-Britt Pontoppidan and her colleagues have shown that the Cordyceps parasitoid not only affects individual ants, but they can also structure the entire host population of a tropical forest in Thailand in terms of its distribution in time and space, and then influence their own distribution: the ‘parasite’s ‘extended phenotype‘. It appeared that the dead ant bodies weren’t randomly distributed in the Thai rainforest floor. Instead they were in large aggregations (the so-called ‘graveyards‘) of up to 26/m2, separated by corpse-free zones. The dead ants had locked onto the undersides of leaves – an example of how the fungus influences its host’s behaviour. The distribution of dead ants appeared to be related to temperature and absolute humidity – things which could influence the survival of fungal spores and thus the chances of an individual ant picking up the infection.
Ant death-grip leaf scars have been documented on 48 Ma fossil leaves, indicating the antiquity of this behaviour (Hughes et al. (2010) Biology Letters online).
Fungal parasites and symbionts play an important role in structuring host plant populations. This study showed that they also affect animal populations … what about human behavior? (see: the Human Afflicting Strain of Cordyceps Fungus).
BTW, Thanks to its medicinal properties, mushroom hunters in Tibet can earn $900 dollars for an ounce of cordyceps.
Pontoppidan M-B, Himaman W, Hywel-Jones NL, Boomsma JJ, Hughes DP (2009) Graveyards on the Move: The Spatio-Temporal Distribution of Dead Ophiocordyceps-Infected Ants. PLoS ONE 4: e4835. doi:10.1371/journal.pone.0004835
Hughes, DP, Wappler , T & Lanadeira, CC (2010) Ancient death-grip leaf scars reveal ant-fungal parasitism. Biology Letters. Published online before print August 18, 2010, doi:10.1098/rsbl.2010.0521
Photo: Cambridge Graveyards, UK © F Martin & A carpenter ant (Camponotus leonardi) whose body has been consumed by the fungus Ophiocordyceps © David P Hughes