Abstract
BackgroundParasites that manipulate host behavior can provide prominent examples of extended phenotypes: parasite genomes controlling host behavior. Here we focus on one of the most dramatic examples of behavioral manipulation, the death grip of ants infected by Ophiocordyceps fungi. We studied the interaction between O. unilateralis s.l. and its host ant Camponotus leonardi in a Thai rainforest, where infected ants descend from their canopy nests down to understory vegetation to bite into abaxial leaf veins before dying. Host mortality is concentrated in patches (graveyards) where ants die on sapling leaves ca. 25 cm above the soil surface where conditions for parasite development are optimal. Here we address whether the sequence of ant behaviors leading to the final death grip can also be interpreted as parasite adaptations and describe some of the morphological changes inside the heads of infected workers that mediate the expression of the death grip phenotype.ResultsWe found that infected ants behave as zombies and display predictable stereotypical behaviors of random rather than directional walking, and of repeated convulsions that make them fall down and thus precludes returning to the canopy. Transitions from erratic wandering to death grips on a leaf vein were abrupt and synchronized around solar noon. We show that the mandibles of ants penetrate deeply into vein tissue and that this is accompanied by extensive atrophy of the mandibular muscles. This lock-jaw means the ant will remain attached to the leaf after death. We further present histological data to show that a high density of single celled stages of the parasite within the head capsule of dying ants are likely to be responsible for this muscular atrophy.ConclusionsExtended phenotypes in ants induced by fungal infections are a complex example of behavioral manipulation requiring coordinated changes of host behavior and morphology. Future work should address the genetic basis of such extended phenotypes.
Highlights
Parasites that manipulate host behavior can provide prominent examples of extended phenotypes: parasite genomes controlling host behavior
The host ant is diurnal at our field site [23] and infected ants (n = 42) appeared even more restricted in their activity as they were never observed in the early morning or late afternoon (15:00-18:00 hrs), in spite of our searches covering these early and late periods of the day
We conclude that prebiting infected C. leonardi ants at this site were only active in the morning and that this observation was not affected by sampling bias
Summary
Parasites that manipulate host behavior can provide prominent examples of extended phenotypes: parasite genomes controlling host behavior. Worker ants infected by fungal parasites belonging to the genus Ophiocordyceps express death grip behavior shortly before dying for no apparent other purpose than to assist parasite reproduction [15,16]. The fungus inevitably kills the ant and must do this outside the colony because ants quickly remove dead nest-mates [19], so that dying in the nest would not allow sufficient time for stalk development and spore release [20]. This host death as a developmental necessity implies that Ophiocordyceps infections would match the functional definition of being a parasitoid [21]
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