Daytime Bites for Zombie Ants

Most ants in the grip of a deadly fungus spend their last moments on Earth around midday.

THE GIST

Ants infected with a deadly fungus stagger about the forest floor until midday.

Around that time, ants bite into the vein of a leaf about 2.5 meters (8.2 feet) off the ground and stay clamped there even after death.

Actual death, not easy to determine with zombies, occurred some hours later, possibly near sunset.

Zombie time in Thai forests turns out to be brunch time.

Ants fatally infected with a fungus that turns them into staggering weirdos were most likely to stumble around from about 9:30 a.m. until noon or 1 p.m., says evolutionary biologist David Hughes of Pennsylvania State University in University Park.

The approach of noon in the forest brought a spike in what may be the ants' most peculiar behavior, Hughes and his colleagues report online May 9 in BMC Ecology. Afflicted Camponotus leonardi ants bite into the vein of a leaf about 2.5 meters off the ground and stay clamped there even after death. Out of 16 ants observed at their last chomp, nine bit the leaf between 11:30 a.m. and noon. All the observed final bites, for unknown reasons, occurred between 11 a.m. and 1:45 p.m. Actual death, not easy to determine with zombies, occurred some hours later, possibly near sunset.

Researchers also got into the heads of the zombies. Dissections suggest Ophiocordyceps fungal buildup may explain the infected ants' erratic gait and convulsions, which probably keep zombies from climbing back into the canopy where they normally live. And muscle atrophy, possibly from rapid post-bite fungal growth, may keep the ants from pulling their mouthparts loose from fibrous leaf tissue once they've bitten in.

Death grips on low leaves give the fungus auspicious humidity to reproduce far from the sun-scorched forest canopy, Hughes has reported. Yet the notion that any parasite is manipulating its host is tricky to prove. "One of the alternative hypotheses in these cases is that the change in behavior is a side effect of infection, not something the fungus 'actively' does to the host," says behavioral physiologist Shelley Adamo of Dalhousie University in Halifax, Canada. So far though, she says, the zombie-maker fungus "fits most of the criteria for parasitic manipulation."

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