In popular culture and horror movies, people’s minds and bodies may be taken over by some mysterious force, transforming them into undead zombies. In reality, this is superstitious nonsense. However, there are indeed “zombie animals” in nature! The real culprits are parasites that affect these animals, so far that the poor victims will hurt or even destroy themselves!
Some parasites “zombify” animals. Parasites are organisms which live inside the body or on the surface of the host animals. As they obtain their survival needs, such as nutrients and protection, from their hosts, the hosts are harmed or even eventually killed. In some cases, the host animals behave in abnormal or crazy ways, which still intrigue scientists with unanswered questions.
Zombie animal no. 1: The carpenter ant that bites a leaf with a dead grip
In tropical forests (such as in Brazil) there is a fungus called Ophiocordyceps unilateralis that lives in carpenter ants (Camponotus spp.). After a fungus spore infects an ant’s exoskeleton, it not only obtains nutrients from the ant but also grows a germtube and hyphae throughout the ant’s muscle network. In three weeks, the ant comes under control of the fungus and behaves abnormally, such as by spending less time in the ant colony and wandering outside the ant nest.
At an appropriate time, the fungus “commands” the ant to climb onto a plant, to a height of about 25cm above ground, bite a leaf or twig, and die there. At the same time, the fungus stalk emerges from the back of the ant’s head, at the junction of exoskeleton plates. When the fungus matures, its spores will spread to nearby soil and plants, and infect other unlucky victims.
It is still unclear why the infected carpenter ants always climb onto their death plants at around noon. As regards why they have to climb to around 25cm, research has revealed that the temperature and humidity at this height is more favourable to the growth of Ophiocordyceps unilateralis. As the ant climbs to this height, the fungus releases some unknown substance that destroys the ant’s jaw muscles, so it bites so tightly on the plant that the muscles can no longer relax. The result is that the fungus propagates at the most optimal spot. How amazing!
Zombie animal no. 2: The praying mantis that drowns itself
Horsehair worms are parasites that spend most of their lives inside the hosts’ bodies. They only leave their hosts when it is time to breed in water. They lay eggs in water, which will be eaten by larvae of aquatic insects such as mayflies and dragonflies. This is the beginning of the parasitic process.
Horsehair worms remain in the bodies of their hosts until the hosts become adults and are preyed upon by other insects, such as a praying mantis. A worm then becomes a parasite of the praying mantis, living in it.
The infected praying mantis feels hungry all the time and keeps looking for food, while the horsehair worms grows inside its body—often reaching up to several times the body length of its host! When a parasite matures, it releases some chemicals that affect the behaviour of the praying mantis, causing it to drown itself in a water body such as a pond or river.
As the praying mantis jumps into the water, the horsehair worms emerges from the end of its abdomen. The host dies while the parasite will later breed, with the offspring bound for new hosts. The story repeats itself.
Zombie animal no. 3: The snail that attracts its killer
Leucochloridium sp. is a flatworm that lives in a snail as its intermediate host. It absorbs nutrients from the snail’s body, and intrudes itself into its host’s eyestalks. The muscles of the infected eyestalks become swollen and extremely brightly coloured.
Now, after being taken over and influenced by a little understood chemical mechanism instead of avoiding its predatory birds, the normally nocturnal snail carries out dangerous moves including exposing itself high up on plant in daylight. Moreover, the infected zombie snail is more active and can move three times faster than normal snails!
As the snail climbs up high, its pulsating, brightly coloured and swollen eyestalks look exactly like plump, juicy worms, which easily attract the final host of the parasitic flat worm—a bird.
Leucochloridium sp. will not kill the host bird, but reproduces inside its body, with the eggs spread to different places through the bird’s faeces. The life cycle of the parasite comes full circle perfectly as the bird’s faeces become food for a snail.