Mind Control In The Animal World
Selfish moves by parasites to manipulate the behavior of their hosts
The orchard orb weaver, known in the biological world as Leucauge argyra, is a small spider found around the world. William Eberhard and his team have been observing its behavior for a few days now in the African palm plantation in Parrita, Costa Rica. A parasitic wasp indigenous to the region, H. argyraphaga, stings the spider and attaches its egg to the spider’s belly. After being stunned for a brief period, the spider regains consciousness and goes about its business of weaving orb webs as usual.
The orb web is an important part of the spider’s life. It is used to capture prey, to house young ones, and for movement. Its proper functioning depends on the precise arrangement of the radial silk strings. The egg of the wasp hatches into a larva which remains attached to its host and feeds on it. A week later, the spider’s behavior changes. Instead of weaving a circular web as it has been doing all his life, it starts weaving a bizarre-shaped web with thick perpendicular silk lines, unsuitable to capture prey or nurture its young ones. It is suitable, however, to house the cocoon of the wasp. Shortly after it has built the web for its guest, the spider gets killed and eaten up by the wasp larva.
To find out how the wasp influenced the spider’s behavior, the researchers detached the larvae from a few spiders before they are killed. The spiders survived and the next web woven by some of them was a regular orb, suggesting the behavioral modification of the spider was a result of a fast-acting chemical mediator released by the parasitic larva.
The findings by Eberhard and colleagues, published in 2001, weren’t the first or the only ones showcasing the ability of parasites to control the minds and behavior of their hosts. A parasite is an uninvited guest who, by definition, harms its host for personal gains. A very ignoble thing to do but in the cut-throat world of living organisms, the desire to survive often trumps nobleness. Parasites use their host in a variety of ways to ensure their survival and growth. Some of the ways, however, are so manipulative and ingenious that you can’t help admiring them, even at the cost of feeling borderline sadist. Consider the single-celled protozoan Toxoplasma, for instance, which makes a rat run to its death so that it can enter its next host, the cat, to reproduce. These tales of manipulation and trickery are countless, some of them, however, particularly stand out.
Grasshoppers for a dive — and death
The larvae of the worm S. Tellini, commonly known as hairworm, are eaten by grasshoppers and cricket where they develop into adult beings. The worm can remain on land for part of its life but to mate and complete the lifecycle, it needs water. Grasshoppers are land creatures through and through and stay out of water for their own good. The worm, by releasing certain proteins in the central nervous system of the host, manipulates the behavior of its host, causing it to dive straight into water bodies and, in all likelihood, drown to its death. Once in the water, the worm leaves the host’s body, matures further, mates, and produce offspring.
How these proteins modify the behavior of the grasshopper is not exactly known but evidence suggests that some of them act as neurotransmitters while others modify its geotactic behavior — the oriented movement of an organism in response to gravity. As it stands, the worms enjoy their first dance to the tune of a drowning grasshopper — the host which enabled them to mate in the first place.
A fearless rat is a dead rat
Cats and rats don’t get along. One eats the other for food — and fun. Toxoplasma gondii is a microscopic organism that can live as a parasite in almost all mammals but sexually reproduces only inside a cat’s intestine. Rats are one of its frequent hosts. Fertilized eggs of the organism are taken up by rats mostly from cats’ feces. Inside the rat, T. gondii matures and forms cysts in its muscles and brain.
To complete its lifecycle and produce gametes, it requires a feline body. Rats tend to stay away from cats and for a good reason — their lives depend on it. How Toxoplasma persuades a rat to get intimate with cats is creepy but ingenious. Researchers have found that the level of dopamine rises in the central nervous system of rats infected with toxoplasma. Dopamine is a neurotransmitter and has been called many names: ‘the molecule of more’, ‘the reward chemical’, ‘the happy hormone’, ‘the hormone of love’. It is known to mediate the emotional aspect of animal (and human) behavior including love, fear, desire, etc. Higher levels of dopamine make the rat fearless and an extrovert — and thus an easy target for cats. As the rat roam around boldly in the vicinity of cats the inevitable happens — the cat feasts, toxoplasma produces gametes in the cat’s intestine; nature has its way.
Alcon butterflies and their hitchhiking larvae
Beauty has fooled many but it’s the nasty-looking caterpillars of the beautiful Alcon blue butterflies that get Myrmica ants. The butterfly lays its eggs on plants. The eggs hatch into larvae (caterpillars) which feed on the plants for some time. When they are mature enough, the larvae fall to the ground. Passing-by ants pick the larvae and, mistaking them for their own larvae, carry them to their nests where they are nurtured.
Why the ants can’t tell the difference between their own and the butterflies’ larvae is due to some sneaky little trickery on part of the butterfly — it’s the way their larvae smell. The Myrmica ants recognize their larvae by sensing a familiar chemical on their surface. The Alcan butterflies have evolved to express a similar chemical on the outer coats of their larvae which tricks the ants into recognizing them as theirs. As a result, the caterpillars are catered for and nurtured at the expense of the ants’ own larvae.
As David R Nash and colleagues found out in their research in Denmark, one species of the ants, Myrmica rubra, have become aware of the trap and now resists taking up the butterflies’ larvae in place of their own. How has the butterfly responded? By expressing chemical signals that trick not one but two ant species: Myrmica rubra and Myrmica ruginodis. Thus, they ensure that if one ant doesn’t pick their larva another one will.
Studies in animal behavior make us realize the cut-throat nature of our world. The extent to which an organism goes to ensure its survival can be overwhelming. Observing nature, life, and survival can be wonderful, horrible, awe-inspiring, and impressive — all at the same time.