Who’s in control ?
Having a dog forces us to walk more frequently, finding a cockroach in a meal makes us lose our appetite, and having ticks or lice makes us scratch (and perhaps wash) more often: Animals can cause us to change our behaviour. But those changes are nothing compared with what some animals (especially small ones) do to others.
Nematode worms of the genus Mermithia castrate their insect hosts. A worm, known as Acanthocephala, matures in freshwater fish, but has larval forms that attack small crustaceans. Following an infection, the latter turn bright red and become easy targets for the predatory fish. Unusually red abdomens in some tropical ants, filled with nematode eggs, are also a consequence of a parasite attack, causing ants to resemble tropical fruits and to get eaten by birds. Thus, the nematodes reach their target.
Feminization of males is observed in terrestrial isopods like Armadillidium when infected with Wolbachia bacteria. And since the bacterium is transmitted in the isopods’ eggs, highly female-biased sex ratios can occur. Strepsiptera (a group of insects with only female individuals parasitizing other, larger insects) modify the sexual features of their hosts and turn them into the opposite sex, actually increasing their host’s life expectancy by curbing their sex drive. In mice feminization of males is the result of an infection by Taenia crassiceps. While male mice produce less and less sperm, females gain weight and develop larger uteruses. But not only sexual orientation can be subject to parasite manipulations: Even more complex behaviours can become parasite-controlled. Rats and mice usually avoid cats, but when infected by the protozoan Toxoplasma gondii (which needs to invade a cat to complete its life cycle), rats and mice develop a fatal, feline attraction, no longer running away from cats.
Another form of behavioural control is exerted by the last larval stages of featureless worms known as ‘horsehairs’ or Gordius. A beetle or praying mantis that has consumed an insect that, as an aquatic larva, had become infected with the first larval stage of such a worm, becomes completely hollowed out by the worm’s second larval stage and then suddenly becomes very thirsty, seeking freshwater. On reaching a puddle, pond, or river, the mature second larval stage leaves the dying beetle or mantis, turns into a 15-30 cm long free-living adult worm, finds other worms to form a “Gordian mating wad” with and releases its eggs. Then the worm dies , but its eggs hatch, the first larval stage enters an aquatic insect, the latter turns into an adult, is eaten by a beetle or some other insect and the cycle begins anew.
Even more ‘manipulative’ is a parasitic wasp, whose larva sucks body fluids from a spider. By the time the wasp larva intends to pupate, it somehow induces the spider to prepare a silk shelter. When the shelter is ready, the larva eats the spider and uses the shelter to pupate in. Caring for somebody else’s brood and developing maternal instincts (even when male) is how Sacculina sp., (probably the weirdest of all parasites in terms of its modified body) manipulates its crab host.
The most amazing parasite-induced behaviour, however, is that of a fluke, who resides in a songbird’s gut and releases eggs that get eaten by the snail Succinea. Inside the snail, the first larval stage generates hundreds of secondary larvae in a sac that is lodged in one of the snail’s two tentacles. The latter swells up, develops a red tip and keeps conspicuously pulsating in green and yellow rings. Infected snails no longer seek shelter and expose themselves. Birds then bite off the tentacle and become hosts to the fluke. For some of the described behavioural changes answers have been found (chemicals from the parasites are released and interfere with the host’s hormonal system), but for many such changes the mechanisms are still far from clear. And how did all these parasitic oddities evolve? According to a former colleague of mine in New Zealand, God must have had a wicked kind of humour to create all these devious parasites.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2015.
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