Parasites influence the behaviour of their hosts. It is known. The tapeworm that cultivates rash behaviour in its host. The tiny bug that emboldens rodents who stand their ground in the face of certain death. The virus that inspires its victims to get their party on. But new research has shown that parasites can also influence uninfected members of their host’s community.
Whether or not this has implications for social behaviour in humans is a more complicated issue. Could these findings be used to predict human behaviour? Or, if one is feeling especially sinister, control human behaviour — without overt coercion?
Sometimes when you’re infected, you know you’re infected. For instance, the African eye worm Loa loa has a way of really getting under your skin, you know? And that way is through the bite of a horsefly or a mango fly. After maturing the larva crawls around just underneath the surface of your skin, wending its merry little wormlike way from head to toe and back again. Fun fact: you can see it moving if you look carefully. Double fun fact: when the adult worm migrates to your face and crosses over your eyeball, you can see it moving even when you’re not looking carefully.
At other times, you are under your passenger’s influence before you even know it’s there. In a 2010 study led by biological anthropologist Chris Reiber of Binghamton University, researchers found that subjects given a shot containing a weak form of the flu virus were considerably more social for the two days immediately after exposure, when compared with their behaviour for the 48 hours leading up to the shot.
“Participants interacted with significantly more people, and in significantly larger groups,” the researchers said in their paper, published by the Annals of Epidemiology. In other words, before symptoms were even showing, infected subjects behaved uncharacteristically gregariously so as to expand the buffet of potential new victims for their new mind-controlling snot-monster overlord.
Of all the behaviour-modifying parasites, the best known and most documented is almost certainly Toxoplasma gondii, which is carried by rats, breeds in cats and — maybe — turns humans batty (or at least into cat-lovers). It acts on the dopamine receptors in the brains of mammals so, in mice or rats, the fight or flight reflex is in effect switched off. This then allows them to be caught easily by the cats, the only known host in which the parasite is able to reproduce.
The jury is out on T gondii’s effects on humans but there is some research linking the parasite to mental illness. T gondii is the reason pregnant women are advised to stay clear of cats: passing an infection on to an unborn child can lead to severe cognitive disorders or even death.
In adults the effects are less horrific but more unusual — and differ according to whether the infected person is male or female.
Infected women “were more warm-hearted, outgoing, conscientious, persistent and moralistic”, according to Jaroslav Flegr in his review of 11 studies published in the Schizophrenia Bulletin journal in 2007. But men “were more likely to disregard rules and were more expedient, suspicious, jealous and dogmatic”. Which is so weird, right?
But then humans are a bit of a dead end for T gondii. We don’t help the parasite reproduce, so it messes with our minds just because it can.
Like rats, stickleback fish also become fearless when infected. One of the ways the tapeworm Schistocephalus solidus reproduces is inside fish-eating birds, which then spread its larvae in their droppings. These are eaten by tiny water-dwelling crustaceans, which in turn infect the sticklebacks that feed on them. And then finally the infected sticklebacks hang around near the surface of the water, making them easy prey for the birds. (“Nants ingonyama bagithi, Baba” it is not, but then being a photogenic lion isn’t a prerequisite for being part of a circle of life.)
All this is known. What was not known until now was how a whole shoal would behave if some but not all of its sticklebacks were infected by the worm. Would the rest of the shoal dart off to safer depths when a threat appeared, leaving the infected fish behind? Or would peer pressure make them stick around and get eaten too — and, if so, what proportion of the shoal needed to be infected for peer pressure to work?
In a study published in the Proceedings of the Royal Society B last week, researchers Nicolle Demandt and Benedikt Saus and their team from the Institute for Evolution and Biodiversity at the University of Münster conducted a controlled experiment involving fish in tanks responding to a simulated bird attack.
They found that:
a) a completely uninfected shoal displayed escape responses and risk-averse behaviour when threatened; b) completely infected shoals did not; c) mostly uninfected shoals did display escape behaviour but infected members of that shoal did not; and — drumroll please — d) mostly infected shoals did not display escape behaviour at all.
Most notable, perhaps, was the quorum of sticklebacks required for (d) to occur was a ratio of 2:1 infected to uninfected. In other words, for the entire shoal to be affected, the infected individuals had to outnumber the uninfected individuals by two to one.
Knowing the threshold or critical mass required to change a group’s behaviour without coercion is very interesting. But it would not be wise to try to use these findings as a predictor for human behaviour — by creating a 2:1 ratio in a social media network, for example by flooding it with twice the number of on-message puppet accounts than real users in an attempt to produce real conformity by artificial means. (Ringing any bells?)
As it turns out, studies have already been done on whether such a threshold applies to people. In 2011, social scientists at the Rensselaer Polytechnic Institute in New York state found that there was indeed a ratio of believers to nonbelievers required for a new belief to be adopted by the majority of a society.
And it wasn’t 2:1, like the sticklebacks.
After all, the stickleback is a simple creature, with no aptitude for critical thinking. Human beings have agency and intelligence; they are able to discern subtlety and make informed judgments based on context, experience and deductive reasoning.
No. For humans the ratio was 1:10.