Mind con­trol for lit­tle mon­sters

We’ve just learned that brain-hi­jack­ing par­a­sites can bend even un­in­fected by­standers to their will

Mail & Guardian - - News - Matthew du Plessis

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 wa­ter­d­welling 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.

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