Behavioural Processes 68 (2005) 279–282 Look what the cat dragged in: do parasites contribute to Western Ecological Research Center, United States Geological Survey, c/o Marine Science Institute, University of California, Received 21 August 2004; accepted 21 August 2004 Keywords: Toxoplasma gondii; Cats; Rats; Culture; Personality If human culture emerges from the modal personality Other parasites are much more common in human of a population, can global variation in parasitism that populations. Some may have the ability to enact sub- affects personality lead to cultural diversity among na- tle, but long-term, changes in behaviour – to the ex- tions? The answer could help explain why people seem tent of changing personality. The most notable exam- to vary so much from one land to another. ple is Toxoplasma gondii. This parasite is primarily of view how parasites manipulate behaviour, in- concern because it can cause pathology under some cluding human behaviour. To quote them, “The rabies circumstances. Obstetricians warn expectant mothers virus lives in the brain, affording the virus ample oppor- that this parasite can pass through the womb and blaze tunity to directly affect host behaviour. Rabid animals through an immunologically immature fetus (resulting do show changes in behaviour, including increased ag- in blindness, permanent central nervous system dam- gression and biting.” Rabies affects a wide range of age or death). Toxoplasmosis is also a significant mor- mammals and the aggressive biting associated with fu- tality source of the immune suppressed. Nevertheless, rious rabies appears to increase transmission. The per- clinical cases are rare compared with latent toxoplas- sonality transformation of infected humans can be hor- mosis that occurs, on average, in 40% of adults (aver- rifying, transforming loved ones into thrashing, baying age estimates vary). Although the medical profession beasts. Not coincidentally, in Europe, past periods of treats these latent cases as asymptomatic and clinically rabies outbreaks correspond to increases in werewolf trials. Although rabies can have a dramatic effect, the indicates that the infected 40% behave in subtle, but present rarity of human rabies cases and the availabil- statistically different ways from the uninfected 60%.
ity of a vaccine, means that the behavioural effects of Seroprevalence varies greatly among nations, meaning rabies are primarily an illustrative curiosity.
that variation in T. gondii can lead to variation in thecollective (modal) personality among nations.
To explain how T. gondii affects human personal- Fax: +1 805 893 8062.
E-mail address: [email protected].
ity, it helps to describe how it gets from host to host.
0376-6357/$ – see front matter. Published by Elsevier B.V.
K.D. Lafferty / Behavioural Processes 68 (2005) 279–282 In its final host, the cat, the T. gondii parasite lives bound to do something because the rat and human brain inside a cell lining the gut. An infected cat sheds nu- are similar enough that the same neurotransmitters act merous oocysts that contaminate the soil or kitty litter.
to alter behaviour. However, the effects of T. gondii in Cats can accidentally ingest oocysts and infect them- humans differ qualitatively from the effects on rats. In selves. But given cats’ notoriously good hygiene, it particular, novelty-seeking behaviour decreases with T. is unclear whether direct transmission happens fre- gondii in men while it increases in rats. In addition, the quently. Toxoplasma exploits an alternative, indirect vastly larger mass of the human brain may be more mode of transmission because oocysts can also in- fect other mammals and birds. In the non-feline host, I am usually confident that I can easily do things the oocyst releases sporozoites that enter cells, divide that most people would consider dangerous (such as rapidly (tachyzooites), and ultimately invade muscle driving an automobile fast on a wet or icy road) states a and brain tissue where division slows or stops (brady- true–false question on the Cloninger’s Temperament zooites). The dividing phase can lead to flu-like symp- and Character Inventory personality test. A parasite toms (fever, lymph node swelling), but soon becomes could affect your answer. At least that is what recent dormant because the host immune system limits di- studies about the link between personality and parasitic vision (and provides lifetime immunity to new infec- tions). Infections can readily transfer to any carnivore ment, a key and heritable factor of personality, is associ- through ingestion of tissue-dwelling cysts in raw flesh.
ated with varying levels of the same neurotransmitters After ingestion, the intracellular tachyzooite and brady- zooite phases reestablish in muscles and the brain.
ple with latent toxoplasmosis score themselves differ- However, if the carnivore is a cat, T. gondii comes full ently in self administered personality profiles than do circle by invading the intestinal lining and producing trusting of others, less respectful of rules, more orderly ically transmitted parasite should be under selection and more jealous. Unlike infected rats, infected men to alter the behaviour of the intermediate host to in- crease the chance of predation by a final host. Studies In stark contrast, infected women are more likely to with rats infected with T. gondii provide compelling score themselves as warm-hearted, easy-going, con- scientious, persistent, insecure and moralistic ( mally view the world as a dangerous place. They avoid Infected women are also more likely to have the light and stay clear of new things. The scent of many friends and romantic partners and like to shop for cats sends them scampering. In contrast, infected rats clothes. Both men and women are more prone to guilt when infected. Personality changes are greater in indi- and first to enter traps T. gondii viduals that have had infections for a long time, helping also specifically reduces a rat’s innate fear of cats and to discredit the hypothesis that personality determines their associated smells; infected rats actually prefer the exposure risk. Perhaps the most dangerous thing about scent of cat urine over the scent of rabbit urine ( . gondii is that it slightly reduces the ability to et al., 2000). The behavioural changes of infected rats seem all about increasing the risk of predation by cats; plain why infected people are nearly three times more T. gondii does not alter irrelevant behaviours related to Modern pharmaceuticals provide a range of popular Although the mechanism of behaviour modification is therapies that can alter personalities to the psycholog- not known, mice infected with T. gondii have elevated ical benefit of patients. Selective serotonin reuptake inhibitors (e.g., Zoloft, Prozac, Paxil, and Luvox) are now widely used to treat depression and anxiety. Now The tricks that T. gondii uses on its rat host stand imagine that you discover you are seropositive for Tox- little chance of working on humans. Still, there is no oplasma. Would you, given the chance, cure yourself cost for the parasite to try. Moreover, the efforts are to get your innate personality back? If you answered K.D. Lafferty / Behavioural Processes 68 (2005) 279–282 yes, unfortunately, no complete cure is available. How variation in personality, this linkage often leads to na- about if you were not infected? Could a dose of toxo- tional stereotypes of personalities (the formal Japanese, plasmosis improve your life? Some women might find macho Italian, precise German, informal American, it tempting. Though it seems unlikely we will be see- etc.) that emerge from the concept of national culture ing a market for toxoplasmosis therapy, it is hard to conclude that uninfected women have more fit person- the field of anthropology explains the association be- tween personality and culture by arguing that culture Many factors influence whether you get toxoplas- shapes the modal personality of a population through mosis. The first has to do with the abundance of oocysts myriad top-down effects of environmental condition- around you. Cysts live longer in humid, low altitude re- ing and experience. However, it is difficult to distin- gions, especially at mid-latitudes with infrequent freez- guish between the cause and effect relationships of 1974; Walton et al., 1966). Homes with or near cats parably plausible that the modal personality of a pop- can help shape culture from the bottom up.
2001) have more oocytes around them. Working with If T. gondii can alter individual personality, and modal personality can shape culture, then T. gondii may an individual’s exposure to oocysts while a taste for be one of the factors that shape culture. Similarly, vari- rare or undercooked meat increases an individual’s ex- ation in climate, diet and other risks of exposure lead to variation in risk. This could contribute, in part, to the bility of contact accumulates with age and is higher for diversity of culture. Perhaps parasites and their effect on behaviour have a bigger impact on our human world see how climate, geography, economic standing, and that presently imagined. Although enough evidence ex- culinary traditions can interact to alter the prevalence ists to conclude that Toxoplasma affects personality, it of T. gondii. It is also obvious that these risk factors is unclear how much of the variation in personality it can explain, and whether these effects can have a de- Differences in risk cause prevalences of toxoplas- tectable effect on cultural variation among populations.
mosis to vary widely among populations. For example, If Toxoplasma does affect human culture, can we con- in the United States, the prevalence of Toxoplasma is clude that Toxoplasma is culturally deleterious? With- higher in the Northeast (29%) than in the south (23%) out it might the result be a France a little less French and Midwest (21%) and lowest in the arid West (18%) more variation among nations. This ranges from 81% have pervasive and powerful effects on the behaviour of in France (where raw beef – steak tartare – is a tradi- their hosts. Particularly for trophically transmitted par- tional dish) and 79% in Brazil (where climatic condi- asites, these behavioural changes appear advantageous tions are favorable for oocyst survival), to 7% in Japan for the parasite. The authors note that humans too are hosts for manipulative parasites. The speculation that differences in personality between my wife and I could this may have important implications for human culture partly result from her having grown up in humid, toxo- is testable and provides direction for where we should plasmosis rich Brazil and me being raised in arid, tox- Ethnic and linguistic groups often exhibit culture References
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