Comment by gojomo | Hacker News Reader

Comment by gojomo | Hacker News Reader

How can they call it a parasite?

It's clearly a commensal symbiote: for cats, it assists them in catching prey.

In rats and humans, its changing prevalance helps adjust the relative expression of different behavioral strategies, faster than inheritance alone could.

For example, the risk-taking and increased libido in rats shifts the infected slightly along the K-strategy/r-strategy reproductive spectrum. In some environments that's a win, others a loss, but the T.gondii increases rat behavior diversity, and thus the species' adaptability.

Similarly, its modulating effects on human suspicion, gregariousness, investment in fitness-signalling (clothing), and obedience/trust sometimes help and sometimes hurt, depending on an ever-changing environment.

What if we 'cured' this and found the resulting human populations were less resilient against war, ecological stresses, and rapid technological/cultural change?

(I'm only half joking. And a meta-parasite is making me write apologetics for other parasites. And this entire comment is just a long way of saying "I, for one, welcome our new Toxoplasmosis gondii overlords" without getting immediately downvoted.)

Semantics point/question:

The T. gondii, as viewed from the cat's perspective might be a symbiote, but from the rat's perspective, is certainly a parasite, isn't it? The T. gondii is benefiting at the expense of the rat. Do biologists classify things as both simbiotes and parasites?

I think some rats also benefit: their adventurousness and increased sexual activity isn't always penalized with death, and even when it is, female rats prefer the infected male rats. So even if they live fast and die young, they may leave more offspring.

Whether this is a net benefit at any one time probably depends on lots of things, and especially relative rat/predator populations. Just as human reproductive preferences seem to change in relation to wealth, population density, and life-expectancy.

Regarding semantics, I do think something can be both a symbiote and parasite. When I first learned of 'symbiosis' it was in the context of mutually-beneficial relationships, but it seems the term can be more general. Wikipedia says scientists sometimes disagree whether 'symbiosis' should only be used to describe 'mutualist' relationships (where both benefit) or also 'parasitic' (one benefits and other is harmed) and 'commensal' (one benefits and other neither harmed nor helped) relationships.

I should have used the word 'mutualist' in my above post, as that's what I was intending to imply. But the conjecture is intentionally fuzzy about whether the benefit is to the individual (it perhaps sometimes is) or the species (it perhaps always is, as long as both infection and non-infection are well-distributed).

think of it as an agent of accelerated evolution

> Do biologists classify things as both simbiotes and parasites?

I don't know about that, but I know ichneumonid and braconid wasps have built exactly that relation with viruses: http://en.wikipedia.org/wiki/Polydnavirus

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