Comments on: Parasites outweigh top predators and castrators do best of all

By: Nathan Myers

Nathan Myers — Thu, 14 Aug 2008 08:53:10 +0000

I cannot help but post this link, given the context. http://www.partiallyclips.com/index.php?id=1165

By: Jon D

Jon D — Thu, 31 Jul 2008 14:06:12 +0000

This discussion is getting quite long! very interesting one too.
To address why the snail populations dont drop to dangerously low levels, I remember reading in Parasite Rex that one of the strategies snails use to combat parasites is switching between sexual and asexual reproduction, depending on whether there are a lot of parasites around at the time.
When they reproduce asexually, genetic variation is low since there are only a limited number of strains of snail, so parasites are able to evolve ways of getting around the snails’ immune systems quickly.
When there are lots of parasites around, sexual reproduction allows for more genetic diversity, so its difficult for parasites that specialize in specific strains to invade the population.

By: Ed Yong

Ed Yong — Wed, 30 Jul 2008 11:26:40 +0000

Subramanyam – It’s unlikely that the relationship between the parasites and the snails would ever become so extreme that the parasites make themselves extinct. If they infect too many snails, there will be fewer hosts around and the parasite population should fall. As this happens, the snails should recover allowing for another boom in parasites and so on. That and the fact that some snails will probably be better than others at fending off parasites – after all, only half of them were infected. The thing with the birds isn’t a symbiosis because the parasites don’t do the birds any favours once they’re actually in their bodies.
Ian – I think you’re jumping the gun in assuming that the snails have self-awareness. That’s actually a surprisingly complex mental trait and one that few animals have. Have a look at this Wikipedia entry for a short primer and some links.
Richard – The *snail* is the *parasite’s* extended phenotype. Because the snail can’t reproduce, it’s evolutionarily irrelevant. To the parasite, the snail is the equivalent of a dam to a beaver or an anthill to an ant or a skyscraper to a human.

By: Ian Tindale

Ian Tindale — Wed, 30 Jul 2008 11:23:10 +0000

Or, it could be as it is in humans, where it is often said that men think of sex every x minutes (depending where you read it). The ‘thinking of sex’ might be a primary driver in most species, and removal of the presence of this controller may leave a free slot into which to insert a foreign control mechanism. This may lead to the question of whether the sex drive is a ‘high level’ driver or ‘primal’, as is often assumed. Religious beliefs frequently assign human sex drives to the ‘animal’ parts of our make up. Maybe it might turn out that sex drive is in fact a high-level, if not top-level control construct, and that all mating animals are driven by it. Take away this libido and you have a fully developed machine or tool that you can re-purpose to your own ends. Not taking it away may result in your control mechanism having to fight with the powerful libido all the time. Who knows. Just a minute, I’ve got to think about sex again – back in a while.

By: Richard

Richard — Tue, 29 Jul 2008 18:08:18 +0000

Could you explain the “extended phenotypes” in this context? I can understand a gene affecting behaviour to encourage a parasite if this affects your competition or predators more than you. But emasculating the host seems so obviously detrimental to the host’s DNA that I’d have assumed there’d be a massive evolutionary advantage to developing ways of stopping or slowing parasite infection.
Or have I just misunderstood and you’re referring to “extended phenotypes” purely from the point of view of the parasite. So disabling the host’s reproduction isn’t a primary aim of the parasite, just a side effect- it’s evolved because it makes the host even more efficient as a factory?
The sort of parasitic load you’re talking about for these snails seems to large I’d have assumed that it would seriously affect their competitiveness, both within and intra species. Even if evolving increased immunity is too tricky I’d intuitively expect tactics like shorter life spans (reducing competition with your offspring when you can’t reproduce). What’s the point in continuing to live when all you can do is poo you’re own killer?
Though I guess the parasite might try to extend your life, but now we’re getting a bit too complicated…

By: Ian Tindale

Ian Tindale — Mon, 28 Jul 2008 15:47:36 +0000

Is all of this really nothing more than highly sophisticated tool use by the parasite? The parasitic fungi is deploying an insect as a tool that will relocate it – it’s now not a moth, but a vehicle.
But does the host (and this we’ll probably never know) still “think” in an ‘acceptable’ and functional manner? Does the host know that it is under such control, and does it realise that the reason it does certain things is not because it decided to but because another agent wanted it to (or was evolved to want it to). I mean, it obviously must know something’s amiss, but it might not attribute the behavioural outcomes to the physical effects.
It might be possible that they live a perfectly functional (but not normal and probably not optimally comfortable) life, where they understand everything they do to be their own impulses and stimuli (assuming we can extrapolate that they might ‘understand’ their own experiential world model in any depth at all. Probably – they can’t be that stupid, they’ve been around since before the dinosaurs, and after).
The more I think about it, the more I think there’s a network-topology style intersection of disciplines waiting to produce innovative research, here:
Bring together this area of parasites and “controlled host behaviour that achieves an end”.
Match up with those studying behavioural deviancy in human cultures, where discussions of what makes a person a ‘bad’ person or criminal or insane often traverse a continuum with ‘environmental’ at one end and ‘hereditary’ at the other.
It’d be interesting (frightening? disempowering? enlightening?) to discover, for example, that much antisocial behavioural aberration in individuals was really not our fault after all. The weevil made me do it.

By: subramanyam.a

subramanyam.a — Mon, 28 Jul 2008 09:06:56 +0000

these parasites are not passed on from parents to offspring. ok. But if the parasites take an upperhand and
infect all snails and castrate them,where will the parasites find new hosts?. One more doubt. I read that some
parasite infected fish easily offer themselves to birds. The birds actually take these fishes, because they can find them easily and save precious energy, even though they have to rear and pass on these parasites. Is this another kind of symbiosis?.

By: Jon D

Jon D — Mon, 28 Jul 2008 03:30:01 +0000

QUOTE: I’ve blogged about Toxoplasma affecting human culture before.
I actually read that post last year when I first heard about Toxoplasma, before before I started reading your blog regularly here!
Parasites are very cool! Botfly larvae give me nightmares though

By: Ed Yong

Ed Yong — Sun, 27 Jul 2008 23:27:06 +0000

I've blogged about Toxoplasma affecting human culture before. As Jon pointed out, there is certainly a fine line between symbiosis and parasitism, and just last month, two papers came out that illustrated this nicely. I actually planned to do a two-part post on this recently, but failed to due to lack of time. So consider this a mini-write-up. The first concerns corals and the symbiotic algae that provide them with energy through photosynthesis. It's a classic example of symbiosis. One group of algae called Symbiodium had 8 different genetically distinct groups and there's new evidence that one of these (A) is actually a parasite. Corals that carry the A algae tend to have poor health and the algae provide them with less carbon than the more common C type. The second paper showed that a parasitic mite could actually be more of a symbiont. The mites suck the blood of a parasitoid wasp, but they also defend it from other hyperparasitoids (see my previous comment). The mites attack the hyperparasitoid larva and the more of them they are, the greater their chances of killing it. This benefit is so great that the host wasps have apparently evolved small structures on their backs to house the mites, even though they lose blood in the process. So we have a symbiote that's actually a parasite and a parasite that's actually beneficial in the grand scheme of things. It's definitely ludicrous to label parasites as evil, and I personally think they are quite beautiful (despite the reflexive wincing they can induce).

By: Jon D

Jon D — Sun, 27 Jul 2008 20:04:44 +0000

QUOTE: It might also conclude in thinking how lucky we are that our minds aren’t controlled by an external agent. (?)
Toxoplasma gondii, a parasite carried by cats infects somewhere between 30-80% of the world’s population. It infects the brains of mammals, and its effect on rats makes them less afraid of cats, which obviously makes them easier to eat and thus pass the parasite on.
Although more research on humans is needed, the effects suggested thus far are various changes in personality and delayed reaction times.
The rest of your comment was very interesting – There is often a fine line between symbiotic and parasitic relationships – I’m obviously talking here about parasites that dont kill their hosts. eg would you call the bacteria living in your guts parasites or symbiotes? On its own, I expect bacteria would generally be a parasite, and of course some introduced strains can kill you. But the microbes in our guts break down carbohydrates and make some of the vitamins and amino acids our bodies need. (Microcosm – another really good Carl Zimmer book!!)