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Comment by Dig1t | Hacker News Reader

> It is therefore possible that the behavioral changes induced by Toxoplasma in humans, including suicidality, are a part of its adaptations to increase the probability that our ancestors were eaten by lions or leopards

What the what

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I am skeptical that most suicidal people, who may idealize or even follow through on shooting or hanging themselves, would willingly let themselves be eaten by a lion. Fight or flight adrenaline in the face of big cats feels like a likely effective cure for suicidal ideation…

> Normally, a protozoan is not very likely to end up in a cat’s gut because rodents instinctively fear cats. In Toxoplasma-infected rodents, however, the fear of cats disappears (Vyas et al., 2007). The mere smell of a cat normally makes rodents flee, but rodents infected with Toxoplasma experience the smell of cat as sexually attractive (Berdoy et al., 2000, House et al., 2011). This makes them seek out cats, making Toxoplasma-infected rodents more likely to end up as cat food. These rodents still avoid other predators as before (Berdoy et al., 2000, House et al., 2011). The reversal of this innate fear reaction helps the protozoan reach its definitive host, a cat, and to propagate its genes to future generations.

I hear theories like "this parasite has evolved to make it's host want to be eaten by lions" and I instinctually think that's crazy, but then I remember all the way more crazy things that nature does, and it starts to sound reasonable. Thanks for sharing!

Don't Google search "parasitic ant fungus" then.

When I first learned how HIV (and retroviruses) worked, it was the same level of amazement. Like "it splices its own genes into your cells' DNA, and when your cells reproduce, they additionally make copies of the HIV virus" is just something that I would have never thought was real.

I think folks like myself, who became an "amateur epidemiologist" during covid (out of necessity, because of the information vacuum) were shocked to learn all these amazing things we weren't ever told. As an aside, the same is true for everyone who is now an "amateur diplomat" post-invasion and read about realism etc. and well-understood and reasonable explanations for "why the U.S. is always at war" that make it seem so logical. Crazy to think there are so many other fields keeping amazing secrets like this.

Nature is fucking terrifying. Every time I see some zoo program on the telly where "oh we're rehabilitating these endangered rabbits and then we'll release 'em back into the wild where they'll be free and happy". Not sure that's how the rabbits feel about it.

If I were a rabbit in one of these programs I'd feign a limp, hide in the corner, anything I could to avoid being cat or wolf fodder, or getting one of the dozens of tortuous diseases and parasites that infect rabbits in the wild.

Wow, this is amazing, I totally didn't know this.

Makes the case for this happening in humans seem more plausible.

Like my sibling comment said, thanks for sharing!

I'd think that likely. Ideation is easy. Actually pulling the trigger is difficult. Survival instincts are strong.

In the paper, they hypothesize that the route of transmission would have been big cats scavenging human remains after suicide.

Of course this, like all evolutionary psychology, is basically unknowable and highly susceptible to just-so story telling

T. gandii and its lifecycle are not in the scope of evolutionary psychology, though. It actually reproduces (sexually) in cat gut and the (cysts of the) host animal actually need to be eaten by a cat (or other host). I agree that EP can lead to dangerous, political, and moralistic ways of thinking.

But the idea that parasites have lifecycles and incentive to survive and reproduce does not deserve such criticism. It's mainstream biology. Toxoplasmosis is a real thing that really affects behavior in wild mice. Read it on Wikipedia.

> https://en.m.wikipedia.org/wiki/Toxoplasma_gondii

>Toxoplasma gondii infection in mice lowers general anxiety, increases explorative behaviors and surprisingly increases a general loss of aversion to predators (without selectivity toward cats).

Whether toxoplasmosis causes suicidal ideation in humans is less clear and more speculative, as you argued. I agree. But the mode of parasite transmission, assuming a psychological effect exists in humans, would indeed be Alice being eaten by a cat and then Bob touching cat shit.

Here is a diagram of the known lifecycle: https://en.m.wikipedia.org/wiki/Toxoplasma_gondii#/media/Fil...

Actually you might be surprised [1] [2] [3]

[1] https://www.independent.co.uk/travel/africa/lion-kills-man-g...

[2] https://www.telegraphindia.com/west-bengal/calcutta/alipore-...

[3] https://www.dailymail.co.uk/news/article-3602819/Naked-man-j...

>The young man broke into the enclosure, took off his clothes and jumped into the middle, horrifying other visitors who witnessed the attack.
>Once inside, the lions reportedly immediately pounced on him and began to 'play' with him.
>At this point, zookeepers intervened in the attack and shot the two lions in order to save his life.
>The director of the zoo, Alejandra Montalva, said: 'We believe that this person entered as a visitor and paid for his ticket.
>The zoo director said she was 'deeply affected' by the deaths of the two lions, a male and a female.
>One witness said the man was 'shouting things about Jesus'. They continued: 'He was screaming religious things.'
>According to the news channel, the suicide note that was found inside his clothes made allusions to the apocalypse as being a reason for his attempt to take his life.

>He added: 'He suffered several injuries and trauma to the head and the pelvic area. 'We have high hopes that will recover and that will be fine.'

How is this guy the victim here? What about the lions?

[Edit: retracted dumb speculation]

Does cat ownership cause toxoplasmosis, or does toxoplasmosis make you want to be around cats?

There aren’t many people that have more than 2-3 dogs. You can’t say the same for cat people.

Come to Georgia. Quite a few people have 10-15 dogs. Even in suburban neighborhoods. I've subjectively seen more "pack of dogs" type people than "cat ladies".

It's a cultural thing; I'm not arguing there's some kind of "dog toxoplasmosis". Wikipedia says the evidence for (feline) toxoplasmosis causing crazy cat ladies is not definitive, either. https://en.m.wikipedia.org/wiki/Toxoplasmosis#Society_and_cu...

That sounds like the bacteria that makes ants hang off of leaves to get eaten by predators to spread the parasite. Similar to that cat pee bacteria thing.

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