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Chapter 1: What fascinating story involves Theodore Roosevelt and piranhas?
In the early 1900s, a group of Brazilian fishermen pulled off arguably the greatest nature scam in history. And the poor soul they pulled it on was none other than the former US president, Theodore Roosevelt, who at the time was fresh off of a failed presidential run and looking for some adventure in the Amazon.
And the locals, well, they wanted to put on a show for our famous American, so they got to work weeks in advance, netting off a section of the river and stuffing it with piranhas, all the while making sure not to feed them a single thing, so that by the time Teddy rolled around, every single fish in the pen was starving and ready to chump on anything that dared move in their general direction.
And so of course, when Roosevelt showed up, like clockwork, the fisherman dragged out a live cow and shoved it into the water, which turned red, frothy, and bloody in seconds, while Teddy stood there watching what he was 100% sure was the most terrifying fish on the planet, where he then proceeded to recount the fish in his best-selling book, Through the Brazilian Wilderness, and calling it, I quote, the most ferocious fish in the world, with her malignant eyes and cruelly armed jaws.
And yet, despite me telling you this, this story is in fact false, even though it's been spread around the internet for years, including by some more reputable sources. So yeah, there were no locals conniving to show a false brutality of Amazon, no piranhas, no cow, it simply didn't happen. The book part was real, but not this embellished quote-unquote adventure, if you will.
And that, my friends, is incredibly fitting, considering that basically sums up what we think we know about piranhas as a whole. So in other words, basically everything you know is wrong, or at least wrong-ish.
And so, despite Hollywood building entire horror franchises around these guys, and convincing kids around the world that falling into the wrong river meant being reduced to a skeleton in under 60 seconds, or dare I even say 10 seconds, this is all really just not true. And so, to understand what piranhas actually are, you have to start with the family they belong to, the Sarasamans.
and the easiest way to picture this family is as a spectrum with a true piranha sitting at one end and then the paku's at the other and roughly 100 species in between the family name itself comes from latin for saw salmon which is a nod to the distinctive saw-like keel running along the belly of every fish in the group and they all share a laterally compressed body that kind of looks like someone took a normal fish and squeezed it from both sides until it was a little too flat for its size
But here's the first very interesting part, which is that this whole family descends from a common herbivorous ancestor, specializing not in meat flesh, but plant flesh, and only later splintering out to different branches that included more, well, interesting diets.
So what you've got today is around 60 species of piranhas, along with dozens of other species split between pacus and silver dollars, all sharing somewhat of the same body plan, but doing very different things with it. And when I say different, I really mean different.
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Chapter 2: What common misconceptions do we have about piranhas?
So basically, here you have a giant prana, or should I say megaprana, that could open turtles like a can opener, crunch through genuine megafauna femurs, which is, for the record, one of the strongest bones in the body. So, uh, good thing it's been dead for several million years. Am I right?
Now, speaking of megafauna, despite their reputation, modern piranhas don't actually attack large vertebrates regularly, with much of its negative reputation coming from records like Roosevelt's, as well as its habit of consuming dead bodies, which can make it look like the piranhas were the original killer and not just a scavenger.
And interestingly, and contrary to what you might think, studies that have actually looked at necrophagous consumption in the ecosystem, i.e. the eating of dead things, have found that it's actually various species of catfish that are responsible for most of the consumption of drowned people, not piranhas.
And if you've ever seen River Monsters, you might have seen the one episode where Jeremy Wade took red-bellied piranhas, put them in a pool, a small pool that is, proceeded to put blood and meat in said small pool, and then literally the mad lad jumped in the water with them. And guess what happened?
the end of river monsters just kidding absolutely nothing nothing at all well actually something did happen which was that they moved to the edge of the swimming pool away from the scary man thing and the fact that they did this also speaks towards another myth of these guys which is that people believe that groups or shoals of piranhas main purpose is cooperative hunting
And yet, there is actually no evidence of this at all, with the shoaling or grouping of these animals actually instead being an anti-predator behavior, not a predator behavior, with the shoaling really just being a strength in number kind of thing.
And this makes a lot of sense when you think about it, especially since, well, they're not that big, and also because their predators are no joke either, as they include caimans, giant river otters, birds, other fish, and river dolphins. And so to survive in a river full of things that will eat you, pranas, shoal. And the composition of that shoal is not random either.
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Chapter 3: How do piranhas fit into the broader family of fish known as Sarasamans?
One study exposed red-bellied pranas to a fake cormorant, which is a type of bird, in order to simulate a predator attack. And what they found was is that fish in groups of eight return to their resting breathing rate much faster than fish in just groups of two, with the smaller groups showing elevated ventilatory rates for much longer after the simulated threat.
Which, just like when you hyperventilate before having to give a public presentation, is a physiological measure of sustained stress. So basically, in a large group, the perceived risk per individual drops, and the fish can chill more quickly.
The shoal, by the way, is also socially stratified, with the older, bigger individuals occupying the central positions where exposure is lowest, while the younger, smaller little guys get pushed to the edges, i.e. the worst location, where predation risk is higher. So basically how human society works too. Just kidding, kind of.
But overall, looks like Hollywood is taking another L, as they got this pretty much exactly backwards, because that quote-unquote menacing death cloud shoal you see in movies is in reality just a giant huddle of nervous fish trying not to die.
And another wrinkle in the big eat-you-alive scary fish picture is their reproduction, which adds another behavioral layer that directly explains many piranha attacks.
Now, you have to understand, piranhas are generally seasonal breeders, with the males during the naughty season excavating bull-shaped nests in soft sediment, with the female then depositing her eggs in adhesive clumps that cling to submerged vegetation in the nest.
And during this time, the soon-to-be parents guard their starter home with all the aggressiveness you'd come to expect of piranhas, attacking pretty much anything that enters the nest zone. And yes, that includes people, mainly their feet, that are trespassing on their home.
So fun, not so fun fact, many of the documented prana bites on humans seems to be associated with defensive behavior during the breeding season, not deliberate hunting or the likes.
With one 2025 review looking at 711 documented prana bites over a 10 year period and finding that roughly 82% of them were classified as mild with single bites instead of repeated ones, which is exactly the pattern you'd expect from a defensive bite, not a feeding strike. And guess what? About 30% of attacks
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