Sharks are, by any definition, incredibly badass. They’re the only animals around that can claim to have chowed down on honest-to-goodness dinosaurs; they induce what is genuinely known to biologists as a “landscape of fear” in the places they inhabit – even if a “seascape” might be more accurate – and they do it all to a backing track of heavy metal.
Which makes it all the stranger that they’re so easy to incapacitate. Seriously: all you have to do to win against a shark is flip ‘em upside down, and you’ve basically hacked their mainframe – they enter a state of what’s called tonic immobility, in which they are, for all intents and purposes, playing dead.
And here’s the extra weird part: nobody knows why.
Playing dead
In evolutionary psychology, people often talk about the so-called “four Fs”: fight, flight, feed, and – ahem – mate. They’re the most basic drives an animal can have, and they often come into play when we’re at our most stressed – it’s why mama honey badgers will face down prides of lions without batting an eye, and why the neighborhood kitties skedaddle so quickly when you’re just trying to say hi.
It’s also, for a handful of animals, why stressful situations make them kinda… break. A physical bluescreen, if you will. A good example is the possum – in fact, the technique is often literally called “playing possum” for the species’ tendency to keel over in the face of danger – or the ever-entertaining fainting goat, so named for its species (goat) and its most notable activity (fainting).
On its face, of course, suddenly freezing up and all but losing consciousness seems like a terrible survival mechanism – and indeed, tonic immobility is “one of the least-understood behaviors across the animal kingdom,” note Jodie Rummer and Joel Gayford, respectively a professor and PhD student in the department of marine biology at James Cook University, in a new paper studying the phenomenon in sharks.
“The prevailing hypothesis for the adaptive value of TI [tonic immobility] posits that it is a defensive tactic against predation,” they explain – and this idea does at least have some empirical evidence to support it. In a couple of experiments from the early 1980s, for example, and mind that puts us before the era of robust ethics for animal experimentation, domestic cats were introduced into an arena containing two quail. When one bird was immobile, the cat almost always ignored it in favor of its mobile comrade: “After TI was induced in a bird, the cat would leave it to stalk the remaining moving bird,” the researchers reported at the time. “A bird would be re-attacked if it came out of TI during a trial. In one case, a bird was re-attacked, killed, and partially eaten after it came out of TI and moved while the second bird was still in TI.”
As a hypothesis, then, it does kind of hold water – but there’s just one problem. Sharks are very rarely the metaphorical quail in their interactions with other species – so why would they experience tonic immobility?
Scaredy sharks
If you had to take a stab at which of the four Fs a shark would go for in extremis, you’d probably think “fight”, right? After all, they’re famously big, toothy, and pugnacious; not for them would be the fawning or fleeing instinct. So it’s kind of unexpected, then, that they’re so easy to switch off.
“In sharks, rays and their relatives […] tonic immobility is triggered when the animal is turned upside down,” Rummer and Gayford wrote in a recent article for The Conversation.
“It stops moving, its muscles relax, and it enters a trance-like state,” they explained. “Some scientists even use tonic immobility as a technique to safely handle certain shark species.”
Scientists flip over a sawfish to be able to work on it in the water.
Image credit: NOAA
It is, to use a technical term, weird. There are a few things that prey on sharks, but playing dead wouldn’t help them escape – in fact, it’s an active hindrance to the sharks’ survival, as orcas are known to exploit their weakness to being flipped upside down, immobilizing them before ripping out their delicious livers. Other proposed reasons for tonic immobility fall short, too: it’s not related to reproduction, despite occasionally being seen during mating, since the effect doesn’t differ between males and females – and, in any case, “remaining immobile could make females vulnerable to harmful or forced mating events,” the two researchers pointed out.
So what could the explanation be? To find out, Rummer and Gayford went holistic: they recruited 47 sharks from 14 different species and attempted to induce tonic immobility in each of them. They included sharks from different habitats; diverse sizes; even different steps up and down the food chain – if this strange catatonic state had developed for any useful reason, surely such a wide net would reveal it.
Instead, the answer appears to be the precise opposite. Rather than developing for a reason, tonic immobility in sharks seems to have hung around long past its usefulness.
A bad habit
It turns out not all tonic immobility is created equal. Some sharks experience it for a long time; some take longer for it to set in. Some species don’t experience it at all. And, as the researchers discovered, predicting which species does what is… basically impossible.
“No major intraspecific differences in [the response] were observed,” the paper notes. Phylogenetic logistic regression – a statistical technique the pair had previously used to confirm sharks’ geometric perfection – found no connection to either shark size or place on the food chain; similarly, the pair “recovered no evidence of significant relationships” between a shark’s immobility response and its home habitat.
In total, “our results suggest that neither depth ranges, body size, biogeography, nor habitat preferences can satisfactorily predict the phylogenetic distribution of [tonic immobility] in Chondrichthyes,” the paper summarizes. So where does it come from?
The answer, they suggest, is simple: it’s just a leftover evolutionary relic from times long past.
It’s an explanation that would please William of Ockham. After all, it’s easier for a few branches of a growing evolutionary tree to forget a behavior – tonic immobility “has been independently lost at least five times through chondrichthyan phylogeny,” the paper concludes; “at least once within each of Chimaeriformes, Myliobatiformes, Hexanchiformes, Orectolobiformes, and Carcharhiniformes” – than it would be for every other branch to independently invent it.
And it explains why shark species for whom tonic immobility would be a real danger – for example, those who live in reefs or shallow waters – don’t exhibit it, while deep-water species who would rarely be flipped in the wild still have it. “In the absence of any significant fitness benefits or costs, there is no particular reason why [tonic immobility] should be lost in other lineages,” the paper explains.
Now, it’s still a hypothesis, and the pair stresses that further research is needed to confirm it more strongly. But the explanation… well, it makes sense, in a way that previous suggestions can’t seem to manage. Essentially, it seems, sharks can’t fight back when flipped upside down for the same reason you can’t do complex statistics in your head: because a hundred million years ago, their ancestors never needed to – and hey, we’ve survived this long, haven’t we?
“Rather than a clever survival tactic, tonic immobility might just be ‘evolutionary baggage’,” the pair concluded. “A behavior that once served a purpose, but now persists in some species simply because it doesn’t do enough harm to be selected against.”
“It’s a good reminder that not every trait in nature is adaptive,” they wrote. “Some are just historical quirks.”
The study is published in Reviews in Fish Biology and Fisheries.
Source Link: When You Hack A Shark, You're Exploiting A Glitch Billions Of Years In The Making
