Category: Sharks

Weapons made from shark teeth are completely badass, and hint at lost shark diversity

By Ed Yong | August 13, 2012 11:32 am

I was at the Ecological Society of America’s Annual Meeting when I saw this tweet:

As you might imagine, I did check out that talk.

For those of you who are wondering how you weaponise shark teeth, which are already regenerating, serrated meat knives at the business end of a streamlined, electric-sensing torpedo, here’s how. You drill a tiny hole in them, and then bind them in long rows to a piece of wood to make a sword. Or a trident. Or a four-metre-long lance. And then, presumably, you hit people really hard with them.

That’s what the people of the Gilbert Islands have been doing for centuries. Sharks are an ingrained part of their culture and their teeth have been an ingrained part of their weapons. Tiger sharks feature heavily – they have thick, cleaver-like teeth that can slice through turtle shells so they make a good cutting edge. But the weapons also include the teeth from spottail, dusky and bignose sharks (you can identify species from their teeth), and none of these actually live around the Gilbert Islands today.

Drew, who studied 124 of these weapons, says that their teeth reveal a “shadow diversity” – traces of sharks that disappeared from the surrounding waters before we even knew they were there. I wrote about this story for Nature News – head over there for the full details.

CATEGORIZED UNDER: Conservation, Environment, Fish, Sharks

How the sawfish wields its saw… like a swordsman

By Ed Yong | March 5, 2012 12:00 pm

If you ever saw a sawfish, you might wonder if someone had taped a chainsaw to the body of a shark. The seven species of sawfish are some of the wackier results of evolution. They all wield a distinctive saw or ‘rostrum’, lined with two rows of sharp, outward-pointing ‘teeth’. But what’s the saw for?

Barbara Wueringer has an answer: the saws are both trackers and weapons. They’re studded with small pores that allow the sawfish to sense the minute electrical fields produced by living things. Even in murky water, their prey cannot hide. Once the sawfish has found its target, it uses the ‘saw’ like a swordsman. It slashes at its victim with fast sideways swipes, either stunning it or impaling it upon the teeth. Sometimes, the slashes are powerful enough to cut a fish in half. Even less dramatic blows can knock a fish to the sea floor, and the sawfish pins it in place with its saw.

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Sharks gone walkabout – how Australian great whites ended up in the Mediterranean

By Ed Yong | November 16, 2010 7:00 pm

Great_white_shark

In the 18th century, Europe started sending boatloads of white settlers to Australia. But unbeknownst to these colonists, Australia had sent its own white contingent to set up colonies in Europe, around 450,000 years earlier. These migrants were sharks – great white sharks.

When Chrysoula Gubili from the University of Aberdeen compared the DNA of white sharks from around the world, she found a big surprise. The great white is the most genetically diverse shark studied so far but the Mediterranean fish are only distantly related to nearby populations in the North-West Atlantic, or even in South Africa. Their closest kin actually live half a world away in the Indo-Pacific waters of Australia and New Zealand.

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Seals do it with whiskers, sharks do it with noses – tracking fish with supersenses

By Ed Yong | June 10, 2010 7:00 pm

Harbour_seal

Predators that swim after fish all have to accurately track the movements of fast-moving prey, often in murky waters. Different groups accomplish this feat with different abilities – sharks use their keen sense of smell, while seals depend on touch, thanks to their long, sensitive whiskers. Now, two new studies reveal just how good these supersenses are.

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MORE ABOUT: nose, seal, shark, whiskers

Pocket Science – a nursery for giant sharks, and why mum's voice is a good as a hug

By Ed Yong | May 11, 2010 7:26 pm

Not Exactly Pocket Science is a set of shorter write-ups on new stories with links to more detailed takes by the world’s best journalists and bloggers. It is meant to complement the usual fare of detailed pieces that are typical for this blog.

PhonecallMum’s voice as good as a hug

For many children facing times of stress and worry, there are few things more soothing than a hug or a kind word from mum. Peer into a child’s bloodstream, and you can see this comforting effect manifesting itself at in the molecules that pass by. A simple hug can release hormones that combat stress and strengthen the bond between parent and child but according to a new study, this effect can happen without any contact. The mere sound of a mother’s voice, channelled down a phone line, can trigger the same biochemical changes in a distressed child.

Leslie Seltzer from the University of Wisconsin-Madison asked 61 girls, aged 7-12, to complete a series of exercises involving public speaking and maths challenges, all done in front of an audience. Even many adults would find this stressful and the children were no different. Their saliva betrayed higher levels of the stress hormone cortisol. If mum stepped in, these levels were back to normal within an hour, regardless of whether she was allowed to hug her child or merely to speak to her down the line. Either way, the comforted children also showed raised levels of oxytocin, a hormonal jack-of-all-traders with roles in solidifying social bonds, controlling stress and even, it’s said, love and trust.

Studies in mice have suggested that only touch can release oxytocin, but Seltzer’s work suggests that in humans, a voice can have the same effect. A gentle word from human creates the biochemical equivalent of a supportive nuzzle from a mouse. So far, Seltzer has only looked at communication between mothers and young daughters. It will be interesting to see if a comforting voice can have the same effect on boys or adults, or if it’s delivered by a father, a friend or even a sympathetic stranger.

Reference: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.0567

A nursery for Megalodon, the world’s largest shark

Megalodon was the largest shark of all time: 16 metres in length, 50-100 tonnes in weight, and possessing the strongest bite of any animal. But like every other animal, Megalodon grew from humble and vulnerable beginnings. A new set of fossil teeth suggest that it used a strategy that its living relatives share – baby sharks huddled in shallow waters where they found not only ample food, but also shelter from predators (including, mostly, other sharks). Today, a set of 21 teeth recovered from Panama’s Gatun formation have revealed the clearest evidence yet that the world’s greatest shark used such a nursery.

Catalina Pimiento from the University of Florida uncovered the teeth and confidently classified them as Megalodon chompers based on their shape. However, all of them were surprisingly small. They compared the teeth to other Megalodon specimens from younger and older rock formations to show that the animal wasn’t evolving towards a smaller size as the millennia ticked past. They compared the individual teeth to full sets to make sure that they weren’t just looking at the smaller rear teeth from larger sharks.

Instead, Pimiento says that the teeth belonged to juvenile sharks. Using a model based on great whites, she estimated the length of each tooth’s owner, and found that they fell within the length estimates for juvenile sharks. Even as a newborn, Megalodon could have reached 2 metres in length. That’s still pretty sizeable, but small enough to make a meal for other mega-sharks, including the great hammerhead, the snaggletooth and other adult Megalodons. Today, other sharks including Megalodon’s closest relative, the great white, uses similar nurseries, where pregnant females swim in the company of newborns, who stay there for their first weeks, months or years of life. It’s a strategy that has apparently been around for at least 10 million years.

Megalodon-teeth

Reference: PLoS ONE http://dx.doi.org/10.1371/journal.pone.0010552

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Pocket Science – a psychopath's reward, and the mystery of the shark-bitten fossil poo

By Ed Yong | March 15, 2010 8:30 am

Not Exactly Pocket Science is a set of shorter write-ups on new stories with links to more detailed takes by the world’s best journalists and bloggers. It is meant to complement the usual fare of detailed pieces that are typical for this blog.

The rewarding side of being a psychopath

Nucleus_accumbens_psychopat.jpgWhat goes on in the brains of psychopaths? They can seem outwardly normal and even charming, but tthese people typically show a lack of empathy, immoral behaviour and an impulsive streak. Joshua Buckholtz found that the last of these traits – impulsivity – may stem from a hyperactive reward system in the brain and unusually high levels of the signalling chemical dopamine.

When given small doses of amphetamines, people who come out as more impulsive on tests of psychopathy also released more dopamine in a part of their brain called the nucleus accumbens. This region plays many roles in feelings of reward, pleasure and addiction. This link between it and the impulsive side of psychopathy remained even after adjusting for other personality traits. Even the prospect of winning money, as opposed to a physical drug, triggered a hyperactive response from the nucleus accumbens.

When a psychopath imagines a future reward, the explosion of dopamine in their brain provides them with incredible motivation to get that reward. This extra motivation could underlie the increased drug use and the impulsive streaks that accompany the condition. It could even explain some of the antisocial behaviour – dopamine’s most familiar as a chemical linked to feelings of reward and pleasure but studies in mice suggest that its presence in the nucleus accumbens is vital for aggression.

Previous research in this area has focused on the emotionally cold side of psychopathy, which may stem from problems in other parts of the brain like the amygdala, involved in emotions, and the ventromedial prefrontal cortex (vmPFC), involved in fear and risk. The impulsive side of the disorder has typically been overlooked but it predicts many of the problems associated with psychopathy, including drug abuse and violent criminal behaviour.

Reference: Nature Neuroscience http://dx.doi.org/10.1038/nn.2510

Image by Gregory R.Samanez-Larkin and Joshua W. Buckholtz

Why did the shark bite the poo?

Coprolite.jpgThe specimen on the right is a most unusual one. It’s a coprolite, a piece of fossilised dung. That’s not unique in itself; such specimens are often found and they tell us a lot about what extinct animals ate. But this one has a line of grooves running down its middle. They were made by a shark.

Stephen Godfrey and Joshua Smith found two such specimens in Maryland’s Chesapeake Bay. The identity of the coprolites’ maker is a mystery, but its chemical composition suggests that they were excreted by a meat-eating vertebrate. The identity of the biter is clearer. The duo poured liquid rubber into the grooves to make a model cast of the teeth that made them. These model teeth made it clear that the biter was a shark and the duo even managed to narrow its identity down to one of two species -a tiger shark, or Physogaleus, a close extinct relative.

Why would a shark bite a piece of dung? Tiger sharks are notorious for their ability to eat just about anything, but obviously, neither piece of dung was actually swallowed. No known shark eats poo for a living. The shark may have had an exploratory bite and didn’t like what they tasted. But Godfrey and Smith’s favourite explanation is that the bites were the result of collateral damage – the shark attacked an animal and during its assault, it happened to bite through the bowels. These specimens are the enduring remains of a battle between two predators, as suggested by this wonderful drawing in the paper by T Schierer of the Calvert Marine Museum.

Shark-vs-crocodile.jpg

Reference: Godfrey, S., & Smith, J. (2010). Shark-bitten vertebrate coprolites from the Miocene of Maryland Naturwissenschaften DOI: 10.1007/s00114-010-0659-x

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Widely set eyes give hammerhead sharks exceptional binocular vision

By Ed Yong | November 27, 2009 6:15 am

The hammerhead shark’s head is one of the strangest in the animal world. The flattened hammer, known as a ‘cephalofoil’, looks plain bizarre on the face of an otherwise streamlined fish, and its purpose is still the subject of debate. Is it an organic metal detector that allows the shark to sweep large swathes of ocean floor with its electricity-detecting ability? Is it a spoiler that provides the shark with extra lift as it swims? All of these theories hypotheses might be true , but Michelle McComb from Florida Atlantic University has confirmed at least one other -the hammer gives the shark excellent binocular vision.

Depending on who you believe, there are anywhere from 8-10 species of hammerheads, whose cephalofoils all have different degrees of exaggeration. The bonnethead (Sphyrna tiburo) has a shape that’s more spade than hammer. The scalloped hammerhead (Sphyrna lewini) has a more familiar racing-car-spoiler shape. And the winghead shark (Eusphyra blochii) has the most elongated head of all, up to 50% of its entire body length.

She compared all of these flattened visages with those of two more typical sharks – the lemon and the blacknose. McComb collected her hammerheads by fishing for them off the coast of Hawaii and Florida and housed them in local tanks. She tested their eyes by sweeping arcs of light across them and measuring their responses using electrodes.

She found that hammerhead eyes, though far apart, have the greatest overlap in their fields of view. The winghead shark has a 48 degree arc in front of it that’s covered by both eyes, which must give it exceptional depth perception. By comparison, the scalloped hammerhead has a binocular overlap of 34 degrees, the bonnethead has a much smaller one of 13 degrees, and the lemon and blacknose sharks have the smallest of al with 10 and 11 degrees respectively.

And that’s if the sharks swim straight ahead with their heads completely still. A hammerhead can improve its stereoscopic vision even further by rotating its eyes and sweeping its head from side to side. McComb measured these movements too by filming the sharks swimming around their tanks.

Taking these movements into account, she found that the binocular overlaps of the scalloped hammerhead and bonnethead increase to a substantial 69 and 52 degrees respectively, still outclassing the 44 and 48 degree arcs of the pointy-headed species. The hammerhead species even have visual fields that overlap behind them, giving them a full 360 degree view of the world.

You might think that these visual fields would only overlap some distance ahead of the hammerheads, but not so – their eyes are angled slightly forwards so that ahead of them, their blind spots are just as small as those of sharks with narrower eye-spans. Their main weaknesses are substantial blind spots above and below their heads. Indeed, McComb says that there are some anecdotal reports of small fish giving them the slip by swimming into these regions above and below the hammer.

McComb’s results settle a long-standing debate. In 1942, some scientists have suggested that the hammerhead’s eyes are so far apart that their visual fields couldn’t possibly overlap. Others have argued that the wide spacing actually improves their binocular vision. Despite over 50 years of argument, this is the first study to actually do some measurements with real sharks, and it shows that their binocular vision is indeed improved by their odd heads.

Reference: Journal of Experimental Biology doi:10.1242/jeb.032615

More on sharks:

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CATEGORIZED UNDER: Animal behaviour, Animals, Fish, Sharks

Male and female mako sharks separated by invisible line in the sea

By Ed Yong | February 24, 2009 7:00 pm

Blogging on Peer-Reviewed ResearchIn the middle of the Pacific Ocean, Gonzalo Mucientes has discovered an invisible line in the sea that separates male mako sharks from females. The line runs from north to south with the Pitcairn Islands to its west and Easter Island to its east. On the western side, a fisherman that snags a mako will most probably have caught a male. Travel 10 degrees of longitude east and odds are they’d catch a female. This is a shark that takes segregation of the sexes to new heights.

Mucientes and colleagues from Spain, Portugal and the UK spent four months aboard a Spanish longline fishing vessel.  Amid more typical catches, the boat often snagged shortfin makos and blue sharks. When they did, the researchers meticulously noted the boat’s position, and analysed the shark carcasses on board.

Their results showed a clear “line in the sea”. All in all, the fishermen captured 264 male makos, mostly towards the west of the line and 132 females predominantly in the east. Makos are found all over the world, but this study shows that at a more regional level, their populations are structured to an astonishing degree. 

This segregation is even more surprising when you consider that makos are the world’s fastest sharks. They can clock speeds of up to 45 miles per hour, about eight times as fast as Michael Phelps at his peak. They really shouldn’t have any problems in covering vast tracts of water.

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CATEGORIZED UNDER: Conservation, Ecology, Fish, Fishing, Sharks

Prehistoric great white shark had strongest bite in history

By Ed Yong | August 5, 2008 8:00 am

Blogging on Peer-Reviewed ResearchThanks to Hollywood, the jaws of the great white shark may be the most famous in the animal kingdom. But despite its presence in film posters, the great white’s toothy mouth has received very little experimental attention. Now, Stephen Wroe from the University of New South Wales has put the great white’s skull through a digital crash-test, to work out just how powerful its bite was.

Megalodon.jpg A medium-sized great white, 2.5m in length and weighing in at 240kg, could bite with a force of 0.3 tonnes. But the largest individuals can exert a massive 1.8 tonnes with their jaws, giving them one of the most powerful bites of any living animal.

The jaws exert over three times more force than the 560kg exerted by a large lion, and 20 times more than the 80kg a feeble human jawbone can manage.

Impressive as the great white shark is, one of its extinct ancestors was even more so. Megalodon (aka the megatooth shark aka Carcharadon megalodon), was a monster that may have grown to 16 metres in length and had a maximum weight of anywhere from 50 to 100 tonnes. And according to Wroe’s research, it had the most powerful bite of any animal.

A single chomp could exert up to 18 tonnes of force; even the mighty Tyrannosaurus rex could only muster 3 tonnes of force. Being bitten by a megalodon would be like having three African elephants pressing on top of you with carving knives strapped to their feet. It truly was “one of the most powerful predators in history”.

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How sharks, penguins and bacteria find food in the big, wide ocean

By Ed Yong | March 11, 2008 8:53 pm

Blogging on Peer-Reviewed Research

Some of us have enough trouble finding the food we want among the ordered aisles of a supermarket. Now imagine that the supermarket itself is in the middle of a vast, featureless wasteland and is constantly on the move, and you begin to appreciate the challenges faced by animals in the open ocean.

Basking-Shark.jpg

Thriving habitats like coral reefs may present the photogenic face of the sea, but most of the world’s oceans are wide expanses of emptiness. In these aquatic deserts, all life faces the same challenge: how to find enough food. Now, a couple of interesting studies have shed new light on the tactics used by predators as large as sharks and as small as bacteria.

Big fish…

At a large scale, predators like sharks and tuna rely on chemical cues to give away the location of their prey. Sharks are particularly expert trackers, but powerful though their super-senses are, they can only come into play within a certain range. Over the large distances of the open ocean, they are more like blind hunters, hoping to stumble across some telltale sign of food.

David Sims from the UK’s Marine Biological Association found that many large marine predators use a search strategy called a ‘Levy walk’, although in this case it’s more of a swim. The strategy is formally described by a mathematical equation, but in simple terms, it means that an animal makes several short moves in its search for food, interspersed with a few long ones. The longer the ‘step’, the more infrequent they are.

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