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