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

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

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What goes on in the brains of psychopaths? They can seem outwardly normal and even charming, but these 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?

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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.

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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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