Stripping Down Science (26 page)

For over 100 years, researchers have claimed that fingerprints are there to roughen the skin and help us to get a grip. We're also not alone in having them: koalas, which have a vested interest in holding on tight, are similarly endowed and some South American monkeys possess the equivalent of fingerprints on the tails they wrap around branches to help them hang on as they clamber about in the canopy. Arresting as this argument is, new research has pointed the finger of fate firmly at the door marked ‘myth', by showing that fingerprints definitely don't do for dabs what crampons do for climbers!

Having previously got to grips with the physics of fingernails and why they don't split along their lengths (it's because they consist of a laminated three-layered sandwich of keratin proteins, the middle layer of which runs side-to-side across the nail, stopping cracks from propagating wristward), Manchester University researcher
Roland Ennos
⁹²
decided next to grapple with the real role of fingerprints.

Working with his student Peter Warman, the duo designed a ‘finger-frictionometer' to measure how much friction the pad of a finger could apply to a sheet of perspex being pulled vertically past it. To simulate different grip strengths or applied pressures, weights were used to accurately alter how hard a lightly clamped finger was pressed against the perspex sheet. Fingerprint impressions were also made in each case in order to measure the area of skin in contact with the surface.

The results convincingly show that fingerprints don't provide a traction boost between finger and surface. This is because, Ennos and Warman found, the skin on the pads of the fingers behaves in a similar way to a rubber ball being dragged across a surface. Unlike ‘hard' substances such as rocks, which generate friction when ridge-like irregularities on the surfaces jam together, rubber contains long chains of molecules which form short-lived electrical interactions – called van der Waals attractions – with any surface they touch. So the friction felt when rubber is dragged
across a surface occurs because the existing van der Waals attractions have to be broken and new ones made.

This means that, for rubber, the greater the area in contact with a surface, the more powerful the frictional effect. But rubber also deforms when force is applied, which reduces the amount of contact, therefore cutting down the friction, and this is exactly what Ennos and Warman discovered. They also found from their fingerprint impressions that the ridge pattern reduces the contact area of the finger by 33%. So, given that the finger behaves like rubber and that the greater the contact area the greater the grip, if getting a better hold on things is the aim, then it makes no sense whatsoever to reduce the surface area further with a fingerprint.

But if they're not for tightening our grip on things, what are fingerprints for? Some have speculated that the ridges are there for the same reason that tyres have a tread pattern: to channel water away from the contact areas in order to improve grip in the wet. It's also possible that the ridges, which contain vibration-sensing nerve endings, could help to improve the perception of surface textures, although this doesn't explain
why parts of the body like the palms and feet, which aren't used for touch discrimination, nonetheless still have ridges.

Ennos' preferred explanation is that the ridges and folds form a clever anti-blister system. ‘The pattern will allow our skin to have much greater compliance and that can help to reduce the sheer stresses around the edge of the contact zone,' he says. ‘If you ever do DIY tasks, what you tend to find is that the only bits where you get blisters are the bits, not on your fingerprints or where the big patterns are on your palms, but in areas where there aren't any prints.'

In other words, the ridges and folds provide a store of skin that can be stretched out easily like an elastic band to soak up sudden stresses. Unfortunately, however, the system isn't infallible and hammers and thumbs still frequently come to blows!

Most people believe that, in common with the reptiles around today, dinosaurs were cold-blooded brutes that warmed themselves up with the help of the sun. But some hot new research suggests this may be a metabolic myth as massive as some of the dinosaurs were themselves, because scientists have found that, without being warm-blooded, many of these so-called terrible lizards would have moved more slowly than a tectonic plate!

Herman Pontzer, a scientist from Washington University in America, together with John Hutchinson and Vivian Allen from the Royal Veterinary College in London,
⁹³
made the discovery by using two different scientific approaches to reconstruct the metabolic rates of 14 different dinosaur species ranging from the towering
Tyrannosaurus rex
, right down to the tiny
Archaeopteryx
. Their first method was to take advantage of a relationship spotted
previously by Pontzer amongst animals around today. When he looked at 28 different land-living creatures, including 11 different mammals, eight birds and five different reptiles, he found that regardless of species, merely by measuring the height of an animal's hip joints above the ground, you can predict with 98% accuracy how much energy they would need to burn to get about.

Suspecting that the same should be true for
T. rex
, Pontzer and his colleagues calculated the hip heights of their dinosaurs and then tweaked the formula slightly to make it more dino-friendly, largely by taking into account differences in posture between modern and ancient species. They then multiplied by one of two speeds, fast or slow, and then by the weights of the animals, based on fossil evidence, to arrive at an estimate for the energy costs of movement for each of the 14 dinosaurs.

To back up these estimates, the researchers then used a second approach, this time based on how much muscle would be needed for each of the dinosaurs to move. This was much more complicated, because it involved modelling how each of the different dinosaurs walked and ran in order to work out how each muscle group would
have worked and how much energy it would have consumed. Again, this relied both on fossil evidence, looking at where muscles would have attached to bones, and the muscle mechanics of modern species. Encouragingly, this second set of results agreed very well with the results of the first hip-to-height-based approach.

But the palaeontological problem the researchers then ran into was that the energy requirements they had calculated were nowhere near what a cold-blooded metabolism could muster. Tests on modern-day reptiles running on treadmills, from alligators to iguanas, reveal that these species can only generate energy at the rate of about one-tenth of that achieved by warm-blooded birds and mammals. This means that even for a modest-sized dinosaur, anything more than a sluggish stroll would have been out of the question. In energy terms, the effect would be like trying to light a town with a torch.

So, Pontzer argues, these dinosaurs must have been endotherms (hot-blooded), which is the only way they could have managed to make energy quickly enough to keep them moving. That, or they only went in for sudden sprints of activity followed by a very long recovery period. But this
is unlikely, because if something bigger, hungrier and faster came along while they were recovering, they'd be toast – or, at the very least, lunch!

A big question, though, is at what point in their evolution did dinosaurs turn up the thermostat in favour of hot living? Surprisingly, Pontzer thinks they may have started out that way. The 14 examples he examined are positioned in various places – including at the bottom – of the dinosaur evolutionary tree, at the top of which are perched the warm-blooded birds we see around us today.

So, rather than cold-hearted killers, this discovery suggests that being warm-blooded may well have been the ancestral situation for dinosaurs, bringing with it the advantage of being more agile, faster and potentially quicker-witted than their cooler reptilian cousins. This might also go some way to explaining the massive evolutionary success that these creatures enjoyed during the almost 200 million years they ruled the earth.

FACT BOX

New secrets other than just skeletal structure emerging from fossils

As well as helping palaeontologists to answer important questions about whether or not dinosaurs were warm-blooded, their fossil remains are beginning to relinquish other scientific secrets that had previously been overlooked.

In one extraordinary recent example, Chinese and American researchers managed to reconstruct from fossil remains the real pigmented appearance of a bird-like, feathered dinosaur known as a ‘troodontid', which lived about 150 million years ago during the late Jurassic period. By putting the fossilised remains of the ancient animal under an electron microscope, Quanguo Li, from the Beijing Museum of Natural History,
⁹⁴
was able to pick out the impressions of minute
structures, measuring just one-thousandth of a millimetre across, called melanosomes.

These tiny bodies are also found in the feathers of modern birds and contain the pigment melanin, the same stuff that colours human hair and skin. Different coloured melanosomes are slightly different shapes, so by looking at the colours of similarly structured melanosomes in birds around now, Li was able to work out the likely colour scheme for each of the feathered regions of the troodontid dinosaur. Consequently, scientists can now paint a truly accurate picture of what this beast would have looked like: a dark grey body with a reddish-speckled face, reddish crown and long white legs with distal black spangles.

Another major palaeontological leap forward in recent years has been the announcement by scientists that they have also managed to extract genuine
T. rex
tissue from a fossil, casting doubt on the claim that fossils are just reptile-shaped lumps of rock. This discovery was stumbled upon by North Carolina State
University scientist Mary Schweitzer
⁹⁵
in what she described initially as ‘a lucky accident'. She had placed fossil fragments into a solution to remove some of the minerals, but forgot about them and left the samples soaking for far longer than she had intended. But rather than dissolving like a tooth in cola, the samples yielded something far more exciting: once the minerals had gone, what remained was a matrix of fibrous material resembling the connective tissue scaffold that binds bone together.

To find out whether it really was dinosaur tissue, Schweitzer and her colleagues used electron microscopy when looking for the characteristic stripy pattern of collagen, one of the major bone connective tissue proteins. Having spotted what they were looking for, they then used antibodies programmed to recognise collagen, and these bound to the tissue too. Finally, to confirm that it really was collagen, the team used a mass spectrometer
to work out the chemical sequence of the amino-acid building blocks from which proteins are made. When they compared the protein sequences to modern animals, they found a close match with chickens, frogs and newts, some of the dinosaurs' closest living descendants.

‘This similarity to chicken is definitely what we would expect, given the relationship between modern birds and dinosaurs,' says Schweitzer. ‘From a palaeo standpoint, sequence data really is the nail in the coffin that confirms the preservation of these tissues.'

This suggests that fossils may be more than just a rocky replacement for the real thing. In fact, there may indeed be parts of the real thing still lurking inside. That said, some scientists have publicly expressed doubts about Mary Schweitzer's results, refusing to believe that tissues can survive intact for such a long time. On this occasion, however, this is probably one instance where time really will tell!