Stripping Down Science (16 page)

Now Mark Tester, a plant researcher with Adelaide University,
⁵²
has discovered how to make certain plants salt-tolerant by genetically equipping them with the root equivalent of an inbuilt desalination system. The key to the discovery was a gene called HKT1;1, which makes a protein that can pump sodium, the key component of salt (sodium chloride).

‘This gene transports sodium out of cells,' explains Tester. ‘So what we've done is to discover a way to increase its expression selectively in the cells that surround the bases of the xylem vessels in the roots. Xylem are the microscopic conduits, like tiny pipes, that carry water and salts from the roots up into the plant shoots. What we have found is that when HKT1;1 activity is increased in the roots of a plant we've been using called
Arabidopsis
, the plants pump more sodium salts out of their xylem and into specialised stores within the root tissue.'

The result is that much less sodium makes its way up the plant, preventing the damage that would normally occur in the shoots and leaves. Consequently, the modified plants will grow
happily in highly salt-contaminated soil. So it works in
Arabidopsis
, which is the plant scientist's equivalent of a geneticist's fruit fly and probably tastes equally bad. But what about important food crops? Will they work too?

‘Yes!' enthuses Tester. ‘We've been able to show this same trick also works in rice, and we're currently testing cereal crops, like wheat, barley and maize.' These latter plant groups have turned out to be slightly trickier to work with because the promoter system – the DNA sequence that the team have used to turn on the sodium-pumping gene in
Arabidopsis
and in rice – does not appear to work the same way in cereals. Instead, Tester and his team have had to find an alternative way to boost HKT1;1 activity, but now they think they are within a xylem-vessel's width of it working.

If they are successful, this discovery will be a major step forward, because currently about one-third of the world's food is grown on irrigated land, one-fifth of which is now significantly affected by salinity problems. This is because ‘fresh' water in rivers and streams contains low levels of dissolved salts and minerals and when this is added to a field, the plants use the water (and some evaporates) but the salts are left
behind. This causes them to build up over time, eventually making the soil unusable.

As food demands continue to increase, coupled with the effects of climate change such as unreliable rainfall and coastal flooding, the problem is likely to become much worse. So crops that can tolerate conditions like these may be critical in an uncertain future. But are they safe?

‘We have checked these plants carefully and there is no evidence that the changes we have made are altering the accumulations of other salts or chemicals within the plant with the exception of a small change to the level of potassium,' says Tester. ‘So we're satisfied that these plants do not pose a threat.'

Beating your head against a hard surface can be a sign of frustration, yet for a woodpecker it's a fact of life …

In the late 1970s, a study carried out by Philip May, Joaquin Fuster, Jochen Haber and Ada Hirschman
⁵³
using high-speed photography (capable of taking 2000 frames a second) revealed that the impact deceleration, when a woodpecker's beak travelling at seven metres per second slams into a tree trunk, can exceed 1000 times the force of gravity (1200 g).

With repeated trauma of this magnitude, it's surprising that the bird's head remains attached to its body, never mind the risk of developing a severe headache, concussion or even brain damage. So why don't they seem to suffer any after effects? Indeed, when other small birds sustain head injuries when they accidentally fly into windows, they usually tumble to the ground and appear to be ‘knocked out' for a while before picking
themselves up and fluttering off. Why should woodpeckers be any different? The answer is that evolution has equipped them with a number of adaptations that make repeatedly banging your head against a hard surface 20 times per second slightly more tolerable.

Firstly, woodpeckers have relatively small brains which, in contrast to a human, are packed fairly tightly inside their skull cavity. This prevents the excessive movement of the brain inside the skull, which causes so-called ‘contre-coup' injuries in humans. These occur when the brain bashes into the skull following a knock on the head. In other words, the head stops, but the brain keeps on moving momentarily afterwards.

Secondly, unlike a human brain, the surface of which is thrown into ridges and folds known as ‘gyri' to enable more grey matter to be packed in, the woodpecker's brain has a smooth surface and, through its small size, a high surface area to weight ratio. This means that the impact force is spread over a much larger area, relatively speaking, compared with a human. Again, this minimises the applied trauma. The bird's brain is also bathed in relatively little cerebrospinal fluid,
which also helps to reduce the transmission of the shockwaves to the brain surface.

Finally, and possibly most importantly, the woodpecker makes sure that he minimises any side-to-side movement of his head, and this is where May and his colleagues' fast film footage comes in. The team found a tame acorn woodpecker that could be encouraged to perform for their camera if they first bashed out a few words on an old-fashioned typewriter. They watched as the bird first took aim and delivered a number of ‘test taps' before unleashing a salvo of strikes, but always in a dead straight line.

This approach is crucial because it avoids placing rotational or sheering stresses on the nerve fibres in the brain. Humans involved in car and motorcycle accidents frequently develop the symptoms of ‘diffuse axonal injury' (DAI), where sudden deceleration coupled with rotation literally twists the different parts of the brain off each other like a lid coming off a jar. By hammering in a dead straight line, woody woodpecker avoids giving himself DAI, further minimising the risk of brain damage.

An unresolved issue, however, is that the researchers noted from their photographs that
their study subject also took the precaution of closing his eyes just before each strike. But whether this was to keep wood chips out, or the eyeballs in, is anyone's guess!

In publishing the biological blockbuster that gave us evolution and rewrote the rules of ecology, naturalist Charles Darwin dwelled extensively on birds' beaks. But one of his claims – that toucans are endowed with enormous 10 inchers to attract the opposite sex – turns out to have overlooked a red-blooded effect of a very different kind. Because toucans, scientists now know, are equipped with the world's best built-in biological radiator, which is more efficient at ditching excess heat than even an elephant's ears.

Taking the relative size of a toucan into account, their beaks are up to 40 times larger, in surface area terms, than they should be. In fact, in some specimens, the beak amounts to more than 50% of the surface area of the whole animal. To put that into perspective, a human with similar proportions would have a mouth more than a metre across, much like Mick Jagger.

Although scientists had speculated that this was an adaptation to help the birds peel fruits found in their native Central and South America, or to
serve as the avian equivalent of a nuclear deterrent to ward off potential predators, Ontario-based researcher Glenn Tattersall and his colleagues
⁵⁴
wondered whether this hotly debated oversized appendage might be more to do with heat than fruit, fighting or sex.

Using infrared cameras, the team looked at thermal losses from the birds' beaks over a range of ambient conditions and compared these with the animals' core body temperatures. As the environment became warmer, they found the birds increased blood flow through their beaks, turning them into thermostatic radiators to dump excess heat and keep their body temperatures stable.

Even more cunningly, the amount of bill being used in this way could also be varied. The scientists saw that the beak radiator first turned on when the local temperature exceeded 16 degrees Celsius. At this point, initially just the part of the bill closest to the face warmed up. But as the temperature continued to soar, a progressively larger area of the beak moving away from the face was recruited, until eventually the whole thing
was pumping out heat like an infrared beacon.

The researchers suspect that the toucan achieves this by having several sets of incoming blood vessels that can be opened up in turn along the beak. By diverting blood into these vessels and then through the surface tissues of the bill, warm blood is brought close to the body surface, where it releases heat and cools down in the process. Measurements of the rate of heat loss from the birds showed that, depending upon local conditions and wind speeds, between 25% and 400% of the animal's baseline heat production can be lost from the body in this way. In comparison, an elephant's massive ears allow it to dump up to 91% of the heat it produces internally. This suggests that toucans might also use their beaks like this to ‘keep their cool' when they are flying, since this consumes large amounts of energy and involves significant muscle activity, which produces 10-to-12 times more heat than when the animal is at rest.

To find out, the researchers also filmed a 10-minute toucan test flight. At the time of take-off the bill temperature was 30 degrees, but within four minutes of becoming airborne this had begun to climb and by 10 minutes it was
37 degrees Celsius. So far from being a sex object, a toucan's bill performs a far more important function – as a hot rod!

FACT BOX

Birds

Apart from big beaks, birds also have a remarkably well developed neighbourhood watch scheme, researchers have discovered recently. Cambridge University zoologists Nick Davies and Justin Welbergen
⁵⁵
were interested in understanding how reed warblers, who are frequently targeted by cuckoos, learn to fend off the threat. They noticed that younger birds tended to be frightened off by cuckoos, probably because they have evolved to resemble hawks. Older, clearly wiser warblers, on the other hand, are much more audacious and will noisily mob an encroaching cuckoo to stop it laying an egg in their nest.

The scientists wondered whether the younger birds might be learning to recognise and react to the cuckoo by watching the reactions of their more experienced neighbours. To find out, they placed a series of fake cuckoos at a reed warbler nesting site in the Cambridgeshire fens and also played, through a concealed speaker, the cries made by reed warblers mobbing cuckoos to attract neighbouring animals. Sure enough, the local youth all turned out and perched nearby, watching how the older birds warded off the threat from the fake cuckoo.

Then, to find out how well the locals had learned from the experience, the two researchers subjected birds in neighbouring nests to the same treatment. These animals, which had previously been spectators, reacted far more vigorously to the cuckoo this time, suggesting that they had taken a leaf out of their neighbours' nest and learned to recognise a threat and how to respond to it. ‘This explains,' says Nick Davies, ‘why there are examples of birds becoming less susceptible to cuckoo
parasitism much more quickly than evolution alone would allow. It's achieved through social learning like this.'