Unit 10. Animal adaptations

Script 25. Radiation and evolution

Surviving fallout

Birds can evolve to cope with the lingering effects of nuclear incidents.

The disaster last year at the Fukushima Dai-ichi nuclear power plant, caused by an earthquake and tsunami, scored seven on the International Nuclear and Radiological Event Scale (INES). No worse rating exists. Radiation is harmful to living things, yet the long-term effects of persistently high levels of background radiation on ecosystems are poorly understood. With this in mind, a team led by Timothy Mousseau of the University of South Carolina and Anders Moller of the University of Paris-Sud set out to compare bird species dwelling near the Fukushima plant with those living at the site of another nuclear incident that scored a seven on the INES: the Ukrainian town of Chemobyl, where disaster struck in 1986. Remarkably, they found that some species seem to develop a tolerance for radioactivity over time.

Fukushima and Chernobyl are more than 7,000 km (4,350 miles) apart, but Dr Mousseau and his colleagues soon realized that the two sites had much in common. Both are in areas that have a temperate climate with species that have similar habits and needs. And both are surrounded by a mixture of farmland and forest. Upon closer examination the researchers found that 14 species of bird lived in both regions, including the barn swallow, great tit, great reed warbler, buzzard and Eurasian jay. With so many similarities between the two places, a comparison of the biological responses to radiation in each (recent in Fukushima; long-term in Chernobyl) would surely be illuminating.

To do this, during July 2011, the researchers counted and identified birds at 300 locations near Fukushima that had radiation levels as low as 0.5 microsieverts per hour and as high as 35 (for comparison, dental X-rays rarely expose patients to more than 0.05 microsieverts). Then they compared these results to bird data collected in areas that had the same range of radiation levels near Chernobyl between 2006 and 2009.

Their results, published in Environmental Pollution, show that as radiation levels in an area rose to 35 microsieverts per hour, the average number of birds dropped by almost a third compared with the areas where radiation levels were only 0.5 microsieverts per hour. This makes sense: in those areas with a high level of radiation, living things would tend to die or sicken and fail to reproduce. However, when researchers looked at the 14 bird species that lived in both regions, they found that the same level of radiation was associated with twice as large a drop in bird numbers in Fukushima as in Chernobyl.

The reasons for this are not clear. It is possible that the composition of the radionuclides are proving more dangerous to the Fukushima birds than they are to the birds near Chernobyl. But Dr Mousseau suggests a more likely explanation is that evolution has already been at work near Chernobyl, killing off individual birds that cannot cope with the background radiation and allowing the genes of those that have some tolerance to be passed on. The birds at Fukushima are only beginning to face the evolutionary challenge of living in a radioactive world. (From The Economist, March 03, 2012)

Script 26. Evolution

Something glowing on

The nuclear accident at Chernobyl has created a natural laboratory.

Catching evolution in action is hard. The best-known examples are those where human action, in the form of pesticides, herbicides or drugs, has intentionally made the world a nastier place for some specific group of creatures, and natural selection has pushed back to create resistance. But a group led by Timothy Mousseau of the University of South Carolina, Anders Moller of the CNRS, in France, and Ismael Galván of the Doñana Research Station in Spain has now, in a paper in Functional Ecology, provided an example of selection responding to a human action that was most definitely unintentional: the explosion and fire at a nuclear reactor in Chernobyl, Ukraine, 28 years ago.

Dr Mousseau and Dr Moller knew from previous work that birds living near Chernobyl have better survival rates than those living near Fukushima, in Japan, where a serious reactor accident happened in 2011. They suspected that was because the Ukrainian birds had had time to evolve resistance. They therefore sent blood and feather samples from 120 birds of 13 species they collected from both high- and low-radiation regions around the defunct reactor at Chernobyl to Dr Galván, who looked for genetic damage in them and also analysed their levels of glutathione, an antioxidant that mops up highly reactive (and therefore harmful) molecules created when radiation hits biological tissues.

In those birds taken from low-radiation zones the average concentration of glutathione was 450 micrograms per gram of body mass; in high-radiation areas it was 725 micrograms per gram. Moreover, the higher a bird’s glutathione level, the lower the amount of genetic damage Dr Galván could spot in its cells. Birds in high-radiation zones, then, seem to have evolved to deal with the threat, just as Darwin would have predicted. (From The Economist, May 03, 2014)

Script 27. Paleontology

Four wings, good. Two wings, better

Why Microraptor became extinct.

It is tempting to think of the process of evolution as one of continuous, stately progress towards better-designed organisms. In fact, it is full of blind alleys - as a fossil called Microraptor shows.

At some point during the Jurassic period, a group of dinosaurs evolved feathers. These may have been for warmth, for display or for both. They were certainly not, at the beginning, for flight. But in one line of these dinosaurs evolution modified them for precisely that purpose. This modification led, via Archaeopteryx, a species that lived 150m years ago, to birds.

Feathered dinosaurs did not go away just because they had spun off the birds, though. They persisted right up until the end of the Cretaceous period, 65m years ago, when all (non-avian) dinosaurs met their ends. And well before that, in a part of the Cretaceous called the Aptian, 120m years ago, the whole thing happened again, in the form of Microraptor. Except that this time, the flying animal in question had four wings, rather than two.

Microraptor's existence raises two questions: exactly how did it deploy its four wings, and why does it have no living descendants? The answers, according to Michael Habib and Justin Hall of the University of Southern California, are linked.

Until recently, there were two schools of thought about how Microraptor carried its wings when in flight. One school proposed that it looked like a biplane, holding one set of wings above the other when it flew. The second school suggested that all four wings were coplanar. Dr Habib and Mr Hall, however, think both are wrong.

They see a division of labour between the two sets of wings. The front pair, they agree, provided lift. But they believe that the back pair were for steering. Microraptor's hind wings were radically different in shape from its forewings. Rather than being lithe and graceful, they were short and stubby. But they would have made good rudders, as Dr Habib and Mr Hall have demonstrated using a computer model of them. This suggests they would have allowed the animal to reduce the radius of its turning circle by 40% and almost triple the speed of a turn.

That makes sense. The rocks Microraptor fossils are found in are also full of trees. Clearly, it was a forest animal. Predatory birds that dwell in modern forests, such as the sharp-shinned hawk, are masters of making tight, quick turns around trunks and over branches as they pursue their prey. That Microraptor was a predator is known because one specimen has a bird in its belly. It would surely have benefited from a similar capability.

Why, then, was such an aerial paragon not ancestral to any modern creature? One possibility is bad luck. The mass extinction at the end of the Cretaceous was caused by a collision between Earth and an asteroid or comet. Though some groups of animals did better than others, survival was often at random. However, the fossil record does not prove that any Microraptor-like animal actually made it to the end of the Cretaceous. They probably become extinct well before that. Dr Habib proposes that what actually did for Microraptor was what he calls drag tax. His model suggests the manoeuvrability that its hind wings granted the animal came at the expense of increased drag. Modern birds do not pay the drag tax because their manoeuvrability results from a single pair of wings that are better able to cope with stress than Microraptor's forewings were. That stress-resistance is provided by muscles which are attached to a keel-like extension of the sternum. Microraptor lacked this keel.

According to Dr Habib, birds adapted for forest flight are able to make the same turns that Microraptor made, but without losing nearly as much energy to drag. This, he argues, made bird flight more efficientand that would have made it impossible for Microraptor to compete. (From The Economist, November 10, 2012)

Script 28. Marine ecology

Sea wolves

Sharks, it seems, are necessary for the ecological health of coral reefs.

For decades, rangers in Yellowstone National Park, in the American West, had to cull the area's red deer (known locally as elk, though they bear no resemblance to European elk, known locally as moose) because the animals' numbers were grazing the place to death and thus threatening the livelihoods of other species. Many ecologists argued that the deer had once been kept under control by wolves, which had been hunted to extinction by people. When wolves were reintroduced to Yellowstone, in 1995, these arguments proved correct. The deer population fell to manageable levels, and culling stopped. Wolves, it turned out, played a crucial role in keeping the wider ecosystem intact. Now comes evidence that the same is true for another top predator: sharks.

Jonathan Ruppert of the University of Toronto, in Canada, and his colleagues reached this conclusion by studying data on shark activity around two groups of coral reefs between Indonesia and Australia. They gleaned their information from baited underwater video stations and also from records collected from those reefs between 1994 and 2008 by the Australian Institute of Marine Science. They have just published their results in the Public Library of Science.

The reef groups in question, known as the Scott Reefs and the Rowley Shoals, are close to each other. They experience similar sea temperatures. And both were hit by powerful cyclones and affected by coral bleaching in the mid-1990s. One, however, turned out to be rich in fish species whereas the other is impoverished, and the one with more species seemed to recover faster from the storms and the bleaching. Dr Ruppert believes these differences are the result of the presence in one, and the absence in the other, of sharks.

The impoverished group, the Scott Reefs, has been fished for centuries by people interested in catching sea cucumbers, certain snails and sharks - all of which are internationally traded and fetch high prices. In recent decades the demand for sharks, in particular, has boomed, as China has grown richer and its citizens have been supping more shark-fin soup. The Rowley Shoals, by contrast, are a protected area where all fishing is prohibited.

It is hardly a surprise that fishing reduces a reef's species diversity. But what did surprise Dr Ruppert and his colleagues was exactly how that diversity was diminished. For besides the species that fishermen are hunting (silvertip and grey reef sharks were three times as common around the Rowley Shoals as around the Scott Reefs), many other sorts of animal had suffered.

This was not because they were being caught accidentally. Sea cucumbers and snails are hand·picked by divers, and sharks are caught on lines, rather than in nets, with bait that attracts only carnivores. All this means there is little bycatch. Yet the Scott Reefs also lacked herbivorous species such as parrotfish, though midsized predators, like snappers, were more abundant than in the Rowley Shoals.

Dr Ruppert thinks that eliminating sharks means populations of the midsized predators they feed on rise and those predators' prey then suffer the consequences. Hence the reduced numbers of parrotfish. Their absence, however, has knock-on effects. Seaweed grows more thickly without parrotfish constantly gnawing at it. That growth smothers young coral and probably, though Dr Ruppert cannot prove it in this particular case, makes it harder for reefs to recover from cyclones and bleaching. Healthy reefs, then, seem to need sharks in the way that healthy forests need wolves. It's not much fun if you are a snapper or a deer. But Mother Nature prefers it that way. (From The Economist, September 28, 2013)