Why did the mammoth become extinct?

An extensive project reports that both climate change and humans played a significant role in the decline of the population

Scientists have been arguing for decades about why so many species of large mammals living in far northern latitudes died out towards the end of the ice ages. Were environmental factors and climate change responsible? Or did the growing population of human hunters drive megafauna such as mammoths and woolly rhinos to extinction?

The most extensive attempt to answer these questions, a project involving researchers from 40 universities, reports in the journal Nature that both climate change and humans played a significant role – and their interactions were too complex to conclude that one or the other was more important.

The collaboration, led by Eske Willerslev of the University of Copenhagen, combined the evidence of DNA extracted from the fossilised and frozen remains of ancient megafauna with climate data and the archaeological record of early human settlement.

“Our findings put a final end to the single-cause theories of the Ice Age extinctions...” says Willerslev. “The impacts of climate change and human encroachment on species extinctions really depend on which species we’re looking at.”

The woolly rhinoceros, which died out less than 14,000 years ago, seems to have suffered least from human impact. The ranges of woolly rhinos and humans did not overlap, at least in Europe, says Beth Shapiro of Penn State University: “These data suggest that climate change, and not humans, was the main reason why this particular species went extinct in Europe.” Other species were influenced more strongly by humans.

Over the past million years or so the climate has oscillated between ice ages and warmer interglacial periods. Large cold-adapted mammals fared better during the ice ages but they survived during the interglacial periods by retreating to “refugia” where conditions were still cold enough for them to hang on in smaller numbers.

Populations fluctuated until the end of the last ice age around 14,000 years ago when some species disappeared. “During the most recent warming event, when the last ice age faded into the warm interval we have today, something kept these animals from doing what they had always done, from finding alternative refugia...” Shapiro says. “That ‘something’ was probably us humans.”

When these animals were beginning their final decline, the human population was spreading out across not only their cold-climate habitats but also their interglacial refuges, hunting big game and changing the landscape with agriculture and other activities.

Advances in the battle against old age

A new approach to treating the symptoms of ageing – by eliminating “deadbeat” senescent cells – has given encouraging results in experiments with genetically engineered mice at Mayo Clinic in Minnesota.

The study, published in Nature, provides the first evidence that the accumulation of senescent cells in the body contributes to ageing and that it may be possible to do something about it.

“By attacking these cells and what they produce, one day we may be able to break the link between ageing mechanisms and predisposition to diseases like heart disease, stroke, cancers and dementia,” says co-author James Kirkland.

Cellular senescence is a state of limbo in which cells neither die nor continue to multiply. It is believed to have evolved as a mechanism to prevent runaway growth and the spread of cancer.

The downside of senescence is that affected cells can produce biochemical factors that damage other cells. The immune system tries to sweep out these dysfunctional cells on a regular basis but over time it becomes less effective at “keeping house” in this way. In an elderly human 10 to 15 per cent of cells may be senescent.

The Mayo mice were genetically engineered to produce a cell-killing enzyme called caspase 8. They were then fed a drug that activated caspase in their senescent cells – and eliminated them – but had no effect on normal cells.

The researchers found that lifelong elimination of senescent cells delayed the onset of age-related disorders such as cataracts and muscle weakness. They also showed that removing these cells later in life could slow the progression of already established age-related disorders. The mice showed no overt side-effects.

It is likely to be many years before scientists develop a way to clear senescent cells in people.

In the thicket of it: how birds navigate

How do birds manage to fly safely and speedily through cluttered environments such as dense forest, without colliding with the trees? The answer lies in their perception of “optic flow”, according to research in Australia.

Optic flow is the pattern of moving images created in the brain as the observer passes series of objects or shapes. “Our findings show, for the first time, that birds regulate their speed and negotiate narrow gaps safely by balancing the speeds of image motion, or optic flow, that are experienced by the two eyes,” says Partha Bhagavatula, who led the study at the University of Queensland.

The researchers trained budgerigars to fly down a 7m corridor, which was lined with different combinations of black horizontal and vertical stripes.

The birds flew down the centre of the corridor when optic flow cues were balanced with identical, vertical stripes on either side of the corridor. But they stayed closer to one wall when these cues were unbalanced, such as when there were horizontal stripes on one side and vertical stripes on the other. The budgies flew faster when the tunnels were lined with horizontal rather than vertical stripes.

These findings – published in Current Biology – may have implications for robotics. The speed and accuracy with which birds fly through thickets will help scientists to design vision systems to guide flying robots through densely cluttered environments.

Know your whisky from moonshine

Researchers at the University of St Andrews have discovered how to detect whether a whisky is genuine – and if so how old it is and where it was distilled – through a laser test that requires less than a dram of the precious liquid.

Their method, published in the journal Optics Express, has been patented and is being presented to the whisky industry.

“Counterfeiting is rife in the drinks industry, which is constantly searching for new, powerful and inexpensive methods for liquor analysis,” says Bavishna Praveen, one of the team. “Using the power of light, we have adapted our technology to address a problem related to an industry which is a crucial part of Scottish culture and economy.”

The technology involves putting a drop of whisky on a transparent plastic chip. The sample is illuminated by one optical fibre and the light scattered from it is collected by a second fibre.

The way the light is scattered and its energy shifted by the whisky reveals not only the alcohol content – at least 40 per cent in genuine whisky – but also how it was manufactured. For a single malt, it can even reveal the cask used.

The chip used in the study was originally employed for analysing samples in biomedical research and then adapted for the drinks industry.

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