Science: Theoretically ...

Faces aren't enough to recognise a person, according to researchers at the Massachussetts Institute of Technology, who report in Nature. Countering the long-standing thinking that we remember a face by storing data about relative positions of eyes, nose and mouth, they say that you also need to know about shape and position of their head. They demonstrated the effect using digitally manipulated pictures of Bill Clinton and Al Gore. Where they found people who could remember what Mr Gore looks like isn't recorded.

Why don't models of ozone depletion in the Arctic match reality? Because, say a team in Paris in the latest Nature, the computer models can't reproduce the tiny variations in spatial distribution of the ozone-eating chemicals. These could account for big discrepancies between forecast and observed ozone depletion around the North Pole, they say. While models work well with the Antarctic, the polar air circulation is less uniform in our hemisphere. To back up their suggestion, the researchers used an ultra high-resolution model to show that ozone depletion really is sensitive to small-scale differences. For the winter of 1994-95, the effect is large enough to account for the Arctic ozone depletion being 40 per cent greater than models predicted.

The River Nile's great bend, where it zigzags from its northerly path to head southwest for almost 200 miles before heading north again, is probably due to tectonic forces. A report in Science based on radar-sensing data from the Nasa space shuttle suggests that the southwesterly movement is comparatively recent, caused by geological uplift from faults in younger rock beneath it. The northwards flow dates from Precambrian strata. So it's not civil engineering to make the cruises longer.

Family-linked cases of Alzheimer's disease tend to occur at an earlier age than the "spontaneous" forms. The role of the genetic link - a gene known as PS2 - is becoming clearer. New US research has found that PS2 mutation produces a molecule which makes neurons more likely to commit "cell suicide" (apoptosis). Perhaps, say the team at the National Institutes of Health in Bethesda, Maryland, the molecule makes neurons more sensitive to the "normal insults" of ageing, adding to the toxic burden in the patients' brains.

Massively fat, diabetic and infertile: it's no fun being a mouse deficient in leptin. That's the hormone encoded by the "Ob" (for obese) gene which helps you lose weight by suppressing appetite and stimulating your metabolism. But as dieters will have suspected, leptin isn't the only player in the weight charades. A neuropeptide called NPY, known to regulate energy balance, reaches high levels in leptin-deficient mice. But mice that don't produce leptin or NPY are less obese than their leptin-deficient mates and suffer less from diabetes or sterility. Gene therapy for weight loss? The day might not be far off. It's just that you would have to mediate a lot of genes.