The researchers found two cycles corresponding to the disappearance of rodent species. One lasts 2.4 million years and is linked to variations in the Earth's orbit. The other is a 1.2 million year cycle relating to shifts in the tilt on the Earth on its axis.

The cycles are associated with lower temperatures, changes in precipitation, habitats, vegetation and food availability which are the main factors influencing the extinction peaks, the study published in the journal Nature said.

"Rodents are very sensitive to seasonal changes because they have such a short lifespan," said Van Dam, adding that they represent one of the best mammal fossil records.

At the moment, the Earth is at the beginning of a cycle but the planet's climate system has changed so much in the past 3 million years that it is difficult to predict what will happen in the future.

"The environment is responsible to what happens to species," said Van Dam. "Biological factors are secondary, according to our results."

Infection by T. gondii is usually facilitated through the consumption of raw or undercooked meat, or, food or water contaminated with soil containing cat feces. Infection results in toxoplasmosis, which can lie dormant and go undetected in humans. Human infection by T. gondii is common. Worldwide, rates vary between 20 - 80 percent of the population, with under-developed nations being the worst affected.

The new gender-bending findings, published this week in the journal Naturwissenschaften, show that women harboring dormant toxoplasmosis are much more likely to give birth to boys than women who are toxoplasma negative.

The researchers, from the Czech Republic, analyzed the effect of dormant toxoplasmosis on the probability of a male birth using clinical records of babies born between 1996 and 2004 in private maternity clinics. The records included information on the mother's age, the concentration of toxoplasma antibodies in the mother's blood, previous deliveries and the sex of the newborn. They found that the presence of the parasite in the mother's blood greatly increased the likelihood that these women would give birth to a boy - in some cases by as much as two boys for every three births.

Interestingly, the probability of a male birth increased with higher levels of toxoplasma antibodies. The researchers speculate that the increased survival of male embryos in infected women may be explained by toxoplasmosis' modulating and suppressing effects on the immune system.

Could T. gondii be stealthily engineering a male gender-preferenced world made up of schizophrenics and neuroticshttp://www.aphroditewomenshealth.com/news/20050926235940_health_news.shtml
? Perhaps, but the researchers caution that while toxoplasmosis appears to be the cause of the increase in male births, they cannot establish cause and effect. "An independent confirmation of this conclusion by a manipulative experiment [by experimental infection of animals] is necessary," they suggest.

The children's scores for hyperactivity, impulsiveness and oppositional behaviour were also compared before and after.

After 20 weeks the number of times staff had to restrain the boys had dropped by 46%. The length of time they had to be restrained dropped by 42% and their scores for impulsiveness and hyperactivity improved by 20%, said Dr Stordy.

For nearly all the boys there was a small but significant improvement, except two who did not take the fish oil and showed no improvement. Three showed dramatic improvements. "Their scores moved into the normal range for the population, which is remarkable," Dr Stordy said.

The claims being made for fish oil's effect on children's learning and behaviour have become controversial, with experts criticising supplement manufacturers for overstating the evidence from unscientific research.

But it was the Cotswold community school that approached manufacturers Efamol for supplies of its fish oil-based essential fatty acid supplement Efalex. Dr Stordy said the work made no claim to be a proper trial - only 19 boys were involved. There was no placebo.

Michael Crawford, an expert on essential fatty acids and the brain at the London Metropolitan University, said this sort of evidence was little more than anecdotal. "What we need is serious research." John Stein, professor of physiology at Oxford University, where much of the UK research on omega-3 fatty acids and learning and behavioural difficulties is based, said: "Some of these studies are little more than an advertising exercise. But our own research has shown a link between nutritional deficiencies and behaviour. It all cries out for a large, properly designed trial but people won't stump up the money because you can't patent nutrition."

The school head, Andrew Thomas, said the fish oils were just one of a range of techniques used to help the children, including improving their diets. "Fish oil supplements seem to be making a genuine difference to the rollercoaster of emotions they face every day." NCH, the charity that runs the school, may extend use of supplements to its other homes.