Too many males, fewer females wiped out dinosaurs

Fewer females wiped out dinosaurs
By Dr David Whitehouse
BBC News Online science editor

Too many males may have been the reason the dinosaurs died out 65 million years ago, say Leeds University researchers.
They believe that dinosaurs may have been like modern-day reptiles such as crocodiles whose sex depends upon the temperature before they were born.

The new idea is that the asteroid that struck changed the world's climate causing it to be cooler, which led too many dinosaurs to be born male.

The male-female imbalance would have led to their extinction, they say.

Sun block

You have to feel sorry for the dinosaurs.

There they were, the top of the ecological tree, uncontested masters of the world for almost 200 million years when things suddenly start going wrong for them.

Although there are some that say they were on their way out before the space rock hit, most experts agree that one or more asteroid impacts probably triggered a series of global changes that killed off the dinosaurs and many other species of life on Earth.

Some scientists believe the impacts would have kicked up dust that cooled the air and also triggered volcanic activity that would have created even more dust and ash which would have blocked out the Sun and chilled the Earth.

Bad news for the dinosaurs because it is well known that those at the top of the evolutionary pile are especially vulnerable to ecological changes.

And, if this new theory is correct, it would not have been much fun being a dinosaur during these troubled times even if you had survived everything nature could hurl at you.

If the conditions were not bad enough those that did manage to eke out a meagre living could not find a mate.

Preponderance of males

No one really knows whether dinosaurs were much like other reptiles, or whether they resembled other animal groups, such as mammals, in some respects. Reptiles have a different type of metabolism to mammals and have various ways of determining the sex of their offspring.

In mammals, if a baby gets an X and a Y chromosome, it will be male and if it gets two X chromosomes it will be female, with a few very rare exceptions. Similar mechanisms work for birds, snakes and some reptiles such as lizards.

But in crocodilians, turtles and some fish, the temperature at which eggs are incubated can affect the sex of the developing babies.

David Miller of the University of Leeds and colleagues ran an analysis that showed a temperature shift could theoretically have led to a preponderance of males.

Other studies have shown that when there are too few females, eventually the population eventually dies out.

"The Earth did not become so toxic that life died out 65 million years ago; the temperature just changed, and these great beasts had not evolved a genetic mechanism (like our Y chromosome) to cope with that," says Dr Sherman Silber, an infertility expert in St Louis who worked on the study.

But crocodiles and turtles had already evolved at the time of the great extinction 65 million years ago. How did they survive?

"These animals live at the intersection of aquatic and terrestrial environments, in estuarine waters and river beds, which might have afforded some protection against the more extreme effects of environmental change, hence giving them more time to adapt," the researchers say.

But some experts are not convinced by the idea.

"More than 50% of all species that lived prior to the mass extinction were wiped out. In fact, the dinosaurs were not among the most numerous of the casualties - the worst hit organisms were those in the oceans," said Benny Pieser of Liverpool John Moores University.

"I am afraid sex-selection mechanisms are an unlikely cause for the termination of the age of dinosaurs - despite the sexed-up headlines."
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Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/3647115.stm

Published: 2004/04/21 15:58:24 GMT