Lack of females linked to dinosaurs' demise?

[faze]

Date: Apr 21, 2004 - 01:23 PM
Wednesday, April 21, 2004 Posted: 11:43 AM EDT (1543 GMT)

WASHINGTON (Reuters) -- An asteroid may have wiped out the dinosaurs 65 million years ago not simply by changing the world's climate and causing years of dark skies, but also by causing too many of them to be born male, U.S. and British researchers said on Tuesday.

If dinosaurs were like modern-day reptiles such as crocodiles, they change sex based on temperature, David Miller of the University of Leeds in Britain and colleagues noted. And even a small skewing of populations toward males would have led to eventual extinction.



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. 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.

No one really knows if dinosaurs were more like reptiles, or something closer to mammals. Reptiles have very different metabolisms than mammals and also have various ways of determining the sex of 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.

Miller's team 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 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," said Dr. Sherman Silber, an infertility expert in *****. 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 wrote.