1998 seems to be the year when things started going wrong for the world's snakes.

Numbers of species such as European asps and west African vipers began falling then and in some places have plummeted since, by as much as ninety percent.

Some of the declines are clearly due to the spread of human settlements, but others have occurred in national parks and protected areas.

The scientists behind the study, which is published in the journal Biology Letters, say they have no idea what could be causing the trend.

Climate change is a possibility, as 1998 was an unusually hot year.

A similar decline noted a couple of decades ago in frogs was subsequently found to be due to the spread of a hitherto unknown disease, but there's no evidence yet for anything similar in snakes.

The scientists are calling for other researchers in other parts of the world to put forward any data they may have, in order to better understand the phenomenon and perhaps begin to detect what's causing it.