About 35 years ago, the number of adders, a species of small viper known as Vipera berus, living around the Swedish town of Smygehuk on the Baltic Sea began to drop. Fewer young adders were being born and many of those that were suffered deformities or were born dead.

Scientists writing in the journal Nature say it was obvious that the snakes, which had become isolated from other populations on a small, grassy, coastal strip for about 100 years, were suffering because of inbreeding - there was little genetic variability.

Whilst this problem is reasonably well documented in captive animals, it has not been studied that much in wild populations. However, the researchers from Sydney, Lund and Gothenburg Universities say they have been able to rescue the snake by revitalising the population's gene pool.

Starting in 1992, the team introduced 20 adult males from large and genetically-rich snake groups from elsewhere in Sweden.

Considerable increase

From 1996 to 1999, there was a considerable increase in the population, with 32 recently-matured males being recorded this year. This was the highest number recorded during 19 years of data collection in Smygehuk.

The team also noted a rapid increase in the genetic variability during this time, as well as a fall in the number of still born offspring.

The researchers say the results lend weight to genetic approaches to conservation and highlight the importance of preserving genetic variability in wild populations.

Many animals are on the brink of extinction and could die out because they are forced to breed with close relatives, the offspring of which often die. It could be that a surprisingly little injection of new blood could be all that is needed to save the species.