Before woolly mammoths went extinct thousands of years ago, their dwindling population suffered a series of genetic mutations that hampered their ability to survive, researchers said Thursday.

Woolly mammoths were once among the most common herbivores in  North America and Siberia, but came under threat from increased hunting pressure and a warming climate. They disappeared from the Earth 3,700 years ago.

Experts analyzed the genome of one of the last known woolly mammoths ever found -- a 4,300-year-old specimen from Wrangel Island, off the northern coast of Siberia.

On the island, about 300 of the lumbering creatures were believed to exist even after mammoths went extinct on the mainland some 10,000 years ago.

They compared the genes from this recent specimen to one that was far older -- from some 45,000 years ago -- and came from a population that was much more numerous and robust.

"Here we got a rare chance to look at snapshots of genomes 'before' and 'after' a population decline in a single species," said co-author Rebekah Rogers of the University of California, Berkeley. 

"The results we found were consistent with this theory that had been discussed for decades."

Namely, researchers found far more harmful mutations in the island mammoth than in the mainland one, where breeding partners were plentiful and diverse, and the population was far healthier.

Some of the genetic flaws researchers found could have caused the animals to suffer stomach upset and gastrointestinal woes.

Others likely led to a more satiny, glossy coat than the animal's typical stiff-haired exterior -- a change that could have made them more vulnerable to cold temperatures.

"The animals had lost many olfactory receptors, which detect odors, as well as urinary proteins, which can impact social status and mate choice," said the study in PLOS Genetics, describing the process as a "genomic meltdown in response to low effective population sizes."

To rule out the possibility that the 4,300 year-old specimen was not just an anomaly, co-author Monty Slatkin employed mathematical models to show how genomes will look different when population conditions change.

"With only two specimens to look at, these mathematical models were important to show that the differences between the two mammoths are too extreme to be explained by other factors," said Rogers.

The findings are "a warning for continued efforts to protect current endangered species with small population sizes," said the study.

Cheetah, mountain gorillas and pandas are among the species under threat from small population sizes, which can make it difficult to impossible for a species to overcome mutations that risk their survival.

"Thus we might expect genomes affected by genomic meltdown to show lasting repercussions that will impede population recovery," said the study.

source: AFP