Scientists have worked out what killed off a side branch of woolly mammoths that survived 6000 years longer than most of their species after they became marooned on a remote Alaskan island.Researchers say mammoths persisted for about 8000 years on St Paul Island, a chilly speck north of the Aleutian Islands, after rising seas engulfed the land bridge between Siberia and Alaska.
Quarantined from the stone- age hunters who helped push mainland mammoths to extinction about 12,000 years ago, the prehistoric beasts shared their windswept home with arctic foxes and shrews. But they eventually drank themselves to death, trashing their island liferaft in a desperate hunt for water.
The study, published in the Proceedings of the National Academy of Sciences, offers a new explanation for the demise of isolated mammoth populations in the Bering Strait.
The extinction of plant-eating megafauna elsewhere, including Australia, is usually blamed on the effects of climate change, which reduced vegetation, and early hunters.
But the research team found no evidence of human presence on the island until 1787, while the composition of the vegetation remained “stable” during the period when the animals are thought to have vanished.
Instead, the researchers believe a drying phase in the climate concentrated the availability of drinking water in a handful of lakes. This forced the animals to congregate nearby and strip the surrounding areas of vegetation.
“Like elephants today, when the water became cloudy and turgid, the mammoths probably dug holes nearby to obtain cleaner water,” reported Penn State University, which led the study.
“Both of these things increased erosion in the area and helped fill in the lake, decreasing the available water even more.”
Mammoth remains found on the island have been dated to about 6480 years ago, but other animals may have lived more recently. The team analysed sediments from the bed of one of the lakes to pinpoint when spores from fungi that grow on large animal dung had disappeared.
The researchers also studied “water isotope signatures” from the remains of aquatic organisms living in the lake, and nitrogen isotopes from the bones of 14 newly discovered mammoth skeletons.
All of them suggested progressively drying conditions.
“Multiple reinforcing lines of evidence indicate that reduced freshwater availability triggered the extinction of St Paul mammoths,” the paper says. They vanished 5600 years ago, give or take a century, making their disappearance one of the “best-dated prehistoric extinctions”.
By John Ross
With many thanks to The Australian
Picture shows mammals of the Pleistocene era. Artwork showing wildlife believed to have existed in the Northern Iberian Peninsula during the Upper Pleistocene era (125,000 to 10,000 years ago).
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