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Dwarfing on the Californian Islands

Initially discovered on Santa Rosa, then on Santa Cruz and San Miguel in the Californian Channel Island archipelago, Mammuthus exilis is a dwarf mammoth descended from Mammuthus columbi that lived in mainland America. These modern islands are the exposed peaks of a ‘superisland’ known as Santarosae. The majority of this island was covered when sea levels rose as glaciers melted. This dwarf species stood between 1.4-2.1 m, weighing only 2000 lbs in comparison to M columbi, which stood 4.3 m tall and weighed as much as 20000 lbs (National Park Service, 2009). With no evidence for the existence of a land bridge between the islands and the mainland, it is thought that they swam across. The distance between the mainland and Santarosae would have been less, approximately 6 miles from the mainland at its closest distance (National Park Service, 2009) caused by the ‘glacially lowered sea levels’ (Johnson, 1978). Arguably there must have been a reason why some individuals crossed the water to reach the islands. During the late Pleistocene, the coastline suffered drought, which led to poor vegetation. Odours carried on the wind from the vegetation of the islands may have been detected by the strong olfactory senses of M. columbi, and this may have lured them to the islands. There, in the absence of predators, positive selective pressures encouraged a decrease in size assisting in survival of fluctuating food supplies, gradually evolving into M. exilis. Additionally, when negotiating steep slopes, as would have been the environment of these islands, the lower limbs shorten in response, possibly assisting in the evolving the dwarfism characteristic (Agenbroad, 2001). They survived until the colonization of the islands, ‘first recorded between 10800 and 11300 years ago’ (Agenbroad, 2001). Whether it was solely the arrival of humans or the changing climate that caused their ultimate extinction is still yet to be decided.

References and Resources

  • Agenbroad, L.D. 2001. Channel Islands (USA) pygmy mammoths (Mammuthus exilis) compared and contrasted with M. columbi, their continental ancestral stock. The world of elephants: International Congress, Rome. 473-475.
  • Alexandra van der Geer. No date. Evolution and Extinction of Mammals on Islands. 22/04/2010
  • Johnson, D.L. 1978. Origin of Island Mammoths and the Quaternary land bridge history of the Northern Channel Islands, California. Quaternary Research. 10, 204-225.
  • National Park Service. 2009. Pygmy Mammoth. 18.04.2010.

  • Authored by Emma Kerridge & Chris Rogers

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