Field voles living in archipelago have become large and long-legged

Academy professor says a rapid evolutionary change has taken place

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By Johanna Mannila
     
      The field vole or short-tailed vole (Microtus agrestis) that lives in the outer archipelago off Finland's south-west coast is large and has longer legs than its fellow species living on the continent, indicates a survey conducted in the universities of Helsinki and Uppsala.
      Juha Merilä, Academy Professor at the University of Helsinki, reports that according to standard evolution theory, natural selection favours traits that encourage the spreading of populations into new habitats.
      However, the amount of evidence that would support the theory is relatively small.
     
In Merilä’s survey, the researchers measured and weighed field voles found at two locations on the continent near Stockholm and at six locations in the outer archipelago between Sweden and Finland.
      For three years, they bred offspring of field voles originating from the continent and those from the archipelago in a laboratory.
     
The studied field voles all came from the same stock, as their habitats on the islands rose from the sea after the Ice Age about 500 to 1,000 years ago, whereafter the voles swam to the islands.
      The offspring were bred in the laboratory, in order to make sure that the differences between the populations in size and leg length were genetically heritable traits, and not attributable to for example the nourishment available or to any other factor dependent on their own local milieu.
      The offspring of the archipelago voles grew and became large and long-legged even in the laboratory environment.
      ”The rapidity of the evolutionary change was a puzzling phenomenon”, says Merilä.
     
The results indicated that through natural selection the island voles had genetically adapted themselves to survive in the rugged circumstances of the outer archipelago.
      The larger size and long legs help the voles to spread by swimming.
      The long hind limbs also improve the pushing force and speed up swimming through the water.
      Large individuals are stronger than the small ones and have enough strength to move longer distances both on land and in water.
      Because of their mass, they are not as susceptible to hypothermia as their small fellow species.
     
Gardeners in mainland Finland will be grateful that the evolutionary improvements have not spread to the voles found in back gardens and forests - they are generally regarded as a prize nuisance even in their current guise.
     
     
Helsingin Sanomat / First published in print 6.8.2010


Previously in HS International Edition:
  Vole populations about to collapse in Southern and Central Finland (23.6.2009)
  Pygmy owls drafted in to control unprecedented vole damage in Savo Province (4.6.2009)
  A quarter of young strawberry plants in Suonenjoki region destroyed by voles (22.5.2009)

Links:
  Field Vole (Wikipedia)

JOHANNA MANNILA / Helsingin Sanomat
johanna.mannila@hs.fi