The biology of snow voles seems to contradict scientists' assumption that natural selection favours big animal bodies over small ones. A study of the voles suggests that selection has favoured genetic changes that promote smaller body size.
Larger snow voles (Chionomys nivalis; pictured) produce more offspring than smaller ones. To work out why this hasn't resulted in a general increase in body size, Timothée Bonnet and his colleagues at the University of Zurich in Switzerland followed a population of snow voles in the Swiss Alps for ten years and analysed the animals' genomes. They found that although large snow voles did have a reproductive advantage, this was affected more strongly by environmental factors such as food abundance than by genetics.
Genetically, selection favours voles that complete their growth earlier in the season — perhaps because food is scarce later in the season — resulting in smaller adult sizes.
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In selection, size isn't everything. Nature 542, 141 (2017). https://doi.org/10.1038/542141d
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DOI: https://doi.org/10.1038/542141d
