The water vole (Arvicola terrestris) is an endangered species in the UK, and ecologists Dr Tom Moorhouse and Professor David Macdonald of Oxford University's Wildlife Conservation Research Unit had been monitoring the size of two populations - one in Norfolk and one Wiltshire - when they noticed a 48% decline in the number of females born at the Norfolk site. At both sites vole numbers had been monitored using small traps baited with apple and carrot during 2002 and 2003, voles being released after counting. However, Moorhouse and Macdonald had also fitted radio-collars to 38 of the voles caught at the Norfolk site for in-depth study of the voles' movements.
According to Moorhouse and Macdonald: "Our analysis revealed that the most likely cause for the female decline was a shift in the sex ratio of young raised by radio-collared females. This result has implications for conservation research, especially for monitoring water vole populations."
Skewed sex ratios have been reported in stressed and malnourished females of various species, including water voles. The ecologists suggest this could be explained in terms of the local resource competition hypothesis, which predicts that mothers with access to poor resources will produce offspring of the sex most likely to disperse and therefore reduce local competition for resources.
"Radio-collars clearly have the potential to cause some stress to water voles, and it is possible that this might stimulate sex-ratio adjustment as part of an evolutionary me
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