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 mechanism mitigating impacts of suboptimal habitats, similar to the sex-ratio bias and stress response in food deprived water voles," they say.
Researchers have long been aware that the techniques they use had the potential to cause unexpected effects, and there have been many studies into the effects of radio-collars. However, this is the first study to show an association between radio-collars and sex ratio, although further work is needed to establish a causal link. According to Moorhouse and Macdonald: "We would expect any such effect to be species-specific, but our results will alert those studying other small mammals to look for similar associations. Our findings are a reminder that the assumption that the use of radio-collars does not fundamentally affect the biology of the subjects always requires careful checking. This study emphasises that the effects of commonplace wildlife marking and tracking techniques may be difficult to detect and yet both important and revealing. Clearly, it is both scientifically and ethically important to be aware of, and to strive to minimise, any such effects."