Indirect negative impacts of radio-collaring: sex ratio variation in water voles

1. Radio-tracking is used ubiquitously in studies of wild vertebrates, a fundamental assumption being that tagged animals do not significantly differ, behaviourally or otherwise, from untagged animals. 2. We studied two populations of water voles Arvicola terrestris: one population was live-trapped from April to September for 2 years (2000, 2001) and then concurrently radio-tracked and trapped in a third year (2002). The second population was trapped only during 2002. 3. During 2002, a substantial decline in female numbers in the radio-collared population was recorded, apparently resulting from a male skew in the sex ratios of offspring born to this population. The mean numbers of males and females born during trapping only were 25.7 (SD 2.1) and 29.0 (SD 7.0), respectively. Recruits to the radio-tracked population were skewed heavily in favour of males (43 : 13). 4. Both hypotheses commonly invoked to explain mammalian sex-ratio manipulation refer to the condition of mothers. The altered sex ratio resulted from a 48% decrease in numbers of females born, a decrease similar to the proportion (0.49) of the female population that was collared. No similar decline in male births occurred. This suggests that radio-collaring of females caused male-skewed sex ratios. 5. Synthesis and applications. We conclude that the observed decline in female numbers resulted from male-skewed recruitment sex ratios due to the attachment of radio-collars to female water voles. These results question the assumption that the use of radio-collars does not fundamentally affect the biology of collared water voles.