Shrinking body length in snakes in the United Kingdom: ecological phenomenon or sampling error?

In recent years, studies have been linking climate warming with a decrease in body size across many aquatic and terrestrial taxa including reptiles. The impact that fluctuations in temperature are having on reptile populations is widely recognised, yet surprisingly little is understood about the apparent decrease in reptile body size in contemporary times. In the United Kingdom (UK), grass snakes (Natrix helvetica) and adders (Vipera berus) are not only in decline, but there is some evidence that body lengths appear to be decreasing too. Whether the 'shrinkage' phenomenon is real or is a result of measurer error or collection bias, remains a controversial topic among herpetologists. Comparative analyses using historic data from preserved specimens and contemporary measurements from field caught snakes found that N. helvetica were smaller on average by 14.1%, and V. berus by 11.1% than historic counterparts collected between the late 1800s to 1950 in the UK. It was important to establish whether these findings represented a true reflection of trends over time or whether they had been influenced by collection, sampling and / or measurer bias. Visual surveys of model snakes placed in reptile habitat revealed that more larger models were detected than smaller models, and that an experienced observer found more models than two groups (n=9 and n=10) of inexperienced observers. This supports the finding that detectability and collection is biased by snake size. Measuring snakes is notoriously difficult as evidenced by the considerable amount of literature on the topic. Further experiments were conducted to test the biases associated with multiple measurers, multiple measuring methods and repeatability, accuracy, and precision. There was no difference in measurements when they were made by a traditional method (squash box and string) or an image analysis program (Image J). However, experienced measurers varied in how they reported the size measurements of the same snakes using measuring software. Moreover, the position of the snake in the image influenced size measurements highly significantly. Caution must be taken if measurements are obtained using different methods by different measurers or combining data from live and preserved specimens. It is clear, however, that the smaller body size of contemporary N. helvetica and V. berus and the relation to potential environmental change factors are worthy of further investigation.