Miniaturization of biologgers is not alleviating the 5% rule

1. The use of biologging technology has increased exponentially over the last decade, allowing us to study animal behaviour at a level of detail not previously possible. 2.It is clear from recent meta-analyses that the attachment of such devices can have negative effects on individual animals, particularly their behaviour and physiology. In recognition of this, a commonly applied rule is to ensure that devices borne by flying animals weigh less than 5% of their body mass. Over time, the continuing miniaturization of devices should facilitate the deployment of devices that are an ever-decreasing fraction of animal mass. 3.Despite these considerations regarding device mass, here we show that there has been no apparent reduction in the relationship between body mass and the mass of logging devices over the last 48 years. 4.Using a meta-analytical approach, we demonstrate that the ongoing miniaturization of animal-borne devices has resulted not in a decrease in the relative device mass borne by animals, but instead has prompted researchers to measure smaller and smaller species. 5.We recommend that researchers better exploit the ongoing miniaturization of devices to reduce the relative mass of the devices borne by animals, and avoid blind adoption of the 5% or any other arbitrary rule when designing research programs.