Bio-loggers are miniaturized autonomous devices that record and store quantitative data on animal behavior, localization, energetic and physiology. Used on free-ranging individuals, they allow monitoring naturally behaving (theoretically undisturbed) wild animals, not easily observable otherwise. However, according to the studied taxa, externally attached loggers may impede animal movement, or affect locomotion energetics, social and reproductive behaviours, and/or thermoregulation. They may also increase animal conspicuousness, predation or entanglement risk, and ultimately fitness in case of long-term deployment. Implanted devices may mitigate some of these effects, and can be seen as a good alternative for long-term eco-physiological studies. Using recent studies, I will outline how implanted loggers can improve our knowledge on the physiology of diving and call into question well-known physiological relationships. They can also refine proxies for prey consumption and enable for the first time to get data on immature king penguins during their first period of life at sea. Finally, within the Three R’s (reduction, refinement, replacement) framework, we discuss capture and handling evaluation, and the pros and cons of logger implantation in light of ethical considerations and data accuracy and reliability.        

Keywords: anesthesia, bio-logging, capture and handling, data bias, eco-physiology, free-ranging, implantation