Fig. 1

Electrophysiological methods over time, highlighting the need for non-invasive sleep studies of wild marine mammals. A Schematic diagram showing progressive invasiveness from surface-mounted electrodes (least invasive) to needle electrodes, epidural electrodes placed on the surface of the skull or dura, and subdural electrodes placed beneath the dura in the cortex or in the brain (most invasive). B Diagram showing where the electrophysiological studies from Additional file 1: Table S1 fall in terms of invasiveness and animal mobility. Numbers refer to citations within Additional file 1: Table S1 (Table cites references [1,2,3,4, 6, 9,10,11,12,13, 19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35, 42, 44, 45, 53, 58,59,60,61,62,63,64, 66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81]