Improved behavioral inferencing from biologging tags through magnetometry

Wearable technology is now a primary tool for studying animal movement ecology, enabling key insights into species’ migrations, ecophysiology, and interactions across diverse taxa. However, using sensors to identify specific movement behaviors remains challenging without direct visual observations of the tagged animal. This difficultly arises because sensors only measure movement from a single point on the body, providing limited information about the underlying whole-body kinematics that contribute to behaviors. Although magnetometers are commonly limited to orientation data, we show they can be used to identify and describe the motions of spatially-isolated body appendages via an adhered magnet. Variations in magnetic field strength can then be linked to peripheral body movements to directly measure key behaviors. To illustrate the broad potential of this approach, we conducted four experiments with taxonomically diverse animals, providing measurements of ventilation rates in flounder, scallop valve angles, shark foraging, and squid propulsion. For each species, changes in magnetic field strength were correlated with appendage position to identify and characterize important behaviors that are difficult to measure with traditional tagging approaches. This novel method revealed that scallops modulated valve opening angles on a circadian rhythm. Similarly, flounder operculum beat rate occurred at 0.5 Hz, with most beats reaching only a few degrees in magnitude. When applied to a shark, magnetometry quantified jaw angle and chewing events while foraging. Finally, for a mobile epi- and mesopelagic squid, magnetometry revealed three prominent and coordinated fin and jet propulsion movements during high acceleration swimming. This method leverages comparatively small magnets to enable new measurements on fragile and diminutive structures. Magnetometry expands the scope for exploring new ecological and biomechanical questions in a previously understudied size class of marine species.