Covert muscle activity reveals dynamic freezing states and prepares the animal for action

When an animal detects a threat it must make a split-second choice between fight, flight or freezing (1-3). During freezing, skeletal muscles sustain tension to maintain rigid, sometimes atypical postures, for many minutes at a time (4). Meanwhile the animal must dynamically assess its surroundings to plan future actions and ready its body for movement. The interplay between the neural and somatic systems during freezing remains poorly understood. Here we show that freezing Drosophila melanogaster display a striking novel pattern of leg muscle activation unique to immobility, a rhythmic pulsing in the distal tibia. The muscle, which we show to be a previously undescribed leg accessory heart, displays multiple activity modes and ramps up to movement onset, implying a preparation for movement. The frequency of pulsing is dynamically modulated as the fly integrates external threat or safety cues, and artificially increasing pulse frequency leads to freezing breaks, indicating a causal role in the decision to move. Through the identification of a new Drosophila cardiac organ, this study provides a window into the multiple states which can underlie freezing behaviour, and the physiological changes which the body undergoes to ready the animal to move.