A bolus, for introduction into a ruminant animal's reticulum to register pressure signals transmitted through fluids therein, has a case unit adapted for immersion in the fluids. A mechanical amplifier element extends out from the case unit and is adapted to be surrounded by the fluids in the reticulum and absorb mechanical energy from the pressure signals. A guide part conveys mechanical energy from the mechanical amplifier element to a sensor module in the interior of the case unit. The sensor module transduces the pressure signals into electric signals from which a processing module extracts data representing body movements, a heart beat rate, a respiratory rate, a respiratory depth and/or stomach activity of the animal. A communication module in the case unit receives the data and transmits output radio signals reflecting the data.