A device usable in animal husbandry for monitoring calving in large animals by sensing a sustained raised position of the animal's tail. Said device includes a tail position sensor and radio frequency transmitter. The sensor consists of a bearing plate (40), a raised portion (42), straps (44, 48) and an arm (50) hinged to the bearing plate so that its position is dependent on that of the animal's tail, as well as a harness. The individual transmitter device comprises an electronic assembly including a power supply battery (62), a magnetically controlled power fuse (64) (ILS micro-circuit breaker), an oscillator and shaping circuit (68) for discontinuously generating and modulating a radio signal, and a timing means (69) for delaying the power supply and activating the oscillator only after a predetermined time lag. The harness consists of a yoke (13) and two straps (12) each connected both to said sensor (10) and to the lower portion of the yoke via respective resilient linkages.