Activity of an animal heart is detected with an apparatus comprising a housing (20) within which is contained electronic circuitry (32, 34, 36, 38, 40), a power supply (46) for the circuitry, and an indicator (42, 44) connected to the circuitry which can be operated by the circuitry to generate an alerting signal detectable by a user. In addition there is a pair of sensors (28) electrically connected to convey to the circuitry electrical signals detected by the sensors. The circuitry is operative to analyse signals received from the sensors, and upon determination that such signals are electrocardiographic signals derived from a heart beat to cause the indicator (42, 44) to operate so as to generate an alerting signal.