A livestock animal monitoring device (1) has a temperature probe stem (3) for insertion in an ear canal (EC), and including a thermistor temperature sensor (54). At its other end the device (1) has a stud (40, 48) fastener for attachment to an animal's ear lobe tissue. A housing (2) is connected between the fastener and the probe stem (3) so that the device is retained by the fastener on ear lobe tissue and insertion of the probe stem in an animal ear canal in use. The housing contains a signal processing circuit (57) connected to the temperature sensor (54) and a wireless communication circuit (57) linked to the signal processing circuit for wireless upload of temperature data to a host. The fastener (40, 48) is connected to the housing (2) by an adjustable longitudinal support (5) which sets distance between the fastener and the probe stem, to suit the animal.