An animal monitoring device configured as a bolus for oral administration to reside in an animal's stomach. The bolus has a substantially inert solid body with a hollow inside space to receive the animal monitoring device. The animal monitoring device includes a radio frequency generator, an animal identification information encoder for outputting the animal identification information of the particular animal. The animal monitoring device can further include sensors to detect physiological and non-physiological animal characteristics such as temperature and a sensed animal characteristic encoder for outputting sensed animal characteristic information. The animal monitoring device intermittently transmits encoded animal identification information and sensed animal characteristic information to a receiver which transmits the encoded information as packets to a specialized computer which allows a computer user access to decoded animal identification information and sensed animal characteristic information as numeric values.