A self powered monitoring device for monitoring an animal's temperature and other vital signs. The device may be mounted in the animal's ear, under the animals skin, or in any location which can be used to generate a reliable indication of the animals temperature and other vital signs. The temperature probe for the device is inserted subcutaneously and thermally and/or electrically connected to the housing of the device. Power for the device is provided by a miniature array of solar cells, the power from the solar cells being used to drive digital circuitry which processes signals from the various probes used to gather data concerning the animal's vital signs. The size of the array is chosen to be commensurate with the number of vital signs monitored, with the amount of sunlight, density of herd, and other factors being considered to ensure sufficient power for the device.