Patent US 4,247,758

An animal such as a dairy cow carries a transponder unit which includes a motion sensor. When the animal comes within range of a transceiver unit, it is interrogated and the transponder unit transmits data which identifies the animal and indicates the number of movements of the animal. The number of animal movements increase dramatically during estrus and a record is kept on each animal to determine when this occurs. Data received at the transceiver is processed by a microprocessor based system which operates to insure the received data is correct. It also assembles the data and outputs it to a printer.