A respective animal unit (101, 102) is attached to the animals (A1, A2) in a herd of animals. Each unit (101, 102) is configured to repeatedly: emit a signal (S1; S2) representing the unit (101, 102), register a position (P1; P2) and/or register acceleration data (a1; a2) related to the animal (A1; A2) to which the unit (101, 102) is attached. A set of data read-out devices wirelessly reads out information from the set of animal units (101, 102) into a database. A data processor identifies animals potentially in heat by: - associating a respective time stamp with each entry in the database, each entry representing one event concerning an animal (A1, A2) in the herd, which event relates to an emitted signal (S1, S2), a registered position (P1, P2) and/or registered acceleration data (a1, a2); - identifying candidate mounting animals as members of a first group based on the entries in the database; - identifying candidate mounted animals as members of a second group of based on the entries in the database, and
