A device (101) for signalling a presence of animals inside monitoring units (1) for locating animals, of a type comprising a box body (2) destined to contain animals and provided with at least a pair of access ports (3) and respective mechanical means (4) for opening and closing the access ports (3), the device (101) comprising detecting and signalling means of the presence of at least an animal inside the box body (2). In the invention, the detection and signalling means (102) can be activated to open and close by means of a switch element (103) which can be associated internally to the box body (2) and which can be shaped in such a way as to be activated by each of the mechanical means (4) of the pair of access ports (2).
