An indicator apparatus for use in indicating when an animal is in estrus. The apparatus is configured to be affixed to the rump of an animal to detect and indicate when the animal has been mounted and, thus, when the animal is in estrus. The apparatus is affixed to the animal by an adhesive layer. Layered on the top surface of the adhesive layer is an indicator layer that is in turn covered with a floodcoat layer. The floodcoat layer is adapted for removal upon the mounting of the first animal by a second animal. Removal of the floodcoat layer exposes the indicator layer indicating that the first animal has been mounted, thereby indicating that the first animal is in heat. The floodcoat layer is highly visible and may be seen from a distance.