text: Some prey make use of chemicals originating from predators, using these cues as an indicator of the level of predation risk and changing their morphology if need be. Changes in morphology caused by predator presence is known as predator-induced polyphenism, and occurs across a variety of animals. For example, Daphnia cucullata show formation of "helmets" when exposed to predators or the water they have lived in. Their predators include cladocerans (such as Leptodora kindtii) and larvae of Chaoborus flavicans, a midge. They respond to these kairomones by doubling the size of their helmets, a protective structure. These changes in morphology make them safer from predators.
  Mice are instinctively afraid of the smell of their natural predators, including cats and rats. This occurs even in laboratory mice that have been isolated from predators for hundreds of generations. When the chemical cues responsible for the fear response were purified from cat saliva and rat urine, two homologous protein signals were identified: Fel d 4 (Felis domesticus allergen 4), the product of the cat Mup gene, and Rat n 1 (Rattus norvegicus allergen 1), the product of the rat Mup13 gene. Mice are fearful of these major urinary proteins (Mups) even when they are made in bacteria, but mutant animals that are unable to detect the Mups show no fear of rats, demonstrating their importance in initiating fearful behaviour. It is not known exactly how Mups from different species initiate disparate behaviours, but mouse Mups and predator Mups have been shown to activate unique patterns of sensory neurons in the nose of recipient mice. This implies the mouse perceives them differently, via distinct neural circuits. The pheromone receptors responsible for Mup detection are also unknown, though they are thought be members of the V2R receptor class.