In most animals, a broad range of predator odors can trigger hardwired fight-flight-or-freeze fear responses that are key to each animal's individual survival and the survival of the species. Mice can detect chemosignals associated with a predator's scent that warn a mouse of life-threatening danger. Because of this, mice tend to display innate fear-like behavior in response to diverse predator odors such as cat saliva, major urine proteins (MUPs) from a rat, or the smell of snakeskin. 

— Read more in Smells Can Become Hardwired to Trigger Brain-Wide Anxiety at Psychology Today.