The following text is for questions 37 to 41.

Cats

Cats are carnivorous mammals. They are the most popular pet and people often call them kitty or pussycat. Young cats are called kittens, while their parents are often called queens (female cats) and toms (male cats).

    There are shorthair and longhair cats . However, unspecified breeds are categorised as domestic shorthair (DSH). Cats have extra lower back and thoracic vertebrae. Cats
also have distinct features. They have special free-floating clavicle bones that attach forelimbs to their shoulder. These features allow cats to fit their bodies and heads into any space.

    Cats use different kinds of sounds to communicate. They produce a wide range of sounds including purring, meowing, growling, trilling, hissing, chirping, squeaking, clicking, and grunting. The exchanged sounds are used by mother cats and her kittens to communicate. They are also used between mating cats or to other species: Cats also use different sounds and gestures to protect themselves and their offspring. A mother cat can fight even a larger dog to protect her kittens. She usually hisses furiously and gives frightening warning by showing her claws and making her hair stand on end. 

"They are also used between mating cats . . . . " (The last paragraph) The underlined word refers to . . . .