单选题 When you pat your pet dog, he wags （摆来摆去）his tail. That is his way of saying that he lovesyou. And, if you pay attention, you will see that he uses his tail to say so many things. Every move-ment of the tail means a different thing. If the dog is wagging its tail, it is a sign of friendliness ; if histail is straight, it means he is getting ready for a fight ; and if his tail is tucked （塞） behind his legs, itmeans he is giving up the fight. 

Unlike dogs, cats' tail language is not so expressive. When a cat feels threatened, he puffs him-self up to appear big and his tail shakes with tension. And when he is displeased with something, helashes out（甩动） his tail. 

The tail language of dogs and cats has a little story behind it. Earlier, when dogs and cats hadnot become friends with human beings, they were predators. They used to hunt other animals for theirfood. When dogs went out hunting with their friends, tail language came handy. When they were closeto each other, dogs could use facial expressions to talk. But, for long-distance communication, theyused their tails. Unlike dogs, cats liked to hunt alone. So, they did not need to use tail language toooften. As a result, their vocabulary in tail language is much smaller than that of dog's. 

In the last paragraph, the underlined word "predators" refers to __________ .