What is the lecture mainly about? Why is the ground squirrel’s strategy used only for against rattlesnakes?

Listen to part of a lecture in an animal behavior class.
    Professor:
    Okay. Today we’re going to continue our discussion of animal communication. As we’ve talked about, the mode or method of communication has to be perceptible to the target of communication. Between members of one species, there is usually no problem as they speak the same language you might say. But how about between predators and prey, animals of different species?
    Well, let’s look at the California ground squirrel. These squirrels which live on the west coast of the United States have more than one type of predator and they use different strategies to ward them off. If ground squirrels are threatened by birds or mammals, they usually make lots of noise, alarm calls.
    But another predator, snakes, can’t hear, so ground squirrels have developed different physiological and behavioral strategies to defend themselves and their offspring against snakes, most commonly rattle snakes, and gopher snakes which are non-venomous snakes.
    First, adult ground squirrels have developed an immunity to rattlesnakes’ venom. This enables them to aggressively defend their nests whether the attacker’s venomous or not. They will kick dirt and pebbles in the snakes’ faces, and even attack them, biting enemy, taking a swipe at snakes. In other words, the ground squirrel turns the tables on the rattlesnake and puts it on the defensive side.
    The behavior I want to concentrate on is called tail-flagging. Tail-flagging is when the squirrel makes its tail bushier and flags it, waving it back and forth. It is very effective in scaring off snakes. It’s been proposed that this waving tail reminds the snakes of past encounters that they have with ground squirrels. Recently one group of scientists has discovered that there is an additional component to tail-flagging behavior when the predator is a rattle snake rather than a gopher snake. Now about rattlesnakes’ sensory abilities, in addition to their visual abilities, what you have to know is that rattlesnakes have heat sensitive organs on the sides of their heads. These enable the snakes to sense the presence of warm-blooded animals because of the heat they give off. Okay. This group of researchers discovered that ground squirrels have developed this ability to warm up their tails by several degrees in fact, and do so when threatened by a rattle snake. They probably do this by increasing the blood flow of the tail, not their whole body. Just the tail. And the effect on the rattlesnake?  Well, researchers have conducted a series of trials using robotic squirrels and they found the switch from predatory to defensive behavior in the rattlesnakes was much more pronounced when the robotic squirrels’ tails were heated than when they weren’t. They’re not entirely sure why, but they speculated that the heat makes the squirrel seem larger and therefore a more difficult adversary.
    This response is never used when the ground squirrel is threatened by gopher snakes because well, they don’t have the same organs. The response seems to have evolved just as a deterrent to rattlesnake predators and most interestingly is the first known example of anti-predator behavior that includes a thermo component.