How skulls vibrating help snakes to hear ?

  

Snakes have fully formed inner ear structures but no eardrum. Instead, their inner ear is connected directly to their jawbone, which rests on the ground as they slither. Previous studies have shown that vibrations traveling through the ground—such as the footsteps of predators or prey—cause vibrations in a snake's jawbone, relaying a signal to the brain via that inner ear. 

The mechanical vibrations induced in the head by loud airborne sounds that were just above the snakes' hearing thresholds. Skull vibrations were the same intensity as the minimum mechanical vibrations that the animals could sense. So instead of responding to sound pressure, snakes respond to vibrations transmitted directly from the air to the skeleton.

Young calls the work "extremely nice," but he notes that the team studied only one species of snake. "Given that there are almost 3000 types of snakes, the next question would be how this differs between them." Some snakes, he notes, are known to be better at sensing vibrations through the ground, so their ability to sense sound waves in the air might be reduced. Since many sounds are too weak to cause ground-borne vibrations that snakes can sense, having both abilities helps them detect a wider range of noises. Some salamanders and frogs lack eardrums, too, he notes, and they may listen in the same way snakes do.

Young also says that there are probably other ways that snakes are sensing vibrations in the air and the ground. "We know snakes have some special sense organs in their skin and their head that likely react to vibrations. And we have some evidence that they detect vibration along the length of their body," he says. "This is unlikely to be the final word on how snakes sense sound and vibrations." …Source