Hearing and the Cat
Humans can hear in the 15-20 KHz range, while cats can hear at much higher frequencies. Cats can hear at 65 to 70 KHz.. For instance, small cats rely on this ability to hunt rodents, which emit noises in the 20-50 KHz range.
Cats use their outer ears, called pinnae as directional amplifying antennae. This ability increases their capability to pinpoint the location of sounds. The serval, a fifty pound wild cat that lives on the savannahs of Africa uses this method to its utmost. The serval probably has the highest success rate in hunting by exactly homing in on its prey and striking quickly and with little effort.
The auditory bullae of cats also aid in detecting sounds. In mammals, the bullae are inflated bony projections of the skull that protect the ossicles of the middle ear. Each bulla has two air-filled cavaties an outer chamber formed by the tympanic bone and an inner chamber formed by the entotympanic bone. Some species have modified bullae that allows them to survive in specialized environments.
In the tiger, such acute hearing allows these big cats to distinguish the rustle of leaves in the breeze from the sound of an animal brushing through the undergrowth. It is thought that other animals can be identified from the sounds they make during movement, allowing the tiger to concentrate on preferred prey. Tests have shown that tigers can seek out a grazing animal with less than a five degree error.
When tiger hunting was a common activity some tigers became so wary of the noise of rifles being loaded that they instantly retreated. It seems that a twig breaking beneath a human foot, or the sound of a man breathing from within a hide, is also readily identified.
All subspecies of tiger have white spots or 'flashes' on the backs of their ears; the reasons for these remain mostly unknown.
It has been speculated that these markings are illusory 'eyes' to fool prey, or they may serve to discourage other predators who normally hunt and attack from behind.
Another suggestion is that young cubs use these to find and follow their mothers in tall grass. When a tigress drops into the stalking position, with ears flattened against her head, these spots are obscured so making it more difficult for the cubs to follow and ruin the kill. This suggestion is weakened by the fact that these spots appear on both males and females -- and males have nothing at all to do with the raising of cubs.
It is more likely that ear spots are a signal of aggression. A tiger under threat will rotate the ears in such a way that the spots can be seen from the front so providing a visual warning.