to live healthily. Its ears are sensitive to low-frequency sound, which may be ideal for detecting sounds emitted by termites and ants underground. The pinnae are obscured and covered by hair, so predators cannot grab them in an attack, and prey or foreign material cannot enter, although ticks are known to reside there. The macula of the ear is very large compared to other animals, and is used as a gravity sensor to orient the echidna. The large size may be important for burrowing downwards. The leathery snout is keratinised and covered in mechano- and thermoreceptors, which provide information about the surrounding environment. These nerves protrude through microscopic holes at the end of the snout, which also has mucus glands on the end that act as electroreceptors. Echidnas can detect electric fields of 1.8 mV/cm—1000 times more sensitive than humans—and dig up buried batteries. A series of push rods protrude from the snout. These are columns of flattened, spinous cells, with roughly an average diameter of 50 micrometres and a length of 300 micrometres. The number of push rods per square millimetre of skin is estimated to be 30 to 40. Longitudinal waves are believed to be picked up and transmitted through the rods, acting as mechanical sensors, to allow prey detection. A well-developed olfactory system may be used to detect mates and prey. A highly sensitive optic nerve has been shown to have visual discrimination and spatial memory comparable to those of a rat. The brain and central nervous system have been extensively studied for evolutionary comparison with placental mammals, particularly with its fellow monotreme, the platypus. The average brain volume is 25 ml, similar to a cat of approximately the same size; while the platypus has a largely smooth brain, the echidna has a heavily folded and fissured, gyrencephalic brain similar to humans, which is seen as a sign of a highly neurologically advanced animal. The cerebral cortex is thinner, and the brain cells are larger and more densely packed and organised in the echidna than the platypus, suggesting evolutionary divergence must have occurred long ago. Almost half of the sensory area in the brain is devoted to the snout and tongue, and the part devoted to smell is relatively large compared to other animals. The short-beaked echidna has the largest prefrontal cortex relative to body size of any mammal, taking up 50% of the volume in comparison