Senses
There’s been this enduring myth that snakes are deaf. ~ American biologist Bruce Young
Snakes lack external ears. They have only an ear bone connected to the mandible with no ear drum.
It was long presumed that snakes could detect only ground-based low frequencies. Instead, snakes hear sounds through the air via skull vibrations. The best reception appears to be between 80–160 hertz: the lowest cello notes.
The sensitivity of snakes to vibration is great. ~ American neurobiologists Peter Hartline & Howard Campbell
Generally, snakes are short-sighted. Most lack keen sight. All snakes lack eyelids. Limited snake vision may be accounted for by their descent from burrowing lizards.
Some snakes see well enough, especially nocturnal hunters, who have quite light-sensitive sight. Snakes generally detect movement better than form. What’s to see? If it moves right and is an edible size, that’s all a snake needs to know.
Little is known about how snakes see. One oddity is that snakes have a greater diversity of photoreceptor cell types than other vertebrates, indicating that they may get cross-referential visual information which compensates for direct lack of sharpness. Another peculiarity is that snakes control blood flow to their eyes to optimize sight when they need it most: during potential emergencies.
Whatever deficiencies in vision and hearing snakes may have are compensated for by adept senses of smell and heat sensitivity.
Smell
Snakes have a tremendous sense of a smell, courtesy of 2 physically independent systems: the nose and a vomeronasal organ (VNO): open sacs lined with sensory cells at the roof of the mouth (palate), near the nostrils. The nasal and VNO systems are somewhat disconnected, using different signaling pathways. A combined sense of smell comes together in the mind.
A snake flicks its forked tongue out, picking up airborne scents. The retreated tongue tips traipse into the VNO sacs, whereupon the scent sensation is passed to the brain via potassium ion channels and then mentally interpreted.
Some lizards, such as monitor lizards, also have this chemosensory system. Other animals also have a usable VNO, including salamanders, and several species of mammals: rodents, dogs, cats, pigs, goats, cattle, and elephants, along with several primates, such as lemurs, lorises, and some New World monkeys. Humans, while apparently having the genes, lack VNO genetic expression.
For many terrestrial vertebrates, VNO plays a key role in identifying potential sexual partners, aggressors, possible prey, or predators. VNO is often a secondary system for chemoreception. The common primary means of smell, and the only one people have, is through the nose.
Infrared Detection
Some snakes, including pit vipers, boas, and pythons, have infrared-sensing pits in the nose area. These facial pits are not directly sight-related, though infrared (IR) is a visible spectrum in some animals. In snakes, infrared is detected through heat sensitivity: incoming radiation warms an ion channel which triggers a nerve impulse. The pit membrane is then rapidly cooled by vascularization.
The portion of the snake brain active while processing IR-pit input is the optic tectum, which receives other sensory information, including optical and auditory stimulations, as well as motor and proprioceptive (body-related) perception. This physical centrality coincides with mental integration.
