I hate it when they say “sixth sense”! Days of Aristotle and his Five Senses are long gone. Even we have more than five sensory modalities. Various animals (and even plants) have many more. The original five are vision, audition, olfaction, gustation and touch.
Photoreception is not just vision and is not a unitary modality. There are animals with capabilities, sometimes served by a separate organ or at least cell-type, for UV-reception, infra-red perception, perception of polarized light, not to mention the non-visual and extraretinal photoreception involved in circadian entrainment, photoperiodism, phototaxis/photokinesis, pupillary reflex and control of mood. The “third eye” (frontal organ in amphibians, or parapineal in reptiles) cannot form an image but detects shadows and apparently also color.
Audition in many animals also includes ultrasound (e.g., bats, insects, dolphins, some fish) and infrasound (whales, elephants, giraffes, rhinos, crocodiles etc. mostly large animals). And do not forget that the sense of balance and movement is also located in the inner ear and operates on similar principles of mechanoreception.
Olfaction is not alone – how about perception of pheromones by the vomero-nasal organ (and processed in the secondary olfactory bulb), and what about the nervus terminalis? Some animals have very specific senses for particular chemicals, e.g., water (hygroreceptors) and CO2. Gustation is fine, but how about the separate trigeminal capsaicin-sensitive system (the one that lets you sense the hot in hot peppers)? Chemoreceptors of various kinds can be found everywhere, in every organism, including bacteria.
Touch (somatoreception) is such a vaguely defined sense. In our skin, it encompasses separate types of receptors for light touch (including itch), pressure, pain, hot and cold. The pain receptor is a chemoreceptor (sensing chemicals relseaed from the neighboring damaged cells), while the others are different types of mechanoreceptors. Inside our bodies, different types of receptors monitor the state of the internal organs, including stretch receptors, tendon receptors etc. Deep in there, we have baroreceptors (pressure, as in blood pressure) and chemoreceptors that detect changes in blood levels of O2 or CO2 or calcium etc. Animals with exoskeletons, like arthropods, also possess tensoriceptors that sense angles between various elements of the exoskeleton, particularly in the legs, allowing the animals to control its locomotion.
Pit-vipers, Melanophila beetles and a couple of other insects, have infrared detectors. While snakes use this sense to track down prey, the insects use it to detect distant forest fires, as they breed in the flames and deposit their eggs in the still-glowing wood, thus ensuring they are there “first”. While infra-red waves are officially “light”, it is their high energy that is used to detect it. In case of the beetles, the energy is transformed into heat. Heated receptor cells expand and get misshapen. Their shape-change moves a hair-cell, thus translating heat energy into mechanical energy, which is then translated into the electrical energy of the nerve cell.
Several aquatic animals, including sharks and eels, as well as the platypus, are capable of sensing changes in the electric field – electroreception. More and more organisms, from bacteria, through arthropods, to fish, amphibians, birds and mammals, are found to be quite capable of sensing the direction, inclination, and intensity of the Earth’s magnetic field. Study of magnetoreception has recently been a very exciting and fast-growing field of biology.
On a more philosophical note, some people have proposed that the circadian clock, among other functions, serves as a sensory receptor of the passage of time. If that is the case, this would be a unique instance of a sensory organ that does not detect any form of energy, but a completely different aspect of the physical world.
Finally, many animals, from insects, through tree-frogs, to elephants are capable of detecting vibrations of the substrate (and use it to communicate with each other by shaking the branches or stamping the ground). It is probably this sense that allowed many animals to detect the incoming tsunami, although the sound of the tsunami (described by humans as hissing and crackling, or even as similar to a sound of a really big fire) may have been a clue, too. I am assuming that birds could have also seen the wave coming from a distance, although they needed the warning the least, considering they could fly up at the moment’s notice. The reports so far were from Indonesia and Sri Lanka – places hit the most. It would be interesting to know how the animals fared further away from the epicenter of the earthquake.
Which leads me to the well-known idea that animals can predict earthquakes. While pet-owners swear their little preciouses get antsy before earthquakes, all studies to date see absolutely no evidence of this. Animals get antsy at various times for various reasons, and next day get as surprised as we are when the “Big One” hits. When a strong earthquake hit California in the 1980s, a chronobiology laboratory looked back at the records of their mice and hamsters. Those were wheel-running activity records, continuously recorded by computers over many weeks, including the moment of the earthquake. No changes in the normal patterns of activity were detected. I believe that this was never published, but just relayed from advisor to student, generation after generation, and mentioned in courses as an anecdote.
The key difference here, of course, is between sensing the earthquake as it is happening somewhere far away (as the animals can certainly do) and the ability to predict earthquakes before they happen (which animals cannot do). So, I don’t think there is anything mysterious about the survival of animals in the tsunamis, and the sense they used is certainly not just “sixth”…perhaps twenty-sixth or hundred-twenty-sixth (or whatever criterion one uses for counting them) depending on the species.