This post is a slightly edited version of my December 29, 2004, post written in reaction to media reports about a "sixth sense" in animals, that supposedly allows them to avoid a tsunami by climbing to higher ground.
Every time there is a major earthquake or a tsunami, various media reports are full of phrases like sixth sense and extrasensory perception, which no self-respecting science journalist should ever use.
Sixth sense? Really? The days of Aristotle and his five senses are long gone. Even humans have more than five sensory modalities. Other animals (and even plants) have many more. The original five are vision, audition, olfaction, gustation and touch.
Photoreception is not just vision (perception of images) and is not a unitary modality. There are animals with capabilities, sometimes served by a separate organ or at least cell-type, for ultraviolet light reception, infrared perception (which is also heat perception as infrared light is warm), 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 (detection of sound) in many animals also includes ultrasound (e.g., in bats, insects, dolphins and some fish) and infrasound (in 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 (detection of smells) 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 released 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 inside our bodies, 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, such as 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 (including bed bugs) have infrared detectors. While snakes use this sense to track down prey, the insects like Melanophila beetles 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 (pdf).
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 to 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 also see an unusually large wave coming from a distance, although they would need the warning the least, considering they could fly up at the moment's notice. The "sixth sense" reports (in 2006) were from Indonesia and Sri Lanka - places worst hit by high waters. It would be interesting to know how the animals fared farther 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, 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 finding was never published, but just relayed from advisor to student, generation after generation, and mentioned in courses as an anecdote.
On the other hand, one study - "Mouse circadian rhythm before the Kobe earthquake in 1995" - described an increase, and another study - "Behavioral change related to Wenchuan devastating earthquake in mice" - a decrease in activity of some of the mice kept in isolation in the laboratories. With one study showing increase, one showing decrease, and one anecdotal account showing no change, the jury on this phenomenon is still out.
Mice (or the monitoring equipment) could have shown these patterns for causes unrelated to earthquakes. How much each of the three laboratories was isolated from outside cues (light, sound, substrate vibration, air pressure, radiation, etc.) is also not known but could have been quite variable - it is difficult to build a laboratory that is completely isolated from every possible environmental cue (and in circadian research light and temperature are key cues to isolate from, so many others are neglected).
The key difference here, of course, is between sensing the earthquake as it is happening somewhere far away (as the animals can certainly do), or the ability to sense small "foreshocks" that often precede the strong earthquakes, 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 use is certainly not just "sixth"...perhaps 26th or 126th (based on whatever criterion one uses for counting them) depending on the species.