HOW MANY SENSES ARE THERE?

by Whit Gibbons

May 29, 2005

We generally perceive the world through five major senses--seeing, hearing, touching, smelling, and tasting. Each is a mechanism for receiving information about our environment and collectively they are essential to survival. But in the natural world, many animals do not operate on the same sensory systems as we do.

A paper in BioScience magazine titled "The Behavioral Ecology of Insect Vibrational Communication," by R. B. Cocroft and R. L. Rodriguez at the University of Missouri, opens up an entire new world of a sense that we only have a hint of in our own sensory arsenal. Vibrational communication is a major component of detection of predator, prey, and mates among insects and spiders and is relied on as the exclusive means of communication by more than 150,000 species of insects. In short, when an animal moves along a tree branch, the ground, or in water, vibrations are created. Even though they may be imperceptible to our senses of hearing or feeling, some species have fine-tuned sensory detectors that allow them to decode the message and make decisions about how to react. Should they flee from a predator, attack prey, pursue a mate, or just remain motionless? The information is in the mix of vibrations and is as clear as a written page is to us.

We can broadly relate to the feat of sensing vibrations by putting an ear to the ground to "feel" a thundering herd of horses some distance away. I've seen this done in movies and presumably could do so myself, although I imagine the horses would be close enough for me to catch one and ride it before I would really detect them. A stethoscope is another form of vibrational detection. A beating heart does not make a noise in the conventional sense but is definitely evident with the aid of the stethoscope. Geophones are used to measure otherwise undetectable earthquake tremors or vibrations of machinery. And the James Bond-ish use of laser vibrometers to translate vibrations from human speech against a window pane into recognizable conversation reveals the capabilities possible among animals for detection and interpretation of vibrations.

The sensitivity of some species in detecting vibrational signals is amazing and is rampant among insects. Thousands of species communicate by drumming on plant stems or roots. The messages vary from advertising availability for mating or notifying close kin of a food source or potential predator. The sensitive nature of vibration signaling and detecting is difficult for us to comprehend but must be considered as effective for communication among some insects as speaking and hearing is for us.

Sensory detection mechanisms beyond the five major ones we are accustomed to are actually more diverse than we sometimes realize. Some birds, whales, and sea turtles are believed to use the North Pole's magnetic field to orient in a vast ocean with no landmarks. Duckbill platypuses are able to find their prey in murky water where vision is useless, because they can detect electrical currents created by muscular activity. The heat-sensitive pits between the eye and nostril of rattlesnakes, copperheads, and cottonmouths are capable of detecting infrared light. A rattlesnake can strike unerringly in total darkness at a warm-blooded target such as a rat. Some boas and pythons that eat birds and mammals have similar heat detection pits along their lips.

The most common means of communication among many animals is one that is difficult for us to relate to. Many insects, reptiles, and other animals produce chemicals called pheromones that convey messages. The detection of pheromones is accomplished by mechanisms more sensitive than our senses of smell and taste put together. For example, a snake moving toward a winter hibernation den in autumn may leave a chemical trail that can be followed by recently born snakes. Snakes use their forked tongues to pick up molecules that provide vital information about their environment.

The identification of vibrational communication as a major sensory system inherent among numerous insects can be attributed in part to human technology that has allowed us to detect subtle vibrations. Perhaps future technological advances will lead to the discovery of even more sensory mechanisms among the animals around us.



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