First comes the bat. The bat can avoid objects in absolute darkness at impressive speeds and even capture flying insects (sometimes employing a sort of somersault jump that requires incredible precision and coordination). The bat uses a high-frequency sonar system. We cannot hear it, but bats actually emit a (very high-frequency) call and then listen to its echo. We cannot hear it, but the call is very loud: the high energy is needed to maximize the range. The bat's sonar is a very accurate device: it can pinpoint a target with great accuracy even while the bat is traveling at high speed. This is possible because the sonar is used to paint a detailed picture of the surroundings. Hughes details the amount of information that is contained in the "call and echo" process and how the bat's brain picks up that information: the echo's delay is an indicator of the target's distance, the size of the object determines the loudness of the echo, the Doppler Effect allows to calculate the speed of approach, and the target can be localized by comparing the two signals arriving at each ear, The bat literally "sees" with its ears. Therefore Hughes illustrates in detail how the auditory system of the bat's brain is organized. Its organization is in fact specialized for processing the echo. For example, most of the brain is devoted to processing signals at the frequency that yields the loudest echo. The bat's brain is a sophisticated computer for comparing the ultrasonic calls and their echoes, and then inferring the state of the world. The most spectacular feature of this system is actually that the bat can recognize its own echo, out of the thousands of calls and echoes that are emitted by a swarm of bats.

(For the neuroscience of the echolocation of bats also see "The High-frequency Club" in Seth Horowitz's "The Universal Sense").

The biosonar is not exclusive to bats. Another environment that has very little light is the ocean. The bottom of the ocean is always dark. Mammals have more sophisticated brains than other species, and some mammals do live in oceans: dolphins and whales. Dolphins generate their sonar calls also through their nose, besides their larynx. What is significantly different between bats and dolphins and that some dolphin calls also serve as means of communication, and these "social" calls tend to be in the frequencies that are audible by humans. The dolphin call is also structurally more sophisticated (in terms of frequency components) than the bat call.

Migratory animals can orient themselves and navigate vast territories without any help from maps. An arctic bird (the tern) migrates from one pole to the other in what is the longest possible trip on Earth. Butterflies, salmons and whales are examples of wildly different species that are capable of accurate long-distance journeys (butterflies take more than a generation to complete the journey, i.e. those who begin the journey are not the ones that reach the destination).

Birds are equipped with a sixth sense for the Earth's magnetic field. They fly south in the fall and north in the spring. To accomplish their amazing long-distance feats, birds employ more than one technique. They are equipped with a sun compass and an internal clock (recognizing the position of the Sun is pointless if one doesn't know what time it is); and they are equipped with a celestial compass that can recognize the stars (or, better, the star that is at the center of the sky's nightly rotation). However, their compass is not a "polarity compass" (the common compass that always points north). Theirs is an "inclination compass": a compass about the inclination of the magnetic field relative to the force of gravity. This kind of compass is useful to figure out the latitude but it is useless to determine in which hemisphere you are (because it points to the nearest pole). This means that birds crossing the equator during their migratory journey must be able to switch the way they interpret their compass. Hughes speculates that evolution favored birds with an inclination compass because the ones with polarity compass got extinct during one of the many times in which the Earth's magnetic field flipped and the polarity reversed. This has happened 24 times in the past five million years. It turns out that birds have magnetoreceptors made of magnets in the nose.

Bees scout the territory for food, then return to the hive and communicate the location of food by performing a dance. The bees can observe the dance, decode the location and then reach that location. This process requires a combination of navigation and communication skills. The code hidden in the dance is actually the easy part: the way the bee dances conveys information about the distance of the location and its direction relative to the Sun. Watching the dance basically "programs" the bees to travel to that specific location. Bees know where the Sun is even when they cannot see it because their eyes can see ultraviolet sunlight. The pattern of polarized light in the sky depends on the position of the Sun, and the ultraviolet part of the spectrum carries the best information about the polarization of light. The photoreceptor of bees consists of cells that basically replicate the pattern of polarization in the sky: the better oriented the bee is relative to the Sun, the closer the match between the anatomy of its cells and the pattern of polarization, and the stronger the response that is generated by these cells. Just one 360-degree circle can tell the bee where the Sun is. Its compass is not magnetic but, in a sense, pure pattern matching and energy sensing.

Animals that live in water can use another source of information: electrical fields. Any living being swimming inside a body of water generates an electrical field. That electrical field can be used by other fish to detect who is swimming in the neighborhood. At the same time, some fish are capable of emitting their own electrical current. This current can be used for defense purposes but also as a sort of sonar (to navigate and detect prey). When the current is used as a weapon, it is just one of the many tools that nature provides animals to fight enemies. When the current is used for navigation, it is represents a novel sense. Fish with passive electroreceptors are capable of sensing the electrical field generated by other fish. Fish with active electroreceptors are capable of producing an additional electrical field and of sensing the changes caused in it by the presence of other fish. The passive electroreceptors are ampullary receptors of the skin (called "Lorenzini ampullae") that are common to all fish. The active electroreceptors are more specialized tuberous receptors. Ampullary and tuberous receptors detect different features of the electrical field, respectively low frequency and high frequency features. The ampullary receptors tend to be localized in one area of the body, just like a radar, whereas tuberous receptors are spread all over the body because detecting high-frequency features requires more careful examination of the field. One can speculate that an analysis of the electrical field is enough for a fish to know not only that there is something nearby but also "what" that something is. Different objects cause different variations kinds of field and different variations in the field.

The last part of the book is the least convincing. Here Hughes examines the sense of smell, particularly as it relates to pheromones. Pheromones are chemical messengers widely used in the animal kingdom to communicate all sorts of facts. Because they readily diffuse into the air, they can advertise the message to a broad population. The sophisticated social organization of insects (that are not capable of vocal communication) relies on pheromones. Pheromones are also commonly employed by mammals to influence sexual behavior. This is the only part of the book in which Hughes deals with human behavior too. Hence this is not an "exotic" sense, but just a sense that is popular with the media.

On the other hand, Hughes did not study the "sense" that seems to me the most peculiar: the ability to camouflage. A few animals are capable of chromatic response to the surroundings. They "see" the surroundings and, based on what they see, they are capable of changing the color of their skin. The ones that can do it on the fly have a skin covered with cells called chromatophores that contain pigments. The most sophisticated are not the chameleo (that changes color in response to mood changes) but the cephalopods (such as the octopus) that change color in response to the surroundings. Many fish, reptiles and amphibians also change color in response to a change in environment, although usually not on the fly and not so dramatically. Nobody knows (as far i know) how the animal turns a visual cue into a chemical reaction that changes the color of its skin.

Another interesting subject would be the "color sense". Not all animals see colors. In fact, most mammals are red-green colorblind with the exception of only a few primates, including humans. The bulls in a corrida don't see the red drape: they only see a moving cloth. The best color vision is in birds, fish, and some insects. Hugh Raffles' "Insectopedia" (2010) has a chapter on insect vision. Another extension of the vision sense is night vision, that many animals have. The physics of vision varies dramatically across the animal kingdom.

Finally, it would be interesting to read about the physics of luminescent creatures (fireflies, jellyfish, etc).

TM, ®, Copyright © 2011 Piero Scaruffi