Humans make use of only a subset of the information available to us in the environment. Other animals have sensory systems that enable them to use different subsets and different types of information.
some species can see infrared and ultraviolet light.
When discussing vision, we use the term "visible light," but what we really mean is light visible to humans. Our visible spectrum is a very narrow region of the entire, continuous range of electromagnetic radiation in the environment (see Figure 8.5). We cannot see ultraviolet radiation, for example, but many other animals can.
One of the eight photoreceptors in each ommatidium of a fruit fly is sensitive to ultraviolet light. The visual sensitivity of many pollinating insects includes the ultraviolet part of the spectrum. Some flowers have patterns that are invisible to us, but show up if we photograph them with film that is sensitive to ultraviolet light. Those patterns provide information to prospective pollinators, but humans are not equipped to receive that information (though some other mammalian species, such as mice, are).
At the other end of the spectrum is infrared radiation, which we sense as heat. Other animals extract much more in formation from infrared radiation—especially that emitted by potential prey. Pit vipers such as rattlesnakes have pit organs, one just in front of each eye, that use highly sensitive heat detectors and a simple pinhole camera arrangement to sense and locate infrared radiation. In total darkness, these snakes can locate a mouse, orient to it, and strike it with great accuracy.
elephants can communicate with infrasound. Just as there are wavelengths of light beyond our visual capabilities, there are frequencies of sound that we cannot hear. A researcher observing elephants had the sensation that there was a throbbing in the air around her. When she obtained microphones and recorders capable of picking up very low frequencies of sound, she discovered that the elephants were communicating with sounds below the range of human hearing. The advantage of using low-frequency sound is that it carries over very long distances, so elephants in the wild can communicate even when very far apart.
echolocation is sensing the world through reflected sound.
Some species emit intense sounds that are above the range of human hearing, and they use the echoes from those sounds to create images of their environments. Bats, porpoises, and dolphins are all accomplished echolocators. Some species of bats have elaborate modifications of their noses to direct the sounds they emit, as well as impressive ear pinnae to collect the returning echoes. Although we cannot hear them, the high-frequency sounds they emit as pulses (about 20 to 80 per second) are extremely loud in contrast to the resulting faint echoes bouncing off small insects. An echolo-cating bat is similar to a construction worker who is trying to overhear a whispered conversation on a street corner while using a pneumatic drill. To avoid deafening themselves, bats use muscles in their middle ears to dampen their sensitivity while they emit sounds, then relax them quickly enough to hear the echoes. The ability of bats to use echolo-cation to sense their environment is so good that in a totally dark room strung with fine wires, they can capture tiny flying insects while navigating around the wires.
some fish can sense electric fields. The lateral lines of some fish species, such as sharks and rays, contain elec-troreceptors as well as mechanoreceptors. These sensory cells enable the fish to detect weak electric fields, which can help them locate prey.
The use of electroreceptors is quite sophisticated in species called electric fishes. These fishes have evolved electric organs in their tails that generate a continuous series of electric pulses, creating a weak electric field around their bodies. Any objects in the environment, such as rocks, plants, or other fish, disrupt the electric field, and the electroreceptors of the lateral line detect those disruptions. Such electroreceptors allow electric fishes to find food, avoid obstacles, and locate mates in extremely murky waters, such as those of the Amazon River. Electroreception can also play a role in social interactions. In some electric fish species, each individual in a group emits its electric pulses at a different frequency. If a new fish is added to the group, they all readjust their frequencies.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.