Growing up, we learn that there are five senses: sight, smell, touch, taste, and hearing. For the past five years, The Scientist has taken deep dives into each of those senses, explorations that revealed diverse mechanisms of perception and the impressive range of these senses in humans and diverse other animals. But as any biologist knows, there are more than just five senses, and it’s difficult to put a number on how many others there are. Humans’ vestibular sense, for example, detects gravity and balance through special organs in the bony labyrinth of the inner ear. Receptors in our muscles and joints inform our sense of body position. (See “Proprioception: The Sense Within.”) And around the animal kingdom, numerous other sense organs aid the perception of their worlds.
Detecting Gravity and Motion
The ability to detect gravity and the body’s motion may be one of the most ancient senses. In vertebrates, the complex vestibular system handles this task via the otolith organs and semicircular canals of the inner ear. Invertebrates rely on a simpler structure known as a statocyst to sense their own movement and body position relative to the Earth’s gravitational pull. Even comb jellies (ctenophores), which may have been the first multicellular animals to evolve, have a rudimentary statocyst—essentially, a weight resting on four springs that bend when the organism tilts in the water. [...]
Feeling the Flow
Light, sound, and odors travel through water very differently than they do in air. Accordingly, aquatic animals have sensory systems tuned to their fluid medium—most notably, the lateral line system. Observable as distinct pores that run along the flanks and dot the heads of more than 30,000 fish species, the lateral line is composed of mechanoreceptors called neuromasts—clusters of hair cells not unlike those found in the mammalian ear and vestibular system—that relay information about the velocity and acceleration of water flow. [...]
Mollusks, insects, birds, and some mammals are able to sense Earth’s magnetic field, but how they do so remains a mystery. In the last couple of decades, “most of the research [has focused] on proteins and genetics in the various animals, speculating on possible means of magnetoreception,” says Roswitha Wiltschko, who—along with her husband, Wolfgang Wiltschko—ran a magnetoreception lab at Goethe University Frankfurt, Germany, until she retired in 2012.
Although the details are still unclear, most magnetoreception researchers have converged upon two key mechanisms: one based on magnetite, an iron oxide found in magnetotactic bacteria, mollusk teeth, and bird beaks; and the other on cryptochromes, blue-light photoreceptors first identified in Arabidopsis that are known to mediate a variety of light-related responses in plants and animals. [...]
Many animals are able to sense heat in the environment, but vampire bats and several types of snakes are the only vertebrates known to have highly specialized systems for doing so. Humans and other mammals sense external temperature with heat-sensitive nerve fibers, but pit vipers, boa constrictors, and pythons have evolved organs in their faces that the animals use to detect infrared (IR) energy emitted by prey and to select ecological niches. And vampire bats have IR receptors on their noses that let them home in on the most blood-laden veins in their prey. [...]
Sharks and other fish are well known for their ability to detect electric fields, with some species able to sense fields as weak as a few nanovolts per centimeter—several million times more sensitive than humans. But it turns out that they aren’t the only ones. In recent years, evidence for electroreception has been accumulating all over the animal kingdom: in monotremes (such as the platypus), crayfish, dolphins, and, most recently, bees. [...]