BATTY SENSES: A greater mouse-eared bat senses its surroundings using sound.

F DESCHANDOL AND P SABINE/BIOSPHOTO/MINDEN PICTURES

STANDARDS

NGSS: Core Idea: LS1.D    

CCSS: Reading Informational Text: 1   

TEKS: 7.12A, 7.13A, B.12B P.7C

Sixth Senses

Some animals have extraordinary abilities that help them sense the world around them

ESSENTIAL QUESTION: What types of specialized senses might help an animal hunt at night?

When the sun goes down, creatures of the night go on the prowl. Predators like spiders, owls, snakes, and bats use the cover of darkness to sneak up on unsuspecting prey. Each of these animals has heightened or unique senses, unlike anything humans possess, that allow them to navigate in the pitch-black surroundings. These adaptations range from being able to detect the tiniest of movements to homing in on the exact location of a sound. Such extraordinary abilities help the predators know precisely when to strike to snag a midnight snack. Read on to find out more about these nighttime hunters.

The sun goes down, and creatures of the night go on the move. Predators like spiders, owls, snakes, and bats use the cover of dark. They sneak up on prey and surprise them. Each of these animals has sharp or unique senses, unlike anything humans have. These adaptations allow them to find their way around in the pitch-black surroundings. For example, one adaptation helps animals detect the smallest of movements. Another helps them spot the exact location of a sound. These amazing abilities tell the predators just when to strike to grab a midnight snack. Read on to find out more about these nighttime hunters.

SEEING WITH SOUND

A common myth is that bats are blind. But not only do the animals see with their eyes—they can also “see” with their ears. As bats fly around in the dark and search for insects to eat, “they generate a picture in their brains using sound,” explains Marianne Moore, a biologist at Arizona State University who studies bats. Bats do this using echolocation (see How Echolocation Works).

A common myth is that bats are blind. But the animals can see with their eyes, and they can “see” with their ears. Bats fly around in the dark and search for insects to eat. At the same time, “they generate a picture in their brains using sound,” explains Marianne Moore. She’s a biologist who studies bats at Arizona State University. Bats use echolocation to do this (see How Echolocation Works).

A bat uses its mouth or nose to produce sound waves so high-pitched that humans can’t hear them. The sound waves travel through the air and bounce off anything they encounter in the bat’s environment. The echoes travel back to the bat’s large, sensitive ears, providing detailed information about objects. “A bat can tell which direction an insect is moving, its size, and if and how it’s beating its wings,” says Moore. “Bats can even distinguish between insect species so they can choose which bug they prefer to eat—it’s pretty incredible.”

Even while flying among other bats producing their own vocalizations, a bat can pick out its own distinct echo—as well as find out a lot about its prey.

A bat produces sound waves with its mouth or nose. The sound waves are so high-pitched that humans can’t hear them. These waves travel through the air and bounce off anything they meet in the bat’s environment. The echoes travel back to the bat’s large, sensitive ears. This gives them detailed information about objects. “A bat can tell which direction an insect is moving, its size, and if and how it’s beating its wings,” says Moore. “Bats can even distinguish between insect species so they can choose which bug they prefer to eat—it’s pretty incredible.”

When a bat flies among other bats, they all produce their own sounds. But a bat can still pick out its own echo. And it learns a lot about its prey.

CHIEN LEE/MINDEN PICTURES

SEEING HEAT: A Schultz’s pit viper uses pit organs to see infrared heat radiation.

HEAT VISION

Some snakes can see invisible infrared radiation given off from the bodies of animals as heat. That helps them spot prey—even when it’s pitch black outside.

Some snakes can see invisible infrared radiation. The bodies of animals give it off as heat. That helps the snakes spot prey, even when it’s pitch-black outside.

Pit vipers and certain other snakes get this ability from hollow holes between their nostrils and eyes. These pit organs contain a membrane that hangs down, “acting like an antenna that receives infrared radiation,” says David Julius. He works at the University of California, San Francisco, studying how the bodies of organisms function.

Infrared waves travel through the air and make contact with the membrane of the pit organ. This membrane is covered in the same type of heat-sensing nerve cells that are on a person’s skin. An electrical signal travels from these nerves into the visual center of the snake’s brain. This allows it to visualize heat. Scientists don’t know exactly what snakes are seeing, but Julius says it’s likely similar to images people see when using thermal cameras or night-vision goggles.

What gives snakes this ability? Pit vipers and some other snakes have hollow holes between their nostrils and eyes. These pit organs contain a membrane. It hangs down, “acting like an antenna that receives infrared radiation,” says David Julius of the University of California, San Francisco. He studies how the bodies of living things work.

Infrared waves travel through the air and hit the membrane of the pit organ. This membrane is covered in heat-sensing nerve cells. The same type of cells are on a person’s skin. An electrical signal travels from these nerves into the visual center of the snake’s brain. This allows it to see heat. Scientists don’t know exactly what snakes are seeing. But Julius says it’s probably like images people see with thermal cameras or night-vision goggles.

STEPHEN DALTON/MINDEN PICTURES

FLYING HUNTER: A barn owl nabs a mouse meal.

AMPED-UP EARS

Most species of owls are nocturnal, hunting prey beneath the cloak of night. These birds have huge, sensitive eyes that help them see in low light. But they rely even more on their amazing sense of hearing to catch tiny rodents in the dark.

Most species of owls are nocturnal. They hunt prey under the cover of night. These birds have huge, sensitive eyes for seeing in low light. But they depend on their amazing sense of hearing even more. It helps them catch tiny rodents in the dark.

Barn owls have several hearing adaptations that help them pinpoint scurrying prey—even those hidden beneath a layer of snow. A barn owl’s face is shaped like a concave dish. “This shape helps gather sound,” says Charles Walcott, a former director of Cornell University’s Lab of Ornithology in New York. Concentrating sound amplifies, or increases, its volume.

A barn owl also has asymmetrical ears. One ear is located slightly higher than the other, allowing sound waves to reach one ear sooner. From this slight difference in timing, an owl can tell how far away it is from its prey. To locate the source of the sound, the owl continues to adjust its head. Once sound waves arrive at both ears at the same time, the owl knows its face is pointing in the direction of its prey and is locked onto its target, explains Walcott.

Barn owls have several hearing adaptations to help them find moving prey. They can even detect prey hidden under a layer of snow. A barn owl’s face curves inward like a dish. “This shape helps gather sound,” says Charles Walcott, a former director of Cornell University’s Lab of Ornithology in New York. Gathering sound makes it louder, or amplifies it.

A barn owl also has asymmetrical ears. One ear is a little higher than the other. That way, sound waves reach one ear sooner. The small difference in timing tells an owl how far away its prey is. To find the source of the sound, the owl keeps moving its head. Finally, sound waves arrive at both ears at the same time. Then the owl knows its face is pointing in the direction of its prey and the owl is locked onto its target, explains Walcott.

HAIRY FEELERS: Magnified tactile hairs, like those of wandering spiders from the genus Cupiennius, help spiders sense tiny air movements around their bodies.

MOTION DETECTORS

All spiders have hair on their bodies, but its purpose isn’t to keep the animals warm. Most spiders have poor eyesight, so they rely on tactile hairs to provide information about their surroundings—day or night. The hairs detect air movements, like those caused by the flapping of tiny insect wings, as slow as 1 millimeter per second. “If anything moves around a spider, they’ll likely feel it,” says Eileen Hebets, an arachnologist at the University of Nebraska-Lincoln who studies spiders.

A spider’s exoskeleton, or hard outer shell, also contains thousands of microscopic holes. A thin layer of tissue covers each of these slit sensilla. The membranes act like tiny drums that vibrate when, for example, an insect crawls over a leaf that a spider is standing on. The vibration from the insect’s movements sends a signal through a nerve to the spider’s brain, alerting the spider to the presence of potential threats, prey, or mates nearby.

Such adaptations are particularly useful to nocturnal spiders, like those from the genus Cupiennius, found in Mexico, the Caribbean, and South America. These spiders don’t spin webs to catch a meal. As prey walk nearby, signals from the spiders’ tactile hairs and slit sensilla tell the spiders when to leap toward their victims. Experiments show that these spiders can capture prey even when their eyes are covered.

All spiders have hair on their bodies, but not to keep them warm. Most spiders have poor eyesight. They depend on tactile hairs to provide information about their surroundings, day or night. The hairs detect air movements. For example, they can tell when tiny insect wings flap, as slow as 1 millimeter per second. “If anything moves around a spider, they’ll likely feel it,” says Eileen Hebets. She’s an arachnologist who studies spiders at the University of Nebraska-Lincoln.

Spiders also have a hard outer shell, or exoskeleton. It contains thousands of microscopic holes. A thin layer of tissue covers each of these slit sensilla. The membranes act like tiny drums that vibrate. Say, for example, a spider is standing on a leaf. The membranes vibrate when an insect moves over that leaf. The vibration sends a signal through a nerve to the spider’s brain. That tells the spider that possible threats, prey, or mates are nearby.

Such adaptations are especially useful to nocturnal spiders. For example, spiders from the genus Cupiennius live in Mexico, the Caribbean, and South America. These spiders don’t spin webs to catch a meal. As prey walk nearby, the spiders’ tactile hairs and slit sensilla pick up signals. These tell the spiders when to leap at their victims. In experiments, these spiders captured prey even when their eyes were covered.

CORE QUESTION: Compare and contrast two adaptations given in the text that allow animals to sense prey at night.

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