🌟6TH GRADE ELA SBA 1 Informational (Dolphins) 25-26

A Day’s Work in Laguna

One by one or two by two, they arrive for work. Their job is to fish for mullet, a common species on this part of the Brazilian coast. Some of the workers carry nylon nets in crates attached to their bicycles. Others don’t need nets, because they use biosonar* and powerful fins and tails to fish. Arriving at Tesoura, a beach on Brazil’s Atlantic coastline in a town called Laguna, are two working communities—one of fishermen and one of bottlenose dolphins.

Standing waist-deep in Tesoura’s murky water, the fishermen cannot see the fish that they are attempting to catch with their nets. This is where the dolphins come in. One of the animals, whom the fishermen have named Scooby, herds a group of mullet into the shallow water on the shoreline. When the time is right, Scooby gives the signal—a slap of the head or tail—and drives the mullet toward a fisherman, who throws his net toward Scooby. The fisherman pulls in his net, which is hopping with fish. Meanwhile, Scooby feeds on the stragglers that didn’t make it into the net.

Most people around the world view dolphins as competition for fish, but the situation in Laguna is quite the opposite. For 120 years and possibly longer, the dolphins of Laguna have been helping local fishermen with their daily catch. This type of cooperation is extremely rare. On days when the dolphins aren’t around, the fishermen rarely even try to cast their nets—an illustration of how much the humans rely on their bottlenose partners. In fact, the fish caught with the dolphins’ help support about 200 families in Laguna, by both directly feeding them and helping them earn money. These people’s livelihoods would be at stake if the dolphins disappeared.

About 20 of the dolphins in the local population cooperate with humans in this way. The fishermen recognize these animals by their markings and behavior and have named many of them—Scooby, Filipe (a version of Flipper), Caroba, and more. Researchers who study the Laguna dolphins say it is unclear why some dolphins cooperate and others do not. In fact, even though the dolphins eat leftover fish that the men do not catch, some scientists say it is a mystery why any of the dolphins help out. Dolphins have all the tools they need to fish on their own, they point out.

But this cooperation has become a family tradition, passed from generation to generation. Both the dolphins and the fishermen teach their children how to do their part. In fact, the tradition has spread geographically, too. A few of the Laguna dolphins have shown the cooperative fishing method to dolphins in a community 150 miles away!

Researchers have found that the cooperative dolphins tend to spend time together even when they are not fishing at Tesoura. Complex social behavior and a high level of intelligence are common to all dolphins, but different animals—and different groups of animals—exhibit these characteristics in different ways. The Laguna dolphins happen to be unique because they cooperate not only with each other, as many of their relatives do worldwide, but also with people.

The cooperative Laguna spirit extends to the fishermen themselves. They work in rotating shifts so that no one will get an unfair share of fish. For example, once a man catches two or more mullet in his net, he leaves the water for the next fisherman to take his turn. In addition, many of the fishermen and other locals feel protective and even affectionate toward the dolphins. As it turns out, these incredible mammals are more than colleagues; they are friends.

*biosonar—the process of finding faraway or invisible objects by sending out sound waves, which get reflected back to the sender

The World’s Biggest Dolphin

People rarely use the word dolphin to describe the largest member of the dolphin family: the orca, or killer whale. This highly intelligent, social ocean mammal is notorious as a top predator. It hunts sea lions, squid, fish, seabirds, turtles, sharks, seals, and even larger whales. But the orca should also be well known for more friendly reasons, as we will see.

Perhaps one reason for orcas’ fame is their range. The killer whale can be found in all oceans, on the coastlines of all continents, and at all latitudes—including tropical waters. In fact, second only to humans, orcas are the widest-ranging mammals on the planet! And even though orcas live in salt-water habitats, sometimes they swim up rivers; some have been seen as far as 100 miles inland.

The enormous orca makes many other dolphin species look tiny. It measures up to 32 feet long—almost as long as a school bus and more than twice as long as a bottlenose dolphin. The largest adult orcas weigh in at 11 tons. One of those famous teeth can grow up to 5 inches long. Although the black-and-white patterns on most orcas’ bodies look similar at first glance, each animal has unique features, such as markings, scratches, tears, and scars. One of the ocean’s fastest mammals, the orca can travel at speeds up to 35 miles per hour.

Amongst themselves, orcas are cooperative and social creatures. They live and hunt together in groups called pods, each headed by an adult female. Orcas’ pods are considered the most stable in the animal kingdom, meaning that their members stick together. Since wild orcas live for about 50 to 80 years in the wild, a given pod contains animals of several generations. Each orca pod has its own unique language of clicks and other sounds, called a dialect. Members of a pod can recognize their own dialect from several miles away.

Killer whales have been nicknamed the wolves of the sea because of their close-knit social groups and cooperative hunting behavior. Different pods have different hunting techniques. Some force a group of fish into one area and dart in and out of the catch. Others beach themselves—swim up onto the shore—to scare seals into the water, where more orcas are waiting. But all orcas benefit from the natural advantages of cooperation, keen eyesight, and incredible speed.

Echoes in the Sea

Predators, or animals that hunt, are known for highly developed senses and abilities that help them find prey. For instance, eagles have excellent sight. Dogs and sharks have a strong sense of smell. Cats detect sound remarkably well. Dolphins, which often need to find prey in murky, cloudy water, have a unique ability called biosonar—also called echolocation.

On the simplest level, biosonar is the process of sending out sound waves and listening for the echoes that come back after bouncing off an object. The word echolocation explains it best: using echoes to locate something. Animals that use biosonar—such as dolphins and bats—are able to “see” objects that are otherwise invisible due to darkness, cloudiness, murkiness, or distance.

As a dolphin swims, it constantly sends clicking sounds out of its forehead—sometimes as many as thousands of clicks per second. The sounds return to the dolphin at different levels of faintness after objects reflect them. The dolphin’s clicks are timed precisely so that none of the sounds dolphins send interfere with the sounds they receive. Based on the echoes that enter the dolphin’s ear, the animal’s brain creates a picture of the objects it has located.

Imagine that you are swimming in water so murky that any object farther away than 4 feet is invisible. If you had biosonar, your brain would register images of several objects much farther away. And if you were to swim closer to one of those objects—in order to eat it, perhaps—the echoes would continue to tell you where you are relative to that object.

Biosonar provides dolphins with information that is much more specific than simply, “There is an object 150 feet away.” Dolphins can also locate nearby and faraway objects at the same time. And they can determine the object’s size, shape, density, speed, and identity—in some cases down to the species of fish. In fact, a dolphin’s sonar enables it to differentiate between a table-tennis ball and a golf ball.

Dolphins can even detect objects that are buried under the bottom of the ocean. The U.S. Navy Marine Mammal Program uses the help of dolphins to locate buried objects. It turns out that the animals’ natural sonar is more advanced than anything the navy can build.

Dolphins are not the only marine animals that use biosonar. All whales and porpoises have the same ability. It is the most valuable skill in these animals’ hunting “tool belt.”