The Dolphin in the Mirror (17 page)

The dorsal and pectoral fins of dolphins, like those of fish, function for stability, steering, and fine movements. (In dolphins, the dorsal fin and tail flukes are also involved in thermoregulation.) But dolphins use their fins for much more than hydrodynamics and locomotion. Dolphins will use one fin or another to touch, stroke, rub, caress, drag, slap, carry, and interact with other dolphins and a variety of objects. But even without using their fins, they can creatively manipulate their surroundings—as the bubble rings indicate. Here's another example.

In the early-morning hours at Marine World, after I arrived and before I headed to the research pools, I would generally pass the dolphin show pool just to check in on the other dolphins. I was often puzzled at the sight of a ring of river rocks encircling the central drain at the bottom of the pool. These smooth-surfaced eight- or nine-inch oval river rocks were strewn here and there on the pool bottom by divers so the dolphins could rub their bodies along them. The rocks were among a rich variety of toys and objects the dolphins received; there were also loofalike brushes and bumpy-surfaced mats attached to the pool walls for the dolphins' rubbing pleasure.

Yet overnight, mysterious rock circles were formed. A quick investigation revealed that the divers weren't responsible, so it had to be the dolphins' doing. How did the dolphins move these rocks? Rocks, like other objects, weigh less in water than out of it, so the problem wasn't the rocks' weight. The problem was technique. If they used their mouths, the dolphins would injure their teeth. Pushing with their snouts, the dolphins would injure their rostrums. But Stormy and the other dolphins had found a solution.

It took time, but finally we saw it. Stormy turned upside down, placed the top of her head on a rock, sucked it onto her blowhole, then turned right-side up and swam away with it atop her head. Ingenious! We saw some of the other dolphins carrying rocks this way as well. I never did determine why they were so intent on creating the rings. Perhaps it was a reflection of dolphin aesthetics.

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Bottlenose dolphins live in a great variety of ecosystems, so it isn't surprising that they find different ways of catching prey. But dolphins are opportunistic feeders who use a large number of foraging techniques that are not driven simply by the characteristics of the different ecosystems. Different subpopulations learn different feeding strategies from their mothers. Dolphin inventiveness plays a part too, or so it seems.

One of the more bizarre foraging specializations in bottlenose dolphins is called sponging. Rachel Smolker and Andrew Richards first saw evidence of this innovative practice in September 1984. They were out in a boat in Shark Bay, in western Australia, during the first year of what turned out to be a very long-term study of bottlenose dolphins in the area. After they'd observed a group of foraging dolphins, they were surprised to see a single dolphin briefly surface. Smolker described the incident in her book
To Touch a Wild Dolphin.
"There was the big, lumpy, reddish brown blob that seemed to be attached to the front of the dolphin's beak," she wrote, "extending back toward its face." Smolker and her companion concluded that the dolphin had some kind of disease, perhaps a tumor. "This poor dolphin was suffering from some horrid affliction and still having to forage to ward off starvation," wrote Smolker. "I wondered if it was painful, if the other dolphins might avoid it."
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Soon the dolphin surfaced again. Smolker and Richards identified it by its characteristic fluke. But this time the "big, lumpy, reddish brown blob," the supposed tumor, was absent. It turned out that the "tumor" was actually a conical-shaped piece of basket sponge that grew in deep channels in the bay. The dolphin had apparently harvested the sponge and, for some reason, placed it on its snout. During subsequent years, the research team on Shark Bay grew. And that dolphin became the first of a small number of female dolphins studied for their use of basket sponges for foraging.

Bottlenose dolphins in the Caribbean sometimes forage for fish that are lurking just below the surface of the sandy seabed by using their sonar to locate the prey. Once the dolphin detects the prey, it launches itself into the sand, often burying its head up to its eyes and beyond. When the dolphin pulls out with the fish in its mouth, the sand falls back and forms a crater. After a group of dolphins has finished what is aptly called crater fishing, the seabed takes on the appearance of a moonscape. The elite group of females in Shark Bay forage in the seabed too, in deep-water channels. But they can't adopt the strategy of their fellows in the Caribbean for two reasons. First, the sand in Shark Bay is extremely coarse and would scrape their beaks. Second: scorpion fish. These small, cryptic-colored fish are difficult to spot in the surface layer of seabed sand, and they deliver very nasty stings that any dolphin would want to avoid.

It is a challenge to make direct observations of dolphins using the sponges during foraging, because they are in deep, quite murky waters. And Shark Bay is called Shark Bay for good reason. Nevertheless, intrepid students, led by Georgetown University's Janet Mann, occasionally made research dives. They found that when they used the basket sponges as gloves on their hands, they could explore the seabed safely, ferreting out bottom-dwelling fish such as the spothead grubfish, which the dolphins in the area had been seen to eat.
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Mann and her colleagues suspected that dolphins used their sponged beaks to pounce on the grubfish. There are a few very blurry photographs of dolphins apparently doing just this.

Sponging is a solitary pursuit, and those individual dolphins that do it spend far more time searching for food than those that use other methods. Nonetheless, the females that engage in it produce just as many offspring as females that have more time on their hands, so to speak, so the extra time burden is not at the cost of their reproductive success.
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Mann and her colleagues believe that the solitary, time-consuming nature of sponging might explain why the practice is overwhelmingly done by females. Males, they say, are much too busy forming alliances with other males, plotting their future sexual conquests.

Sponging is a good example of tool use, an activity that is quite rare among nonhumans, mostly, but not exclusively, the domain of chimpanzees. Tool use in the oceans is even rarer than on land. Reports of tool use in a marine mammal such as the non-handed dolphin captured the attention of the media. When Mann and her colleagues published their scientific paper on the discovery of sponge-use by dolphins, reports of "first evidence of tool use in marine mammals" rippled through the popular press.

Genetic evidence suggests that sponging probably originated with a single female a handful of generations ago, a "sponging Eve." She passed it to her daughter through active teaching.
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Sponging is therefore a very good example of social learning; one is tempted to refer to it as an example of dolphin culture.

Most foraging innovations by dolphins are highly cooperative. Mud-plume fishing, for example, or mud-ring fishing, as it is also called, was first seen in Florida Bay, in the Florida Keys, and necessarily takes place in shallow waters.
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A group of around half a dozen dolphins patrols the waters in search of a school of fish. Once it is located, one of the group begins to swim in a large circle around the school, closing in ever more tightly. As the circle gets smaller, the ring-maker dolphin begins to beat its tail on the seabed, generating a plume of mud. The effect is a tightening net of muddy water that spooks individual fish to the point that they start to leap out of the water, where they are caught in the mouths of the waiting dolphins. Mud-ring fishing is very similar in principle to bubble-ring fishing in humpback whales, in which individuals swim in a circle below a school of fish emitting air from their blowholes, which creates a circular net of bubbles.

Dolphins (and whales) are masters at weaving variations on a theme, and so-called barrier fishing is a good example. It, too, involves trapping a school of fish by circling it, and then catching the fish as they try to escape. In barrier fishing, which has been seen off Cedar Key, Florida, the encircled school is driven toward a barrier of waiting dolphins. The trapped fish leap out of the circle, and many of them are promptly swallowed. A study of two groups of barrier-fishing dolphins in that area by Richard Connor and students at the University of Massachusetts at Dartmouth showed a division of labor between driver dolphins and barrier dolphins. In both groups there was just one individual that was consistently the driver dolphin, swimming circles around the fish.

Strand feeding is a cooperative endeavor as well, but it's very different from barrier fishing. I have observed this myself, as have many others, in muddy inlets in South Carolina and Georgia. It usually occurs within a few hours of low tide, when muddy banks are exposed. A small group of at least three dolphins patrols just offshore, looking for schools of fish, such as mullet. When they find one, they line up so the fish are between them and the muddy banks, and they begin to move cautiously toward the fish. Then, in unison, the dolphins surge to the shore, driving the fish up onto the banks. The dolphins follow the fish onto the banks, briefly beaching themselves, lie on their right sides to feast, and then wriggle back into the water. Rick Petricig, a graduate student at the University of Rhode Island, was the first to study this foraging strategy, which the dolphins apparently do both at night and during the day. It appears that the dolphins routinely strand themselves on the right side because the dolphin esophagus lies toward the left. Lying on their right sides, they can still swallow easily. Young dolphins learn this technique from their mothers, another example of social learning.

There are many, many ways that dolphins interact and cooperate. But their cooperation goes beyond their own species, and sometimes includes humans. I am not talking here about the swim-with-dolphins enterprises. I am talking about a form of cooperation that has gone on for at least two thousand years: a symbiotic relationship between dolphins and fishermen. It is a relationship in which both mammals benefit and learn from each other.

The earliest record of such an event was given by Pliny the Elder in the first century
C.E.
He wrote that fishermen would keep watch from the shore for passing shoals of fish that were too far out to reach from land, and if a pod of bottlenose dolphins happened to pass by at the same time as the fish, the fishermen would try to attract their attention by shouting, whistling, or slapping the water. Many years ago René-Guy Busnel, my professor in France, teamed up with the famed Jacques Cousteau to document this kind of activity in Mauritania. There, the fishermen didn't leave the arrival of the dolphins to chance; they engaged a shaman to pray and otherwise communicate with the dolphins so they would come close to the shore where the fishermen were waiting.
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Whatever brought the dolphins to the scene, their subsequent behavior was much the same wherever it occurred, according to independent reports over the centuries from the Mediterranean to North Africa to Australia: The dolphins moved toward the shore in unison, eventually trapping the shoal of fish against the beach. The fishermen used all kinds of means to collect their prey while the frantic fish leaped in all directions, and the dolphins grabbed their share by scooping them up from the air. It is barrier feeding, with humans attached.

However effective this system is, it pales in comparison with the choreographed operation that is initiated and controlled by dolphins near the town of Laguna, in the southern tip of Brazil. Karen Pryor, who led the study, described the whole affair as "highly ritualized, and appears to involve learned behaviors in both men and dolphins." The fishing takes place on the shores of an inlet from the ocean near the center of town. The fishermen, armed with circular throw nets rimmed with weights, line up in the water along the shore, or sometimes in boats. The dolphins patrol back and forth some fifteen to twenty feet offshore, facing seaward, looking for mullet. Pryor described what happened next:

"The dolphin reappears, usually in a few seconds, travelling toward the line of men. It comes to an abrupt halt and dives just out of net range, 5–7 m from the line, thus making a surging roll at the surface, a movement markedly different from normal respiratory surfacings. Men who are in front of the dolphin as it rolls then cast their nets ... The dolphins apparently take advantage of the confusion which the falling nets cause among the fish schools to catch fish for themselves."
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Pryor explained that the fishermen couldn't see the mullet in the water because it was extremely turbid. They observed the dolphins' behavior to know when and where to throw their nets. The men depended on the dolphins' cues, and "nets were not cast behind a dolphin, or in its general vicinity, but only in front of dolphins performing the correct behavioral sequence indicating the arrival of fish." The dolphins' rolls told the fishermen when to cast their nets, where to cast their nets, and, from the vigor of the movement, what size catch to expect. The entire arrangement is on a business footing: the fishermen catch the fish to sell, not for subsistence, and they do not attempt to feed the dolphins or touch them. Only about a tenth of the two hundred or so dolphins that live in and around the lagoon engage in this cooperative fishing. The fishermen recognize and have names for the ones who do and call them collectively the "good" dolphins.

According to town records, cooperative fishing has been practiced there since at least the mid-nineteenth century, with present-day fishermen saying that their fathers and grandfathers fished before them. And it seems very likely that the same familial pattern is true for the dolphins, because the practice is passed from mothers to offspring. (At the time of the study, in the late 1980s, one female, Chinelle, fished alongside two of her adult offspring and at least one grandchild.)