They say the sea is cold, but the sea contains the hottest blood of all.
D. H. LAWRENCE, “Whales Weep Not!”
The sea churns with foam. From the surface emerge shimmering bodies as far as the eye can see. They are dolphins, leaping above the water, visiting our world for a moment before disappearing from view. Again and again they surface—thousands of sleek bodies pushing toward a destination we do not know....
This was my first view of dolphins in the wild. I was a student intern on a whale-watching boat off Cape Ann, Massachusetts. The sight of so many dolphins moving in unison was breathtaking. I almost forgot to record data, but I recovered and thus began documenting dolphin behavior for what will probably be a lifetime.— Kathleen
Common dolphins can travel in schools of thousands, even tens of thousands.1 These ocean mammals, true to their name, are found throughout the world’s oceans, and people often see them in vast numbers. Yet our view from above the water’s surface does not allow us to see or hear them in the three-dimensional ocean world in which they live.
When we see dolphins, it is typically like this: from a boat or from land, dolphins appear for a second or two, and the rest is left to our imagination. This can be exasperating for researchers, especially because dolphins exhibit what may be the most complex, unique, and intriguing communication systems of all animals. But, when we go beneath the ocean surface, where dolphins live, we can find out how they live, how they communicate and interact with others, and maybe even more about who they are as individuals. As we delve into the realm of dolphins, we learn that they are not just members of a group but unique individuals, each expressing a kaleidoscope of physical and behavioral characteristics.
Imagine for a moment that an alien scientist is examining an aerial view of people on a busy street sidewalk. The people all appear to look and act pretty much the same from this vantage point. Only if the alien approached and followed individuals would each person’s physical and behavioral discreteness emerge. And if this observer did not understand our language or could not hear or see in the same frequency range as us, an understanding of our world would be even farther out of reach. As aliens to a fully aquatic lifestyle, we dolphin researchers are in a similar situation. It is not always easy to admit how little we know about dolphin life; then again, that is what keeps our work so perpetually inviting.
Dolphins live in an environment that is foreign, even hostile, to humans, making their study and collection of data on their activity difficult at best. What keeps us coming back for more is not only the quest to understand the puzzle of dolphin behavior and communication but the excitement of glimpsing the world of another social being and the possibility of understanding another type of mind.
We have both attempted to gain a broader and more representative picture of how dolphins live in their underwater world. Our research represents two sides of the same coin. Kathleen investigates how dolphins interact with one another, and Toni focuses on how they interact with human swimmers, divers, and boaters. Our work requires us to spend extensive periods in the water with dolphins, observing their actions using underwater video cameras, still cameras, and writing slates. Although we focus on slightly different topics, our observations complement each other. This is true whether we have observed different species or the same dolphins in the same geographic area.
In The Bahamas, for example, we have both studied free-ranging Atlantic spotted dolphins, which travel in much smaller groups than their “common” cousins. From above the water, we see them swimming side by side as they surface to breathe. The underwater scene, however, tells a different story. These aquatic athletes sometimes swim frantically around one another, coming together to surface and breathe in unison, only again to split off from one another to resume their underwater speed ballet. Tracking the same individuals underwater often reveals a scattered, zigzag pattern, which requires us to follow the same pattern with our eyes as well as our swimming! Dolphins are excellent mimics and move in effortless synchrony. It is hard to keep pace with them. If a dolphin does not want to be near you anymore, she or he can vanish in an instant. Toni has observed dolphins swimming with humans in a manner similar to the way they swim with one another. When a dolphin is patient, the human and dolphin movements can be almost perfectly synchronous except a swimmer’s snorkel is breaking through the water’s surface alongside a dolphin’s blowhole. An interesting observation in this regard is that dolphins often seem to “accommodate” some human swimmers by letting the swimmers set the pace, even when the swimmers are much slower and less agile, and need air much more often.2 Similarly, Kathleen has noticed that dolphin mothers often accommodate their calves in a similar manner. The calves need to stay near the surface longer because they surface more often than mom to breathe. As calves mature, they develop the physique to dive deeper and to expel and inhale more air at the surface with each breath. But until they can do this, mothers often stay with their calves near the surface.
Atlantic spotted dolphins are about the length of Kathleen or Toni when we wear our fins. Large, fluid movements are often mimicked by dolphins when they swim nearby.
By merging our views from above and below the ocean surface, we forge the most complete picture of dolphin life. Looking at how they interact with humans and other species helps us understand how they communicate. Dolphins are highly social animals. They often care lovingly for their young for years and assist peers in distress in a manner exemplary even by human standards. Of course, there are exceptions: Kathleen has observed wild Indo-Pacific bottlenose dolphin moms who would not win any parenting awards for nurturing, attentive behavior. These mothers even had a higher calf mortality rate compared with other females in the group.3 The complexity of dolphin society is evident in how they fight with one another, as well as how they play, mate, feed, rest, and communicate.
Dolphins have long fascinated humans and are the focus of myths and fables in many cultures. Yet modern research has shown that many of these fantastical-sounding tales could in fact be true. And the complexity of dolphin society makes the facts even more fascinating than the myths. Some aspects of dolphin life are amazingly similar to that of other social animals—both aquatic and terrestrial, wild and domesticated. Observing them underwater allows us, as terrestrial social animals, to perceive their world through our senses. We eavesdrop on their lives and compare and contrast what we find to other social species, including humans. It is a moment where land and sea mammals meet . . . if only for a short time.
With their modified anatomy and streamlined form, dolphins are supremely adapted to life in the sea. Like us, they are mammals: they breathe air, are warm-blooded, suckle their young, and have hair before birth. But all of these things occur differently for dolphins than they do for land mammals. Dolphins are cetaceans, members of the order Cetacea. The cetaceans—whales, dolphins, and porpoises—are the most highly evolved, fully aquatic marine mammals, and make up 2 percent of the 4,600 living mammal species.4 Dolphins belong to the suborder Odontoceti, or “toothed” cetaceans. Toothed whales are divided into ten families grouped into three “superfamilies”: Delphinoidea, or oceanic dolphins, porpoises, and monodontids such as the beluga whale; Ziphoidea, or beaked whales; and Physeteroidea, or sperm, pygmy sperm, and dwarf sperm whales. The other suborder of cetaceans, Mysticeti, comprises eleven species of whales which have baleen plates instead of teeth.5 Toothed whales also differ from baleen whales in having a single (instead of a double) blowhole, a highly specialized echolocation system, and a pronounced forehead, or melon.
Toothed whales make up the vast majority of cetaceans. Approximately seventyone diverse species range from the relatively tiny vaquita, or Gulf of California harbor porpoise, which weighs in at about 120 pounds (about 50 kg) and is roughly 5 feet (about 1.5 m) long, to the well-known bottlenose dolphin, white beluga whale, and magnificent killer whale (the largest dolphin) on up to the largest toothed whale, the sperm whale, which can reach 55 feet (18 m) in length. The terms porpoise, whale, and dolphin are often used interchangeably, but size (specifically length) is the criterion anatomists have generally used to apply the common name whale. Porpoises, members of the family Phocoenidae, differ from dolphins in several characteristics. Typically smaller, they also lack a pronounced rostrum (beak) and have shorter, spade-shaped teeth as opposed to dolphins’ more conical, pointy teeth. (Scientific names of species mentioned in the text are listed at the back of the book.)
Dolphins of the family Delphinidae are found in all oceans, most seas, and some bays around the globe.6 Most species have a falcate (sickle-shaped) dorsal fin, cone-shaped homodont teeth, a pronounced beak or rostrum, and a gregarious social structure. They range in size from Hector’s dolphin, about 4.5 feet (1.3 m) long, to the killer whale, about 30 feet (about 10 m) long. Dolphins are found in almost every aquatic habitat on the planet—from coastal to deep-water pelagic ocean zones and from fresh to marine environs. Of the thirty-three species of oceanic dolphins, the best-known and most studied is the bottlenose dolphin (think of “Flipper,” of television fame). Bottlenose dolphins comprise the majority of captive dolphins in aquariums and are the species most often sighted along coastlines.7
As social mammals, dolphins and humans share many traits. We are both highly sociable and communicative, predatory, and intelligent, and we exhibit a variety of complex social relationships. Similarly, the cognitive abilities of dolphins are highly advanced.8 For example, dolphins can recognize themselves in mirrors; only humans, some of the great apes, and elephants have been demonstrated to share this ability with dolphins.9 In other words, although humans and dolphins lack a common ancestor, they have evolved similar cognitive abilities, possibly for comparable social or communicative reasons.
There is growing evidence that cetaceans have culture, similar to that observed in humans and other terrestrial (such as chimpanzees and elephants) and avian (such as parrots, crows, and ravens) species. The complex vocalizations and behaviors studied in different killer whale populations are evidence of distinct orca cultures.10 A form of cultural coevolution has also likely occurred between dolphins and humans in some regions of the world. For example, scientists have documented cooperative fishing on several continents between indigenous peoples and dolphins that appears to have developed over time across many dolphin and human generations.11
The orca (killer whale) is the largest dolphin, and the Hector’s dolphin is the smallest. The bottlenose dolphin is found in all oceans and seas, both near and offshore.
In an unexpected reversal of the typical path that humans and other terrestrial-animals followed, dolphins returned from land to the sea about fifty-five million years ago. There are numerous hypotheses why cetaceans resumed an aquatic lifestyle. The most widely accepted are that the ancestors of modern whales, dolphins, and porpoises returned to water to take advantage of an untapped ecological niche, to use and more readily find a prey resource not being exploited by other species, and to have access to another food source without competition from other land dwellers.12
To get a sense of when and how dolphins obtained their unique and fascinating communication and related physiological systems, we must look far back in time. The cetacean branch of the tree of life starts out about fifty-five million years ago, when the ancestors of today’s dolphins returned to the sea. From fifty-five to thirty-five million years ago, all ancient cetaceans are classified as Archaeocetes; at about thirty-five million years ago, the Mysticetes and Odontocetes—the two modern suborders of whales—first appeared.13 If you traveled back in time fifty-five million years, you would not recognize those early cetaceans as being similar to their modern relatives. Rather, the proto-cetaceans were on a genetic trajectory that would take them, over fifty-five million years, to the species we recognize today. In fact, current theory suggests that one of the modern dolphin’s ancient ancestors might have looked like a cross between a modern wolf, a cow, and maybe an alligator. Dolphins have had their current torpedolike, streamlined shape for about five million years. It’s hard to imagine this, considering that modern human beings have been around for only about two hundred thousand years. Perhaps this extensive time for adaptation to the sea is one reason why dolphins have developed such a sophisticated system for sharing information—that is, communication.
Of the six families of Archaeocetes, two were the Pakicetidae and Ambulocetidae. The pakicetids, the oldest members of the lineage, were probably more semiaquatic than fully aquatic and had hind limbs that were close to fully intact. The ambulocetids, the second oldest family, also had hind limbs that were much more than “limb buds,” but they were probably more aquatic than the pakicetids. The hind limbs of ambulocetids seem to be modified for an aquatic existence. As you move from fifty-five to thirty-five million years ago, and from pakicetids and ambulocetids through the other four families of Archaeocetes, there is increased reduction of the hind limbs and a more fully aquatic lifestyle.14 Over time, about nine million years, these large feet became permanent flippers called a fluke, or tail, that was (and is) used to propel cetaceans through the water. The basilosaurids and dorudontids (not dinosaurs, though the names sound like it), now extinct, are the oldest ancestors of cetaceans to show solid evidence of fluke swimming.15 Loss of hind limbs did not occur overnight but was a gradual progression as these animals adapted to an aquatic way of life; reduction of the hind limbs matched whales’ and dolphins’ need for functional locomotion in the water.
The anatomy of a dolphin readily shows its streamlined form.
Anatomical streamlining for life in the sea included other modifications to the evolving dolphin body plan. Most mammals have two joints in the thumb (if present) and three in each of the fingers. Dolphins have several elongated digits per finger, even though the external view of their “fingers” more closely resembles a mitten. Imagine your hand in a mitten, but place your thumb inside with all the fingers, so that your mitten is more of a “mitt.” A dolphin’s flipper is homologous to the typical mammalian forelimb, for example to the human arm and hand, but it is covered by a webbing of blubber and skin that helps it function as a paddle to assist with steering through the water. Evolution has also imparted cetaceans with an elongated, or telescoping, skull, as well as a skull that meets the spine at 180 degrees, unlike the 90 degrees of most terrestrial mammals.16 As the skull migrated from a right-angle connection, the cranial bones elongated, shifted, and migrated to different positions: for example, the nares (nasal openings) are now atop the skull, as opposed to in front of the skull, and act as a built-in snorkel. The dolphin’s torpedolike shape reduces drag when swimming and indirectly helps reduce heat loss. In addition, modern dolphins show no distinction between the vertebrae of their lower spine, specifically the lumbar, sacral, and caudal vertebrae. They have significantly more vertebrae than most other mammals, which assist with trunk-generated, or axial, propulsion. This is a stark contrast to most mammals and is actually much more comparable to that of a snake.17
Molecular DNA studies confirm that the cetaceans’ closest living relative is the hippopotamus.18 Let’s build some evolutionary perspective: think about hippos and their amphibious way of life. The hippo skull exhibits many traits in common with cetaceans: telescoping, upward positioned nostrils with flaps and a blubber layer under the chin for conducting sound.19 Hippos can produce and receive sounds amphibiously, that is, both above and below the water’s surface.20 The hippo typically situates its ears, eyes, and nostrils above the water’s surface, keeping its mouth and throat underwater. Researcher William Barklow distinguished nine categories of hippo sounds that were broadcast simultaneously in air and water.21 Behavioral responses to other hippos and to playback experiments suggest that these social animals react to both the underwater and the in-air components of amphibious calls. They probably use sound to mark their territories and to mediate social confrontation. The ability of each hippo to know the location and concentration of other hippos in an area probably facilitates efficient grazing and use of pools.22 A better understanding of how hippos use their sounds to coordinate social activity might provide insight into the mechanisms by which acoustic communication evolved in their modern-day oceangoing cousins.
Mesonychid condalarths are an extinct sister group of modern cetaceans. These wolflike carnivorous mammals inhabited coastal waters. Both physical appearance (morphology) and molecular data indicate ancestral ties among artiodactyls (modern even-toed ungulates), other even-toed ungulates like the mesonychids, or the modern-day cows or hippos, and dolphins.23 The multichambered digestive system of dolphins and ungulates, such as the hippopotamus, elephants, deer, and cattle, are examples of overt anatomical similarities. Distinct links can also be found in their modes of communication, as we saw in the study of hippo sounds.
For many years, scientists did not know that dolphins vocalized in ultrasonic frequencies sometimes far above what humans can hear without the aid of electronic equipment. Early recording devices had a limited acoustic range. Only when recorders and hydrophones improved did researchers discover that dolphins were using ultrasonic frequencies. A similar discovery occurred with regard to baleen whales, many of which emit sounds far below what the human ear can perceive. So it was not surprising when biologist Katy Payne, a former whale researcher, discovered that elephants also vocalized outside the human hearing range.24 In the 1980s, she and her colleagues discovered that elephants communicate in infrasonic frequencies that are so low that humans often cannot hear them, although infrasound sometimes can be felt by the body! This discovery both relaxed and intrigued elephant researchers, who had been mystified by the pachyderms’ seemingly silent yet synchronous coordination of activities—even at distances of many miles. Elephants use sounds below 20 Hz to coordinate their movements and activities across great distances. We find that blue whales do the same thing with infrasound over hundreds of miles, allowing communication across ocean basins. It is intriguing to consider the range of species using similar vocalization methods, each adapted to specific anatomical and environmental needs.
As it happens, what humans have learned about sonar (ultrasonic frequencies) and applied to medicine helped me (Toni) write some of this book. On a research trip in The Bahamas, I broke my foot. Undeterred, the next day I entered the water with my swollen foot. I was with about a dozen other snorkelers. Two spotted dolphins approached us and headed straight for me. I felt an intense “buzz” of echolocation, which vibrated through my injured foot then dissipated as it traveled up my leg. The dolphins then moved on to investigate the rest of our group, as if my foot was the only interesting thing about me. No one else indicated that they were echolocated on during that encounter. Even though the dolphin sonar did not miraculously heal my broken bones, I was healed through the wonders of modern technology with a device that helps heal bone using ultrasonic frequencies similar to dolphin sonar.
The dolphin brain has been the object of popular wonder, speculation, and, until recently, limited scientific research. Here we focus on the evolution of the delphinid brain. Given that there is a fifty-five-million-year divergence between cetaceans and their closest living relatives, one would expect brain structure to be quite different between the two. Research has confirmed that many extinct cetaceans had relatively small brains.25 Over time, the cetacean brain increased in brain-to-body weight ratio and structural complexity, making them comparable to human brains in these two regards. Two significant evolutionary changes are often correlated with this advance. The first occurred near the origin of the Odontoceti from the Archaeoceti, about thirty-five million years ago. The second occurred approximately fifteen million years ago with the rise of the current Delphinoidae, the superfamily including Delphinidae and Monodontidae. Today, the absolute weight of the dolphin brain (when examined apart from its body) is a tad bit larger than that of the human brain.26 In fact, when considering encephalization quotients (EQ), it is interesting to note that the dolphin EQ is second only to that of humans in all the animal kingdom, including primates. Neuroanatomist Lori Marino and her colleagues have shown that the dolphin brain has a complex cortical cytoarchitecture, a point of comparison to the primate brain.27 Dolphins require complex auditory neural processing to garner information from echolocation and other acoustic communication. This could explain their large brain size, but there is currently no evidence that dolphins have an abnormally large auditory area in the cortex dedicated to echolocation processing. Dolphins also have sleeping and waking patterns that are dramatically different from most terrestrial mammals and that likely require additional neural processing.
The evolution of an increased brain size in dolphins has been compared with the cortical complexity of living primates. There must have been good reason to evolve such a metabolically costly organ; otherwise, one would expect many more animal species to have large brains. Determining what the pressures were to drive this increase in cortical processing ability is the key to understanding why relatively few species have large brains. An increase in overall dolphin brain size and complexity seems to have conveyed a selective advantage for survival in the sea. Causes for the observed increase in dolphin brain size may be related to social ecology and communication.28 Each dolphin taxonomic family, like those of many primates, is socially gregarious. Some species exhibit several convergent behavioral abilities such as mirror self-recognition, comprehension of artificial symbol-based communication systems and abstract concepts, learning, and the intergenerational transmission of behaviors (that is, culture).29
That both primates and dolphins have evolved large, complex brains is evident in our multifaceted social behavior and cognitive abilities. Why we have done this remains to be determined. Likewise, identifying how sophisticated, gregarious social behavior might be guided by complex brains remains to be examined. But the continued journey promises a rewarding outcome if we stay the course.
Dolphin biology permits dolphins to communicate—indeed to live and thrive— through a variety of sensory modes in a world foreign to the average human.30 Their eyes, ears, respiratory system, circulatory system, skin, and blubber layer, to name a few, are all essential to their successful communication. Dolphin eyes are located on the sides of the head. Each eye has about a 180-degree field of view because dolphins can “pooch out” their eyes to the sides. This ability gives them limited binocular vision when they look below their chin. Around Mikura Island, Japan, on days with calm seas, I (Kathleen) have watched dolphins speed-swimming upside down just below the surface of the water. After a few observations I realized that the dolphins were tracking flying fish just above the water’s surface. By traveling upside down and looking up, the dolphins could track the fish with their binocular vision. This foraging strategy seemed to ensure that the dolphins got dinner; when I was watching, the hunting dolphin never missed a flying fish as it reentered the water. Spherical lenses allow dolphins to focus both in air and underwater (human lenses are flattened).31 Dolphins do not have tear glands, but they have other glands that secrete a viscous solution that seems to protect the eyes from the effects of saltwater submersion. This solution may also reduce frictional forces on the eyes at high swimming speeds.32 One of the most unusual adaptations of the dolphin eye is its split, or double, pupil, which gives dolphins vision both above and below the water during daylight.33 In bright light conditions, the two dark pupils might also help with depth perception.34
Can dolphins see in color? We’re not sure. There is evidence both for and against a dolphin’s ability to see colors.35 Even though bottlenose dolphins have color receptor cells in their eyes, whether these receptors allow them to see color or simply enhance their ability to detect objects is unclear.36 The color receptors, or cones, may actually function for increased visual acuity in low-light and bright light conditions.37
Species that live in relatively clear water need good vision, which is important for visual exchange of signals.38 Visual communication occurs through use of an extensive variety of postures, positions, actions, and morphological features.39 Various species have different color patterns. Atlantic spotted dolphins are born without spots and gain pigmentation as they age, which allows for recognition of individuals as well as general age categories.40 Killer whales show distinct black-and-white color patterns between their bellies and backs.41 Dusky dolphins have a white belly that fades into gray sides and a black back, with a faded wide stripe or two for good measure.42 Dolphins use their color patterns during direct and indirect communication with one another as well as when hunting or corralling prey. For example, foraging dusky dolphins will flash their white bellies at fish schools while cooperatively corralling them into a ball. With this action, a dusky dolphin is using several signals—posture, behavior, and morphological traits. While swimming in pairs, spinner dolphins will rotate and tilt their bodies toward and away from partners, using their pigment patterns to convey information.43 The alternating presentation of the dark cape and white belly may allow synchrony among animals during dives and turns, especially when dolphins move as a group.
Dolphins can “pooch” out their eyes to gain more peripheral vision. Yet they have binocular vision (where both eyes see the same thing) only below their rostrum and throat. Each eye can see almost 180 degrees to its respective side.
Dolphin skin is smooth with a rubbery feel. It is exceptionally sensitive, highly enervated, and well vascularized. Dolphins slough their skin regularly, and the outermost layer, or epidermis, is renewed every two hours, almost nine times faster than the human rate.44 Dolphins are very sensitive to touch and are often observed touching or in tactile range of peers.45 Dolphins may touch flukes, pectoral fins, teeth, and rostrums to a variety of places on the body of another dolphin. As in other mammals, dolphin touch can be an intimate communication between individuals or a form of play or aggression. Some forms of behavior are distinguishable only by the ensuing context. As a human analogy, think about when men exchange pats on the back: on the football field, a pat on the back might mean “congrats, nice catch”; in a dark alley, a pat on the back might be a prelude to a fistfight.
As mammals, dolphins must have hair, and they do, but only before birth. If you look closely, hair follicles are present on a dolphin’s rostrum, but the hairs are usually gone. Lacking hair, dolphins would seem to have trouble staying warm in the water. The sea is a cold place for mammals, because water reduces heat from a warm-blooded body at least twenty times faster than air. Yet dolphins are able to maintain a healthy body temperature through a variety of adaptations: a decreased surface-area-to-volume ratio (accomplished through evolution by streamlining their shape); increased metabolic heat production (eating more); modified heat exchange systems (a circulatory system with radiator capabilities); and increased insulation (a blubber layer). Blubber not only insulates and improves dolphins’ hydrodynamic shape, but provides a storage facility for energy during periods of fasting. The lipid content of the blubber layer directly affects thermoregulation; dolphins found in colder climates have a higher lipid content as compared to species with a tropical distribution.46 Thus, for retaining heat, the actual thickness of the blubber may not be as important as the quality of the fat layer.
Resident killer whales are found in family groups called pods. All offspring stay with mom for life, with individuals recognized by their black-and-white patterns and family groups identified by vocal dialects.
Dolphins are thus well suited to staying warm in the sea, but can they get too hot? And, if so, how do they counteract their ability to retain heat? The answer lies in a mechanism called a biological radiator. The circulatory system of all marine mammals has a striking feature—a rete mirabilia (Latin for “miraculous network”).47 Though many animals have retia, these systems are particularly advanced in deep-diving marine mammals. The retia function as countercurrent heat exchangers, or radiators. These radiators maintain a heat differential between oppositely directed blood flows, which increases the amount of heat transferred. Heat is retained in the core around vital organs in cold conditions and transferred through the blubber and skin in warmer climates. These radiators are present in the flippers, dorsal fin, flukes, and genital area. The retia keep the testes and the fetus cooler than the rest of the body, a key factor for species reproduction and survival.
Dolphins can hold their breath for up to fifteen minutes and are streamlined for efficient, swift movement through the water column (a conceptual column based on the horizontal layering of properties within the water).48 When a dolphin dives, oxygen is shunted to smaller capillaries, the bloodstream, and the muscles. Oxygen is bound to myoglobin, a protein that binds four times more oxygen than does hemoglobin. Myoglobin is found in higher concentrations in the muscles related to locomotion, specifically in areas that produce greater force and consume more oxygen during swimming.49 The increased amount of myoglobin present in marine mammal muscles makes possible longer and deeper dives by dolphins as compared with land mammals.
Studies on dolphins’ swimming skills and deep diving capabilities have revealed that they do not actively flex their tail and back muscles when descending to or ascending from depth.50 The dolphins use pressure changes and gravity differences between shallow and deep water; they kick a few times and simply glide down or up, depending on their direction of travel. This “kick and glide” method allows dolphins to conserve energy and still dive deeply for long periods, particularly for foraging, socializing, or investigating something in their environment. Many scuba divers and snorkelers follow the same kick and glide movements with their initial descent during a dive as well as during their return to the surface.
Speaking of kick and glide naturally reminds me (Kathleen) of the power available in a dolphin’s peduncle, or tail stock. Mystic Aquarium, in Mystic, Connecticut, has a marine mammal rescue program through which dolphins that have come ashore are retrieved and, if possible, rehabilitated. One time when four Atlantic white-sided dolphins were stranded, I was helping to restrain a subadult (teenage) female being given fluids and food. My job was to hold down the spot where the flukes meet the peduncle—a dolphin’s business end. I was kneeling on the flukes with one knee on either side of her peduncle with my hands holding tight to the peduncle just in front of my knees. Not your typical kneeling posture, but it served the purpose, or so I thought until she decided she was not happy with the treatment. As she was receiving fluids, she lifted her fluke, and me, about a foot off the ground! I remember balancing on the narrow peduncle of that dolphin while trying to get the attention of the other volunteers to add weight to her fluke. It was quite comical in hindsight, no pun intended. As never before, I understood the power of a dolphin tail, whether in or out of the water.
The next time you visit the ocean, take a deep breath and dive down for a few seconds. You can use the kick and glide method to save energy. You will hear that the sea is far from silent. The underwater arena is noisy, and humans are ill equipped to identify where an underwater sound is coming from. We have evolved to hear on land and to obtain directionality to sounds and their sources on land. Sound-source directionality is determined by an internal calculation in which your brain compares the arrival time of the sound at each ear. Because water is far denser than air, the perception coefficient is quite different between the two. Sounds travel about four and a half times faster and farther in water than in air. Therefore, to locate a vocalizing dolphin underwater, humans would need to do one of two things: increase the size of our ears by four and a half times or increase the distance between both ears by this same amount. Dolphins, however, are perfectly adapted to the underwater acoustic environment. They can hear and produce sounds from about 1 kHz (kilohertz) to more than 120 kHz, depending on the species.51 In fact, dolphins have hearing and sound production capabilities that well exceed those of humans.52 The human hearing range is roughly between 100 Hz and 20 kHz.
Mountains, deserts, grasslands, coastal regions, cities, suburbia, and the plains are just a few of the varied habitats that humans call home. Similarly, our oceans, seas, and even a few rivers offer an ecological multitude of options that dolphins might call home. Some species are littoral, spending their lives near a coastline. Others are pelagic, visiting the shallow coastline regions only occasionally and instead preferring the open ocean, far from the beach. Dolphins are almost always found in groups. Littoral species are found in groups ranging from five to ten individuals, whereas pelagic species are found in herds numbering in the hundreds or thousands.
In the past decade or so, cetologists have begun trying to correlate dolphin social life, behavior, distribution, and group size with their habitat. Bottlenose dolphins are found in all oceans and seas, along coasts and offshore, so they encounter a wide range of habitats and prey items, and other environmental conditions. Bottle-nose dolphin feeding strategies vary according to the type and distribution of prey, which in turn is related to habitat.53 Near Hilton Head, South Carolina, bottle-nose dolphins engage in mud bank feeding.54 In this method, several dolphins will coordinate to chase a school of mullet onto the shore; there is much splashing and chasing and lots of fish jumping into the air. The dolphins beach themselves on their right sides and grab fish in their teeth before shimmying back into the water. In Shark Bay, Australia, a few bottlenose dolphins also practice beach hunting and seem to learn the process from relatives.55 In Golfo Dulce, Costa Rica, Alejandro Acevedo-Gutierrez observed bottlenose dolphins chase and capture fish (such as yellowtail jack) that were too large for them to swallow whole. As he watched, a dolphin would grab a fish by the tail and slam it against the water’s surface, breaking the fish into smaller, easier to eat pieces.56 Indo-Pacific bottlenose dolphins around Mikura Island feed on squid and small mackerel in the deep, productive water between Mikura and Miyake Islands. Closer to shore, these same dolphins snack on takabe and tobiuo. Takabe are a small schooling fish that the dolphins chase and capture regularly. Tobiuo are flying fish and require coordination and skill to catch: a dolphin will swim belly up just below the water’s surface, paralleling the flying fish’s flight pattern; when the fish tires or runs out of a sufficient air current and drops back into the sea, the dolphin is waiting with open jaws.
Although all dolphins are social animals, all dolphin societies are not the same. The smaller groups of littoral species merge and split more frequently when foraging, playing, socializing, traveling, and resting. Water temperature and depth, prey type and distribution, underwater visibility, and ocean currents are just a few of the variables that can differ for each species. The four species of freshwater dolphin in South America and India, for example, live in riverine systems that are often highly polluted, filled with particulate matter that reduces underwater visibility, and subject to heavy boat traffic. These factors, coupled with the dolphins’ aloof, shy behavior, inhibited scientific study until the past ten to fifteen years. It turns out that Amazon river dolphins use a lek-mating strategy reminiscent of some terrestrial hoofed mammals and birds, in contrast to the more promiscuous lifestyle exhibited by many oceanic dolphins.
Dolphins are usually found in groups ranging in size from three to four individuals to hundreds or thousands depending on the species and location.
Habitat not only affects a dolphin’s dinner time, menu choice, and even mating strategy but also how each dolphin behaves as an individual, at least for some actions and signals. For the past ten years, I (Kathleen) have been investigating how dolphins use their pectoral fins to share tactile signals. Why? I’m interested in deciphering whether static contact and rubbing with a pectoral fin have different meanings.57 I am also curious whether contact with different body parts has different meanings and whether initiator and receiver roles affect the meaning of flipper contact. While conducting research, my students and I discovered that Indo-Pacific bottlenose dolphins around Mikura Island, a dormant volcano with a rocky boulder shoreline, use pectoral fin contact in subtly different ways than do The Bahamas’ Atlantic spotted dolphins, which live over a white, sandy ocean floor. We often see spotted dolphins rubbing their bodies in the sand—maybe to scratch a nagging itch or slough off parasites. But the bottlenose dolphins at Mikura don’t do this. In the interest of science, as well as to gain a dolphin’s perspective, I rubbed my hands and arms on both bottom surfaces. As you would expect, the sand was smooth and soft, almost feathery, whereas the rock surfaces were so jagged and rough as to be almost untouchable without damaging my skin. I can fully understand why the Mikura dolphins rub their bodies on the rocks only when a thick, leathery mat of tosaka seaweed covers the surface. Thus, it also appears that the environment, in this case the sandy versus rocky bottom, might affect the expression or use of a particular behavior. Think about this: if you have an itch in the middle of your back and you are alone, then you might rub your back on a doorjamb or similarly smooth object. But, if someone is nearby, you can ask that person to scratch your back.
As we’ve mentioned, many dolphin species are gregarious, with both consistent and fluid social relationships.58 Bottlenose dolphin groups usually average ten to twenty-five individuals, but the gender, relative age, and identities of a dolphin’s companions often change throughout an individual’s life.59 Many small dolphin species’ societies have been compared with those of chimpanzees in having a fission-fusion dynamic. That is, dolphins spend a lot of time in small groups traveling, foraging, and playing and come together to form larger groups for socializing, coordinated foraging, and other activities. When finished, dolphins break back into smaller groups with the same or different membership as before. There are exceptions to this societal format in some dolphins. Larger dolphins lead somewhat different social lives. Bottlenose dolphins in Sarasota Bay, Florida, and in Monkey Mia, Shark Bay, Australia, have been studied for several decades and exhibit the classic mother-calf relationship, in which the young remain extremely close to their moms for several years. Bottlenose dolphins from Florida and Australia are known for the strong, durable friendships that male dolphins form, often lasting a lifetime.60 These male pairs form coalitions during the mating season and either coordinate or compete for access to breeding females.
Killer whales have a matriarchal society—young orcas stay with mom for life. Long-term studies have revealed three distinct populations of killer whale in the northeastern Pacific, the waters of British Columbia, Washington, and southeastern Alaska. Two different populations, or ecotypes, inhabit the same coastal waters, the “transients” and “residents,” yet they remain socially and genetically separate, differ markedly in seasonal distribution, social structure, and behavior, and even exhibit such different physical traits as the shape of the dorsal fin.61 The third orca ecotype typically remains offshore, and little is known about this population’s diet or behavior.
The behavior of free-ranging dolphins reflects a dynamic interplay of aggression, social and sexual interactions, alimentary and exploratory behavior, play, flight and predator avoidance, and assisted locomotion.62 Aggressive behavior among dolphins is not uncommon and may occur along with sexual behavior.63 In addition to reproduction, sexual behavior is involved in social bonding and dominance.64 Care by the mother or other assisting subadult or adult is frequently documented in most dolphin groups.65
Dolphins play throughout their lives, although juveniles play significantly more than adults. Dolphin play is defined as any behavior that is not directed toward the satisfaction of hunger, travel, or other biological requirements.66 Although scientists once held that free-ranging dolphins do not play as much as their captive counterparts, this conclusion was based on limited underwater observations of free-ranging dolphins.67 Atlantic spotted dolphins in The Bahamas and Indo-Pacific bottlenose dolphins around Mikura Island engage in daily play.68 The motivation to play in dolphins is in fact so strong that play with objects has been used as a sole form of positive reinforcement for training captive dolphins, as well as a form of environmental enrichment for dolphins when not interacting with their trainers.69
Play behavior serves many functions in social groups: asserting, determining, and establishing relationships and perhaps self-pleasure and development of motor skills.70 Play activities vary greatly in both form, function, and frequency relative to season, gender, individual differences, species, and social and environmental contexts.71 In dolphins, mimicry, manipulation of objects, chasing, bow-riding, and different types of jumping, turning, and tactile interactions have all been identified as play.72 An increase in motor activity that does not provide any other apparent benefit has also been identified as a primary characteristic of play behavior in dolphins.73
Rubbing flippers (pectoral fins) can be a greeting between dolphins that have reunited after a short time apart.
The study of marine mammals is a relatively new field. Studies of dolphin anatomy, physiology, distribution, population dynamics, ecology, and behavior have been ongoing for almost a century. Yet systematic quantitative data on delphinid behavior and communication have accumulated only within the past forty years or so.74 Data are gathered from a variety of viewpoints: surface observations from boats (including whaling vessels), land-based stations (such as cliff tops), aircraft (planes, hot-air balloons, blimps), and captivity and, since the early 1980s, underwater observations. Detailed observations are conducted on about a dozen of the more than twenty genera of dolphins (oceanic and freshwater) and porpoises that have been identified in the wild. Further, almost all of these studies have been conducted on coastal or near-coastal populations.75 Coastal bottlenose dolphins, harbor porpoise, and short-finned pilot whales are regularly studied in many geographical areas.76 Atlantic spotted dolphins in The Bahamas, Hawaiian spinner dolphins in Hawaii, dusky dolphins near Argentina and New Zealand, humpback dolphins off South Africa, and killer whales in several locations are currently being investigated extensively.77
With the exception of the studies of Hawaiian spinner dolphins, Atlantic spotted dolphins, and Indo-Pacific bottlenose dolphins at Mikura Island, most intensive studies have been conducted from above the water’s surface. Although many studies have been conducted on dolphins in captivity, results have not yet been extrapolated to free-ranging populations.78 Similarly, studies of dolphins in the wild can provide insight into observations of dolphins in captivity. In either setting, observing dolphin behavior is invigorating and thrilling for each of us. Imagine starting your day with the sights and sounds of dolphins instead of a cup of coffee.
In remembering an ideal dolphin day . . . I (Toni) recall being awakened by the sound of a research assistant whispering loudly, “Dolphins!” With the sun barely up, I throw on my bathing suit instead of my bathrobe. As second nature, I grab the underwater housing with camera prepared for action the night before. After placing a mask over my barely open eyes, I slip quietly into the ocean. This is my favorite way to study dolphins as well as to start my day. When waking up to dolphins, my mind becomes quickly alert from the adrenaline that, even after decades, provides a rush of amazement whenever I see these marvelous beings.
One morning, before entering the water, I noted lots of spinner dolphin mothers and calves close to shore. Although I wanted to observe their underwater behavior, I also didn’t want to interrupt their rest or other life-nurturing interactions that might be occurring. So my fellow swimmers and I intentionally swam away from the group, hoping to record vocalizations and watch them from a distance. I heard nothing, at least nothing within my hearing range. But I saw something approaching slowly underwater: at least half a dozen female spinner adults, each swimming with a fairly small calf. I quietly got the attention of my human pod and we dove slowly down together. The dolphins were moving so slowly that we could barely detect body movements from the adults, though the little ones propelled their flukes more noticeably. In our minds, the mothers seemed to be bringing their young to introduce them to us. They glided below us and with precise synchrony angled themselves to look at us with their left eyes. As I videotaped, I intentionally angled my body toward them in reflection of their posture, hoping they would sense that I was mirroring their actions. We swimmers found ourselves moving in synchrony almost effortlessly with one another and with the dolphins for several more dives, all slowly gliding together punctuated by the sound of almost simultaneous snorkel and blowhole breaths and the occasional click of a camera. To me, this was a spontaneous dance between humans and dolphins.
I (Kathleen) often feel the same when I watch dolphins interact with graceful ease through the water column. While making the movie Dolphins for imax theaters with MacGillivray Freeman Films, I used an underwater scooter to keep pace with a group of spotted dolphins. The feeling was exhilarating. For the first time, I could keep up with dolphins that arced through the water and spun on a dime. I led them in turns twice before I again was the one lagging behind, but even though I was not the leader of the pod anymore, the dolphins hung out with me for about two hours. Invigorated, excited, thrilled, awed, mischievous, and, ultimately, exhausted are only a few of the feelings that coursed through me that afternoon. I was invited to the dolphins’ dance and delighted by kicking up my heels for fun.
Our accounts represent more than encounters between humans and dolphins. Within each of us there is also an internal dance between two selves: one self is the dedicated, objective researcher who documents every nuance of dolphin posture, behavior, interaction, vocalization, and more with exacting precision; the other is the little kid who is simply enamored by dolphins and wants to cavort with them until the sun sets. As scientists, we have both a yearning and a responsibility to share something more than our personal experience of these incredible animals of the sea with the terrestrial world. Fortunately, over the years, we have learned to unite these different selves. Our desire to “join” the pod and play has been refined so that, rather than hindering our observations and subsequent analyses, it actually enhances and supports it.
