The social network of Risso's dolphins
Like humans, dolphins rely on their social connections. But the social habits of Risso’s dolphins set them apart, as Eric Wagner reports.
Start with a maths problem. A person on Ilha Terceira in the Azores is looking at the ocean from the edge of a cliff. If this person is sitting in a green plastic chair (as I happen to be), then he is roughly 65 metres above the sea, and the horizon is almost 30 kilometres distant.
He holds a pair of Pentax PCF WPII binoculars, 20x60 power, 2.2° angle of view. These give him a 38-metre field of view at a distance of 1,000 metres. The ocean has a three-metre swell, with force 4 winds from the southwest. Whitecaps are everywhere. What are the chances of seeing the one-metre-tall dorsal fin of a grey Risso’s dolphin four kilometres away? Probably not too good, right? Try telling that to Annabelle Kok.
“Maqo, Maqo, this is Annabelle,” Kok says over a handheld radio. (She is next to me, and her circumstances are the same as mine, except she is standing.)
“Go ahead, Annabelle,” Fleur Visser’s voice crackles in reply.
“Yes, I have Risso’s,” Kok says. “It looks like a group of five or six about four kilometres from shore, west of the lookout, probably three kilometres from you.”
“Great!” Visser says. A tiny speck of a boat roars to life. Visser stands in the bow, her knees absorbing the steady whump of the waves, her eyes sweeping the heaving sea. After a few minutes, scientist and cetaceans converge. Visser readies her tools – computer, hydrophone, compass, stopwatch, laser range finder, acumen – all of which she will need if she is to untangle the secret lives of one of the strangest dolphins in the world.
At three to four metres in length, Risso’s dolphins are medium-sized as whales go. Named after Antoine Risso, the French naturalist who first described them, they belong, with other marine dolphins, to the Delphinidae family of toothed whales. Although they lack the typical dolphin beak they have kept the inscrutable half-smile. They are found all around the globe, but since they are not enormous or dramatically social or easily reached by boat, they have managed to remain relatively anonymous – until now. That they entered the scientific ledgers as “fat grey fish” – their Latin name is Grampus griseus – might also have something to do with this.
But such subtle disparagement is a sign that scientists may have been too quick to skip over the finer details of their lives. Risso’s dolphins are not fish, nor are they unduly fat. (That said, some guides refer to them as “robust”.)
At birth they are a soft grey, the colour of smoke, but their bodies lighten as they age. This is partly because their skin colour fades, but they also spend a lot of time raking at each other with their teeth, whether in fights or as part of their day-to-day interactions. Elaborate patterns of scars cover their bodies, suggesting that Risso’s dolphins sometimes see other Risso’s dolphins as little more than canvases for their wild renderings. “It was like people thought they weren’t that interesting for a long time,” Visser says. “And then they looked at them more closely and realised they were interesting.”
Visser is a biologist at Leiden University, in the Netherlands. (She also heads her own organisation, Kelp Marine Research.) She is in her mid-30s and athletic, with long, straw-coloured hair often stiff with salt from a day at sea, and the cheerful squint of someone who spends a lot of time on the water. She is softly spoken and data-driven, but not at all averse to pitting her will against seemingly intractable obstacles, such as inclement weather. (“Maybe it will clear by noon,” she will say. No. “Maybe at 2.” No. “Maybe 4:30.” No. “OK, we might as well call it a day.”)
On hot, windless afternoons, when the sun beats down, she will hop into the ocean for a quick swim around the boat – an activity the full daring of which becomes plain when a hammerhead shark drifts past a little later. Visser was not drawn to the dolphins because of any great spiritual affinity. She was called to them by an item in a newspaper. She was studying for a degree in marine biology and was looking for a thesis subject when her grandmother sent her an article.
The story was about a pair of Dutch artists who, a few years before, had moved to Ilha Pico, in the Azores, 1,370 kilometres off the coast of Portugal. The pair had started investigating the dolphins they would see swimming in front of their home. They were looking for students to help.
For Visser, the opportunity was enticing. Little was known about the biology of Risso’s. In contrast to most dolphins they are deep divers, disappearing for 15 minutes or so several hundred metres underwater, usually in pursuit of squid. They thus prefer the deeper waters that are usually beyond the dip of Europe’s broad continental shelf.
There is no continental shelf in the Azores – all the islands jut almost straight up from the Mid-Atlantic Ridge – and so the dolphins will come within a few kilometres of land.
Visser went to Pico in 2003. Over the next five years she worked there for up to six months at a time. She and her fellow researchers would stay on the water for hours on end, trailing after groups of dolphins. They photographed them, noted what behaviours they could, kept track of who was swimming with whom. The more they watched, the more they began to suspect they were seeing social structures and behaviours that were unique among whales.
Biologists recognise two main social strategies among toothed whales: fission-fusion societies and matriarchal pods. The former are most common among oceanic dolphins, such as the bottlenose. Like flash mobs, dolphins might come together by the hundreds or even the thousands, either to feed or to socialise, sometimes for as long as a few days or as little as a few hours. Then they go their separate ways, either as individuals or in small groups. (Fission-fusion societies are found on land, too. Chimpanzees and orangutans follow this model.)
Matriarchal pods, on the other hand, are stable groups structured around one or two older females. Killer whales and sperm whales are the best-known cases. (Groups of elephants and spotted hyenas are similarly ordered around a dominant female.) In such pods, several generations of female relatives and their offspring form a single group, sometimes of up to 50 animals. These pods can have distinctive traits. Different groups of killer whales, for instance, communicate using their own dialects of clicks, whistles, and squeaks.
With Risso’s dolphins, Visser saw something different – not quite the flash mob but not quite the stability of a matriarchal pod. The dolphins did appear to form long-term groups, but the groups were diverse. Some clusters were composed only of larger, older adults – others, of mother-calf pairs. Sometimes these clusters would come together to form larger gatherings of 50 dolphins or more, but such events were temporary. The clusters, though, were stable – individuals might be sighted together for years. The clusters were also segregated by sex and age. Males stayed with males, females with females, adults with adults, and juveniles with juveniles. These, Visser surmised, made up the basic units of Risso society.
“Nobody had ever documented anything like that,” she says. Especially surprising were the clusters of mature males. Except for short-term youthful liaisons, males of most mammal species usually avoid each other.
Another day, another maths problem. A person is watching dolphins from a boat. The boat rides low in the water, and so the person has an effective sight distance of about half a kilometre. If Risso’s dolphins spend about 15% of their day at the water’s surface, and if they have a tendency to vanish only to reappear a few hundred metres away, and if you can only see their fins and a slim wedge of their backs anyway, then how will you study their intricate social workings?
Visser’s protocol combines the simultaneous monitoring of surface and underwater activities at the level of both the individual dolphin and its group. It is based on the focal follow, a method developed to study primates. “Most studies work at the level of the individual,” she says. But individuals, especially those of a social species, rarely make decisions in a vacuum, and so she scales her observations up to include the group. Behaviours among group members have to be coordinated – dolphins need help defending territory, or finding food, or caring for their young while they hunt. Interpreting the nuances of how an individual relates to the group, however, is tremendously difficult with whales.
I am with Visser, Kok, and Machiel Oudejans as they shadow six dolphins, all mature males. Visser knows this group well. She sees them often, going from one side of Terceira to the other. She calls them the “Usual Suspects”.
The sun is bright, the wind a breath, the sea glass. The Usual Suspects potter along at a pace somewhere between what Visser calls “slow travel” and “resting”. They move in tight formation, no more than a few body lengths apart. Some are so close together they almost touch, rising and falling in perfect unison.
“I think they just like each other,” Visser says of such pairs. “Risso’s can have close friends, regular friends, more distant acquaintances maybe – their social network.”
Oudejans guides the boat, trying to stay about 100 metres from the dolphins. Visser has chosen one that is easily identifiable – a hulking, scarred male. As the focal animal, he becomes the pivot she uses to evaluate the rest of the group’s behaviour. Every two minutes, when he surfaces to breathe, she will take an analytical snapshot, calling out a series of parameters, “Line 2, synchrony 1, milling 0, distance 0,” and so on. The line indicates whether the animals are swimming in single file or abreast; synchrony, the degree to which they have synchronised their movements; milling, the percentage of animals that are travelling in a different orientation than the majority of the group; and distance, how far to the nearest subgroup.
The boat chugs along, the sun beats down. Visser calls out her numbers, I type them into the computer. This goes on for almost three hours, the dolphins following a more or less unchanging course. It is curious. Dolphins in general are supposed to have such vibrant inner lives, and yet these males are, as Oudejans says at one point, “like the guys you see sitting outside a store, playing dominoes”. But then I consider: If a group of behaviourists were to record my movements every two minutes, they would see me perched against the gunwale of a small boat staring until my eyeballs ache and my skull feels packed full of cotton. I barely move, except to peck out a note on the keyboard, or stuff a handful of peanuts into my mouth, or grab a quick drink of water. The behaviourists could therefore be forgiven for thinking that I am not having the time of my life.
People and whales share a long history, but the systematic study of whale behaviour is still in its infancy. Little was known about them until they became commercially important. Whalers, for example, may have been the first to appreciate the strong bond a mother had with her calf, but only because it helped them catch the mother – she would never leave a harpooned calf. In the case of dolphins, stories of their antics as the stars of marine aquaria spurred more rigorous investigations into their sensory capabilities, the ways they communicated with one another, how smart they might be. But dolphins themselves remained otherworldly. Some researchers thought trying to communicate with them would be good practice should aliens ever land on Earth. In another instance, in the mid-1960s, American neuroscientist John Lilly gave them LSD to see how they would respond. (According to Lilly, both man and animal enjoyed some “wonderful trips”.) Biologists made their first tentative forays to study free-ranging whales in the late 1960s. When they learned they could identify individual animals by the unique markings on their bodies, fins, or flukes, a whole world opened up to them. Now biologists are taking advantage of high-tech gadgets such as GPS and acoustics tags to collect vast amounts of data, and using powerful computer algorithms to analyse them.
Examples abound. In Shark Bay, Western Australia, a group of bottlenose dolphins will cover their beaks with basket sponges for protection while they roust up fatty, nutritious bottom-dwelling fish from among the sharp rocks and corals. It is the only known case of tool use in whales. The sponging dolphins are all relatives — a recent genetic analysis traced the practice back almost 180 years, to a single female ancestor. But the behaviour itself is not genetically determined – it is learned. Females teach it to their daughters and, less frequently, to their sons. The importance of the social component became apparent when researchers applied a technique used to study human social networks. They showed that sponging dolphins are more likely to form cliques with other sponging dolphins than they were with non-spongers. It was, they wrote, “very much like a human subculture”.
The most provocative ideas about whale society have come from Hal Whitehead, a biologist at Dalhousie University in Nova Scotia, Canada. Whitehead has studied sperm whales since the 1970s, following their pods for months on end from a sailboat. After years of careful observation, he determined that groups of sperm whales had their own dialects of clicks, called codas. Moreover, these clans, as he called them, kept to their own territories, employed different foraging methods, and even tended to their calves in particular ways.
In 2001 Whitehead and Luke Rendell, one of his students, published an article in Behavioral and Brain Sciences in which they argued that, increasingly, evidence showed whale societies possessed what people might call culture. They defined culture as “information or behaviour acquired through some form of social learning”.
The research community – biologists, anthropologists, psychologists, philosophers – was intrigued, but sceptical. Critics contended that Rendell and Whitehead were trying to co-opt one of the world’s thorniest words by twisting its definition. They pointed to the challenges of inferring cultural processes from field studies alone. They wondered whether the scientists were making the leap from “complex behaviour” to “culture” too willingly.
“It was a lonely outpost of an argument,” Rendell, now at the University of St Andrews, says. If he and Whitehead had to do it over again, he says, they would be more cautious in their interpretations. But his overall conviction remains unchanged. “All evidence strongly suggests that cultural transmission is really, really important in the way these animals make the living they do,” he says.
In 2013 Rendell co-authored a paper in Science documenting the spread of a unique feeding behaviour through a population of humpback whales off the US East Coast. Humpbacks there had long employed a method called bubble-feeding, where they swim below schools of herring and blow clouds of bubbles around them. The herring panic and bunch into a tight ball. The humpbacks then rampage through, mouths agape and throats distended, gulping fish-filled water.
Then, in 1980, a lone whale modified the behaviour. Before blowing bubbles, he slapped the water with his tail. It was a practice better suited to catching sandlance, a type of small fish that bunches when startled. For the next eight years observers watched as more humpbacks picked up the practice until, by 1989, nearly 50% of the population had adopted it.
Rendell and his team surmised that the innovative whale was responding to an increased abundance of sandlance. But how did the behaviour spread? Did mothers pass it to their offspring? Was there any difference in transmission between males and females? Neither turned out to be important. The most significant factor was whether a humpback was seen with another humpback that was known to lobtail. It was the first time anyone had documented this type of cultural transmission through a social network, and also explained its value. “The animals,” he says, “were simply learning from individuals they had spent a lot of time with”.
As more of these long-term projects start to mature, Rendell predicts the rate of insights will accelerate. Visser, Oudejans and I are following a group of 10 to 12 Risso’s, all mothers and calves, as they languidly trace the coast of Terceira. We have settled in for what seems to be another marathon of not-too-much, but after an hour or so the dolphins start to distance themselves from one another. It is subtle at first, the space between them growing little by little, but yes, they are going off in pairs, giving each other room. Dozens of metres soon separate them. At the wheel, Oudejans straightens up. Visser, too, tenses. “Watch carefully,” she murmurs.
Without warning, one of the dolphins surges forward, its tail pumping furiously. Its dorsal fin scythes through the water and then disappears in a roil of wash. “Torpedo!” Oudejans calls out. “Mark!” I have just started noting the relevant information when another dolphin surges forward, and then another, and then another, each of them racing through the water and then vanishing. After a couple of minutes the sea is empty and calm, save for the calves and one female that has stayed behind to babysit.
Oudejans hands me some headphones so I can listen to the hydrophone drifting behind the boat. Somewhere deep beneath us the Risso’s are now rocketing around in pursuit of squid. I put the headphones on and hear the steady staccato of their echo-locating clicks, which, Oudejans tells me, are the searching sounds they make as they hunt – a squeal means that one has zeroed in on some prey. The drama far underwater has an intense sense of mystery, the eerie sounds at once alluring and alien. I ask Visser what prompted the Risso’s to forage now, in this place, at this time. Their behaviour certainly looked coordinated – the dolphins all seemed to break apart at the same time. Visser says that, in the past, when she and Oudejans have listened to the hydrophone’s playback, they have heard no specific signal, and sometimes no sound at all. “Other social whales, yes, are usually very vocal,” she says. “But the Risso’s are often silent.” She smiles. “We are really only at the beginning of understanding what makes them decide to go at this moment, or how they know what the others are thinking.”
It brings up for me one final problem – how well can a person come to know a wild animal? Visser and Oudejans have roughly 20 years of experience with Risso’s dolphins between them. Earlier I had asked them to estimate how much they thought they knew about their subjects’ lives. “I would say about 50%,” Oudejans said. Visser demurred. “I think 20%. Maybe 10%.”
Far be it from me to contradict either of them, but I feel like it must be closer to 1%, if that. Most of what they have described constitutes the bare outlines – Risso’s are still cookie cutter dolphins. By degrees they paint on shades of colour. The Risso’s quick dash before the foraging dive is one such brushstroke. It is a small act – a prelude to the serious business of feeding. But like an anthropologist slowly coming to know the customs of a tribe, Visser is excited because it is one of the first times she and Oudejans have decoded a peculiar Risso’s behaviour. So far as anyone knows, no other whale does this. Observe the Risso’s for another 10, 15, or 20 years, and what else will they do that no other whale has done before?
I am in the throes of these reveries when the first dolphin pops back up. About 30 seconds later another bobs to the surface, and then another. Within three minutes or so, all the dolphins are accounted for. They reform and return to their idling pace, looking quite content to follow their present course for another few hours.
Visser hands me the computer. She hits the button on her stopwatch, starting the two-minute countdown. She fixes her gaze on the dolphins. “OK,” she says. “Back to work.”