The puzzle of pilot whales
They strand on our shores in greater numbers than any other species of whale. Scientists believe they know why, but there is much about these animals that remains an enigma, and the strandings continue to happen.
Every year, their tragedy plays out on our shores. We see news footage of their sleek, coal-black bodies rolled and tumbled by the surf and strewn along sandy beaches like driftwood logs. We hear the piercing, plaintive whistles of their distress. In dozens, in hundreds, they lie bereft in helpless disarray.
People kneel beside them, propping them up, covering them with towels and bed sheets, pouring buckets of water on their lacquered skin. Some rest a hand on their slick flanks, say words of encouragement and solace. Marine mammal rescue experts in fluoro vests marshal the dozens of volunteers who have come to the beach to help. They manoeuvre the living animals on to inflatable pontoons and push them back out to sea. The dead are left for burial in the dunes.
We ask why. What kind of animal are these, which we rarely see except when they come to grief—increasingly a shared grief, theirs and ours. If we are so disposed, we might ask who these creatures are—warm-blooded like us, possessing brains of deep complexity and wondrous size. Who are these ocean minds?
They have been called blackfish, the howling whale, the bottle-head whale, but we know them as the pilot whale. They get that name from a misconception: that there is a leader, a cetacean navigator who pilots the group, even unto death. That’s not the case, but the name has stuck.
We say “pilot whale”, but, like the mako shark (or New Zealand’s freshwater eels, for that matter), they come in two types based on fin length. By far the more commonly seen species in cool temperate waters like ours is the long-finned pilot whale, Globicephala melas, which means “black with a bulbous head”. The short-finned pilot whale, which lives in warmer waters, fares even worse etymologically. Globicephala macrorhynchus means “bulbous head with a big snout”.
That cephalic swelling, so noticeable in pilot whales, is called the melon. It is a tissue mass of wax and fat that functions like the lens of an eye, only for sound, focusing and modulating the whale’s calls and clicks. All toothed cetaceans have this bulb. These animals live in a world drenched with sound. Sound is the basis of their navigation, their feeding, their communication, and some would say their culture. They use short-range echolocation clicks to hunt in total darkness, like bats, and long-range lower-frequency sound to interact with each other and with the ocean realm.
Baleen whales, the other great family of cetaceans, do not have an equivalent of the melon. They have no need to echolocate their prey. Jaws agape, they gulp vast shoals of krill and fish like living seine nets. But for them, too, the oceans are a boundless auditorium that they fill with voice and song.
Baleen whales rarely mass-strand. It is the toothed whales that beach themselves in large numbers, and New Zealand has one of the highest rates of herd stranding in the world. Almost half of the world’s 89 species of cetacean have stranded on our shores. The commonest species to strand is the pygmy sperm whale—though it is mostly individuals that strand. For mass strandings, however, no species comes close to the long-finned pilot whale. The largest stranding event ever recorded in New Zealand—perhaps in the world—took place in 1918. Though details are scant, an estimated 1000 pilot whales came ashore at Long Beach, on the western side of Chatham Island.
Between 1976 and 2000, 165 pilot whale stranding events were recorded in New Zealand, though only half of these were herd strandings; the other half involved single individuals. In all, 6000 whales stranded, and a third of these were successfully refloated. (See sidebar ‘Coming to grief’).
Why whales strand is an enigma that has perplexed people since ancient times. “It is not known for what reason they run themselves aground on dry land,” observed Aristotle 2500 years ago. “At all events, it is said they do so at times, and for no obvious reason.”
More than a dozen theories have been advanced, from the technical (geomagnetic anomalies) to the psychological (a subconscious longing for land). In 1991, Mark Brabyn, a marine mammal researcher with the Department of Conservation, compiled and analysed all New Zealand whale stranding records, looking for patterns and seeking a cause.
One of the patterns Brabyn found is that live strandings are clustered at certain “stranding hotspots”, while the beaching of individual dead animals occurs uniformly around the coastline (reflecting the widespread distribution of whales in New Zealand waters). This fact alone allowed Brabyn to reject several theories. For instance, if there were a land-seeking drive in cetaceans—a genetic memory based on the fact that the ancestors of whales and dolphins lived on land some 40 million years ago, before returning to the sea—then animals following this impulse would be expected to beach themselves without regard to location.
Similarly, if herd strandings are triggered by a “key whale”—a cetacean rangatira whose distress through getting into difficulties in shallow water induces the tribe to mass-strand in solidarity—then these occurrences should be widespread, argued Brabyn. In reality, they occur repeatedly in the same spots.
Could such locations be the cetacean equivalent of elephant graveyards? The legendary Jacques Cousteau thought it was possible that elderly whales, “sensing that death was near, summoned their last ounce of strength to cross thousands of miles and breathe their last on ancestral burial grounds”. But Brabyn argues that the whale-graveyard hypothesis cannot explain why young, healthy whales strand in large numbers, and not just ailing elders.
Brabyn also eliminated harassment by predators (such as orcas) and illness caused by parasites or disease as inconsistent with the clustered occurrence of herd stranding. “Sick whales should strand anywhere,” he noted.
Could whales be stranding because of navigational malfunctions based on anomalies in the Earth’s magnetic field? That theory was put forward by a British researcher who found that all of the 70 live strandings in the British stranding record occurred where geomagnetic anomaly contour lines ran perpendicular to the coast. However, no such relationship has been found in New Zealand. “Where the geomagnetic contour lines ran perpendicular to the coast, the number of strandings was similar to that occurring where the contour lines ran parallel to the coast,” noted Brabyn. “Whales seem oblivious to direction of geomagnetic contour lines.”
What emerges as the strongest factor in herd stranding is perhaps the most intuitive: that long, gently sloping beaches are natural “whale traps”.
The idea is that offshore species, unfamiliar with this kind of topo-graphy and its associated tides and currents, find themselves getting shallower and shallower, until it’s too late to retreat.
This explanation fits with data that show that it is offshore species that strand the most. Inshore cetaceans, such as coastal dolphins, are presumably knowledgeable about coastlines and the dangers they present, and rarely live-strand. (Hector’s dolphins, for instance, never strand as herds the way pilot whales do.)
But why should a gently sloping beach be such a deadly snare for pilot whales? These animals are, after all, experts in echolocation. Could they not readily detect the danger and beat a retreat to deeper water?
Researchers in Western Australia have offered a bioacoustics explanation of why toothed whales’ echolocation capabilities might let them down. They focused their study on Geographe Bay, near the south-western tip of Australia, where several mass strandings have occurred, including 320 long-finned pilot whales in 1996 and 120 false killer whales in 2005.
Along this broad crescent of coastline the slope of the beach is rarely more than half a degree, and often much less. Two metres of fall in a kilometre of distance from shore is not uncommon. The researchers argued that in such gently sloping bays an echolocation click projected towards the shore attenuates through multiple reflections to a point where it is not detectable to the animal that produced it. In essence, the whales send out a sonar “ping” but don’t get an echo—and this may suggest to them that there is no obstacle ahead.
“The reflections contain important information about the location of the shoreline,” the researchers write. “Successful detection of a shoreline from reflections may only occur at a point where the cetacean is at a high risk of stranding or has already stranded.”
Failure to detect the proximity of the shore—or detecting it too late—could result in confusion and disorientation and in navigational errors that trigger the stranding of the entire herd.
Compounding the problem of a super-low-angle shoreline, say the researchers, is the presence of microbubbles in the water—relatively long-lived tiny bubbles produced by rainfall, storm turbulence and even the respiration of marine algae. These could contribute further to the whales’ loss of navigational perception. They would be operating in an acoustical fog.
Many of the beaches where pilot whales strand regularly in New Zealand share similar topographical properties to those of Geographe Bay—Farewell Spit being one example. In addition, several common stranding beaches are bounded by protruding spits of land, which may add to the whales’ navigational challenge in finding a way back out to sea. A further contributing factor is that in such locations the tide often goes out rapidly. If the herd is already milling about in some confusion or uncertainty they might simply get stuck.
Suicide or altruism?
Physiological factors alone seem inadequate to explain some aspects of stranding, such as the fact that refloated whales, even when guided away from a death-trap beach, often show great determination to beach themselves again.
Observing this apparent death-wish has led some researchers to suggest that stranding is cetacean suicide, and that just as certain locations are hotspots for humans who decide to take their own lives (bridges, for instance), gently sloping sandy shores are “suicide beaches” for whales. Yet the suicide theory tends to fall apart when you consider the large number of whales that are successfully refloated. The rescue rate in New Zealand is now between half and three-quarters of all individuals that strand. From the few overseas studies that have tracked refloated pilot whales by satellite, these animals successfully rejoin herds and appear to resume their normal lives.
Rather than suicide, it is more likely that the bonds of social cohesion cause refloated whales to return to shore if others of their group remain in distress. It is for this reason that refloating stranded whales is now carefully managed to ensure animals leave the beach together, or in large enough groups to offset the impulse to return.
Some researchers believe that social cohesiveness is a key reason for the evolutionary success of cetaceans. It may have developed as a response to predators, suggests Brabyn. “Living in cohesive schools and developing supporting behaviour (social whales actively support sick members of their herd) enabled whales to overcome predatory pressures,” he notes.
These social bonds may even transcend species boundaries. In 2008, it was widely reported that a mother and calf pygmy sperm whale, which had stranded and been refloated at Mahia Beach, south of Gisborne, but seemed unable to find their way to deeper water, were rescued by a resident bottlenose dolphin. The dolphin, which locals had named Moko, appeared to lead the two disoriented whales past a sandbar and into a channel that took them safely out to sea.
Earlier this year, it was reported that humpback whales tried to prevent a pod of orcas from eating a gray whale calf that the orcas had just killed off Monterey in California. An earlier incident from Antarctica was cited in which a Weddell seal, fleeing from an orca attack, was carried protectively on the chest of an upside-down humpback whale until it could scramble to safety on to an ice floe.
Such behaviours have been interpreted as instances of interspecies altruism. Whether they are or not, evidence of social interactions within and between cetacean species continues to accrue. For offshore species such as pilot whales, gathering behavioural data is costly and demanding, but from what little is known about these animals, they appear to have an intricate and long-term social structure in which mothers are dominant. Herds can be up to 1000 strong, but these “super pods” seem to be temporary associations of smaller extended family units of a few dozen individuals. It may be that the smaller kin groups join up for feeding or breeding opportunities, or for enhanced security from predators—the same safety-in-numbers benefit that fish gain from schooling. Both male and female offspring remain with their mother, even after reaching sexual maturity (which for females is around eight years of age, and for males 12).
Older, post-reproductive females also remain with the group, and may play the role of nannies. Given that mothers nurse their offspring for three years or more, but need to make deep feeding dives that are beyond the range of their calves, having extra care-givers on hand could be vital. Female pilot whales may live into their 60s, while the life-span of males averages 35 to 45. Perhaps, as is the case with their land-based equivalents—elephants—older matriarchs are the holders of cultural memory, retaining knowledge of foraging areas and migration pathways and passing this on to the young.
The same matrilineal group structure seen in pilot whales is found in other cetaceans, including sperm whales, false and pygmy killer whales, melon-headed whales and rough-toothed dolphins—all species that are known for mass stranding.
Because of the association between social cohesion and stranding, Noumea-based marine mammal biologist Marc Oremus and three colleagues decided to analyse the DNA of 1033 whales from 12 pilot-whale stranding events in New Zealand and Tasmania between 1992 and 2006 to determine their kinship connections.
They were able to conduct this research because small samples of skin from the stranded whales had been collected by scientists and preserved in the New Zealand Cetacean Tissue Archive at the University of Auckland. This archive, one of the largest of its kind in the world, covering 36 species, is yielding important insights into the life histories of many poorly known cetaceans. (See sidebar ‘The library of flesh’).
The pilot-whale results were surprising. The stranded animals were more genetically diverse than expected, implying that matrilineal units were mixing freely within larger herds. Finding multiple “matrilines” within a single stranding “challenges the hypothesis that mass strandings are driven primarily by kinship-based behaviour”, the researchers say. If group cohesion is not based on kinship, it may have come about through reciprocal altruism—a term evolutionary biologists use to describe “you scratch my back, I’ll scratch yours” arrangements between organisms.
In addition to their findings on genetic diversity, the researchers discovered a curious anomaly: mothers and their unweaned calves did not strand in close proximity to each other, and in some cases the mothers of calves were not among the stranded whales at all. Oremus and his colleagues speculate that disruption of kinship bonds could be a contributing factor in stranding. Some kind of social breakdown through competition or aggression may lead to confusion, loss of navigational certainty and the eventual beaching of the herd.
This finding has implications for managing whale rescues. Rescuers have often inferred that a calf’s mother will be nearby on the beach, and when refloating occurs, the calf and adjacent adult females are kept together. If mothers strand at some distance from their offspring, that protocol makes less sense.
Research like that of Oremus and his colleagues tends to underscore how little we know about what is really going on in the ocean. No one is even sure what draws pilot whales to inshore waters in the first place. Are they coming close for food? It could be that they follow their target prey—squid and octopus, with occasional mackerel, herring, cod, turbot, hake and dogfish—into continental-shelf waters. The fact that pilot whales have been caught in jack mackerel trawls off the west coast of the North Island gives slight support to this possibility.
Perhaps the animals’ prey preference changes with the seasons or with their breeding cycle. But often the stomachs of stranded pilot whales are found to be empty, so other factors may be involved in the movement of herds from offshore to inshore—from the safety of distant seas to the dangers of the coastline.
There certainly seems to be a seasonal component to that movement, with animals coming closer to the coast in summer and moving away in winter. But inshore movement doesn’t necessarily correlate with frequency of stranding. In Northland, most strandings are in winter, perhaps associated with the season’s strong, persistent onshore winds that compound wave turbulence and environmental noise into a cacophony of echolocation problems.
In Golden Bay, on the other hand, most pilot-whale strandings occur in summer, between October and January, with January the peak month, perhaps pointing to an entirely different mechanism at work there.
If warm currents are bringing pilot whales close to shore in summer, where the animals travel during the rest of the year remains anyone’s guess. There has been no satellite tracking of pilot whales in New Zealand, and because their usual habitat is many kilometres offshore, finding them by boat is needle-in-a-haystack difficult.
Or so it was thought, until Jochen Zaeschmar, a graduate student at Massey University’s Coastal-Marine Research Group, took up the challenge and began to look. Zaeschmar was well positioned for the task: he operates a yacht-based ecotourism business in the Bay of Islands, a popular gamefishing destination where charter launches regularly travel 10 to 20 kilometres offshore in search of marlin and other trophy fish. Zaeschmar and his colleagues set up a tollfree number so that gamefishing skippers could report sightings of pilot and false killer whales (0800 FAR OUT), and the calls started coming in. As the calls came in, the researchers went out to investigate.
“The first surprise was that pilot whales are a lot more common than most people think,” Zaeschmar told me when I visited him on his steel-hulled ketch at Opua. “They’re not random strangers, coming only occasionally into the 12-mile zone. They’re here all the time, and the more time you spend out there, the more pilot whales you see.”
In part, Zaeschmar’s work on pilot whales arose because game-boat skippers and other boat operators kept misidentifying the species he was actually trying to study: false killer whales. The two species are easily confused, and Zaeschmar ended up with more pilot-whale data than false-killer-whale data as a result. So it made sense to broaden the research to include both.
False killer whales (also known by their genus name, Pseudorca) are much rarer than pilot whales, and they form long-term resident groups in New Zealand waters. After several years of observing and photographing them, Zaeschmar says he “knows them all now”.
“With pilot whales it’s the opposite,” he says. “Every time you see them, it’s a new group.”
There are occasional repeat sightings within a season, but, unlike the situation with false killer whales, it’s rare to see the same individuals from year to year.
“The standard social unit seems to be between 30 and 50 animals, but groups are constantly coming together then splitting apart again,” says Zaeschmar. “At sea, it’s not unusual for us to see 200 to 300 whales at a time. You wonder, ‘Are they together, or what?’”
These large groups aren’t always a picture of cetacean harmony. On several occasions, Zaeschmar has witnessed animals tail-slapping, jostling and head-butting each other. “You sometimes see blood,” he says. It’s usually males that are showing aggression, and it’s probably related to mating, he thinks. An overseas study has shown that male pilot whales are much more likely than females to have broken jaws. They may be using the melon as a battering ram to beat rivals into submission.
A further twist in the population dynamics of pilot whales is that some groups may be more migratory than others. “In the northern hemisphere, researchers have found that some pilot whales are nomads, while others are locals,” says Zaeschmar. “Maybe the same thing happens here.” It could be that the nomadic groups, lacking the familiarity with coastal conditions that the locals possess, are more likely to strand. That’s something Zaeschmar hopes future research will shed light on.
“As always, more questions than answers,” he says.
On a boat out of sight of land, it can be easy to think that all is well in whales’ oceanic universe. But increasingly the human world encroaches on the cetacean world. We assist them back into the sea when they strand, going to Herculean lengths to do so, but sanction the industrial-scale removal of their food by fishing trawlers and the sonic disruption of their habitat by military sonar, seismic surveying and ubiquitous propeller and engine noise—not to mention the human-induced global warming that is increasing the acidity and temperature of the seas they swim in.
Warming seas are already having an impact on pilot-whale populations in the northern hemisphere. Short-finned pilot whales—the warmth-loving member of the two pilot-whale species—appear to be moving into waters used by their long-finned relatives, and producing hybrid offspring with them. If the trend continues, the two species—which diverged more than half a million years ago—might eventually become one.
A more immediate concern is the impact of human noise on the animals’ wellbeing. There have been multiple international reports of military sonar causing the deaths of whales and dolphins. Necropsies of affected animals have shown haemorrhaging around the ears and gas-bubble lesions in internal organs, possibly the result of decompression during sudden ascents triggered by sonar disturbance.
At the less-traumatic end of the response spectrum, loud sounds or sudden changes in sound could be causing confusion and disorientation in whales and dolphins. University of Auckland whale ecologist Rochelle Constantine says that just as human hearing can be compromised by loud noise, temporary or permanent loss of hearing in whales can result from exposure to disruptive sounds under water.
“There are a lot of areas in the ocean where not just sudden blast trauma occurs—the kind that does physiological damage to the ears—but also less-deafening sounds that are still loud enough to cause chronic stress, disrupt group cohesiveness and force the animals to move away,” she says.
If marine mammals are having to leave feeding and breeding areas they have used for generations in order to avoid intrusive underwater sounds, the conservation implications could be severe.
The wider issue is that with so much noise in the sea now, whales may not be able to communicate so effectively. It would be like trying to have an intimate conversation at a noisy party. That could be a minor inconvenience, or a potentially life-threatening environmental change. We just don’t know.
What we do know is that just as our flightless birds, giant insects, ancient reptiles, alpine plants and Gondwanan beech forests are a unique and irreplaceable treasure that New Zealanders are called on to protect, so, too, do we have responsibilities towards the whales and dolphins that dwell in our seas. They come here in such numbers and in such diversity because our marine environment is a nexus of warm currents, cool upwellings, deep canyons, tall seamounts—a veritable melting pot of oceanographic features, all of which attract cetaceans and their prey. And our islands happen to lie in a direct line between Antarctica and the islands of the South Pacific—an ancient migration path of the great whales.
Whales returned to the sea 40 million years ago. We marvel at the consciousness they have developed. Perhaps the sea is inviting us to return, in our consciousness, to the place where life began—returning as guests and guardians, rather than as dominating masters. Returning not because it has something we want, but because it is something we need. Watch the tears in the eyes of volunteers at a whale stranding, or the rapture on the faces of guests on a
dolphin-watching cruise, and you will see a connection being made. Our seas and our selves—a mystery, a wonder, and a tie that binds.