The Mystery of the Disappearing Elephant-Seal Dialects
A northern elephant seal needs to remember the calls of his rivals. An encounter between two males, fighting to control female harems, can be bloody—skin marked by an opponent’s canines, chunks torn from the trunklike nose, wounds on the chest shield. Such battles are rather rare only because less violent cues are often enough to deter an adversary.
Vocal displays are fundamental in these ritualized confrontations, and each male in the population has his own unique call that serves as an ID. “You can think of them as drumbeats,” says Caroline Casey, a researcher at the University of California at Santa Cruz. If a male can remember and recognize the vocal signature—characterized by this drumming rhythm—of those he has previously confronted, he can avoid energy loss in the best scenario, and death in the worst.
In the late 1960s, while studying the northern-elephant-seal population along the coasts of Mexico and California, Burney Le Boeuf and his colleagues couldn’t help but notice that the threat calls of males at some sites sounded different from those of males at other sites. “It was just so obvious. It would be like me distinguishing a dialect from people who live in Alabama as opposed to people who live around Boston,” recalls Le Boeuf, who has studied the marine mammals ever since and is affiliated with UC Santa Cruz.
That was the first time dialects were documented in a nonhuman mammal. Fifty years later, however, those dialects are lost.
Half a century ago, Le Boeuf and his colleagues were documenting the size of the elephant-seal population, which had shrunk to almost nothing during the 19th century. The population now counts more than 210,000 individuals, three times as many as in 1969. Such a dramatic expansion, after the species had been so close to extinction, might have affected how these males talk to one another.
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Little is known about the abundance of northern elephant seals prior to 1840. “There were apparently quite a lot of them,” A. Rus Hoelzel from Durham University told me, but “then they were noticed as a nice resource for oil.” As with other seals and whales, they were hunted for their blubber. The oil obtained from this thick layer under the skin served as fuel for lamps or was used to make soap. A single large male could provide up to 210 gallons of oil.
By 1850, northern elephant seals were scarce. Two decades later, individuals were barely seen, even on Isla Guadalupe, a volcanic, somewhat lonely island in the northwest region of Mexico, where most of the seals had escaped from poachers. It is, even today, not easily reached. But the remoteness of the place did not discourage collectors, whose interest in the species increased as the seals became rarer.
“They were keen to get samples,” Hoelzel said, adding that “1892 was probably the nadir.” That year, an expedition to Isla Guadalupe found nine individuals: a surprise, given that the seals were presumably extinct. The collectors killed seven of them for the Smithsonian’s museum collection. Decades later, Alfred W. Anthony, who had been part of the team, argued that this type of action was “considered justifiable at the time, as the species was considered doomed to extinction … and few, if any, specimens were to be found in the museums of North America.”
By the turn of the century, the population had been driven down to a tiny number—maybe 100 individuals, perhaps even fewer than 20. All the northern elephant seals that exist today are descendants of the small herd that survived on Isla Guadalupe.
The first decades of the 20th century proved to be kinder to the species, and in September 1922, the Mexican government sent a patrol boat to Isla Guadalupe “to post a large sign in both Spanish and English, informing those that might land at that point that a heavy penalty followed the killing or capture of any elephant seals,” Anthony wrote. A month later, the island was declared a biological reserve. The number of northern elephant seals has only increased since then.
As that tiny population grew, northern elephant seals started to recolonize former breeding locations. It was precisely on the more recently colonized islands where Le Boeuf found that the tempos of the male vocal displays showed stronger differences to the ones from Isla Guadalupe, the founder colony.
In order to test the reliability of these dialects over time, Le Boeuf and other researchers visited Año Nuevo Island in California—the island where males showed the slowest pulse rates in their calls—every winter from 1968 to 1972. “What we found is that the pulse rate increased, but it still remained relatively slow compared to the other colonies we had measured in the past,” Le Boeuf told me.
At the individual level, the pulse of the calls stayed the same: A male would maintain his vocal signature throughout his lifetime. But the average pulse rate was changing. Immigration could have been responsible for this increase, as in the early 1970s, 43 percent of the males on Año Nuevo had come from southern rookeries that had a faster pulse rate.
This led Le Boeuf and his collaborator, Lewis Petrinovich, to deduce that the dialects were, perhaps, a result of isolation over time, after the breeding sites had been recolonized. For instance, the first settlers of Año Nuevo could have had, by chance, calls with low pulse rates (assuming that variation existed within the original colony on Isla Guadalupe). At other sites, where the scientists found faster pulse rates, the opposite would have happened—seals with faster rates would have happened to arrive first.
As the population continued to expand and the islands kept on receiving immigrants from the original population, the calls in all locations would have eventually regressed to the average pulse rate of the founder colony. In the decades that followed, scientists noticed that the geographical variations reported in 1969 were not obvious anymore. But nobody explicitly tested the differences among the multiple sites again. In the early 2010s, while studying northern elephant seals on Año Nuevo Island, Casey noticed, too, that what Le Boeuf had heard decades ago was not what she heard now. “It was an amazing opportunity to study how their vocal behavior had changed over the recovery from near-extinction,” she told me.
Casey and her collaborators teamed up with Le Boeuf to reanalyze the historic recordings and record the vocal displays of modern males in those same sites. By performing more sophisticated statistical analyses on both sets of data, they confirmed that dialects existed back then but had vanished. Yet there are other differences between the males from the late 1960s and their great-great-grandsons: Modern males exhibit more individual diversity, and their calls are more complex.
While 50 years ago the drumming pattern was quite simple and the dialects denoted just a change in tempo, Casey explained, the calls recorded today have more complex structures, sometimes featuring doublets or triplets. Think, she said, of each drumbeat-like vocal display of each male as a name. In the 1960s, the males would have had names such as Jan, Dan, or Sam, which would support recognition but still sound pretty similar to one another. That was presumably not a problem, as a male had “to keep track of five dudes in his little social network,” Casey said. These days, however, with many more males to encounter, there might be a Jan or a Sam in the population, but also a Gilbert or a Trevor. Without these new signatures, “it would be really difficult to distinguish everyone,” Casey added. The diversity of names—rhythms—nowadays allows modern males to keep track of their 25 to 30 competitors without making deadly mistakes.
The development of dialects in northern elephant seals was perhaps just a snapshot of a behavior happening during the first decades of population expansion and recolonization. In contrast with that geographical variation, the individual diversity we hear now might be more representative of the vocal communication of these marine mammals. “I think that what we are seeing today is probably more closely matched to the vocal behavior of males prior to the bottleneck,” Casey said. It’s only a guess, she acknowledged, as there are no records of how the males were talking to one another before the extermination driven by humans.