Insights
In the late 1960s, while studying northern elephant seals along the coastlines of Mexico and
California, Burney Le Boeuf and his team stumbled upon a remarkable discovery. They observed
that male elephant seals at different sites produced threat calls that varied in distinct ways,
particularly in their rhythm. This observation marked the first recorded instance of dialects in a
nonhuman mammal.
The northern elephant seal population had narrowly avoided extinction in the 19th century, with
the species’ survival depending on a small group that remained on Isla Guadalupe. As this small
population gradually expanded, the seals began to return to their former breeding grounds. It
was at these newly recolonized sites that Le Boeuf noticed that the vocal displays of males
differed more significantly from those of the original colony on Isla Guadalupe, particularly in the
pace of their vocalizations.
To investigate whether these vocal differences, or dialects, were stable over time, Le Boeuf and
his colleagues focused on Año Nuevo Island in California, where male seals were found to have
the slowest pulse rates in their calls. From 1968 to 1972, the researchers returned to this site
each winter to monitor the seals’ vocal patterns. Their findings revealed a gradual increase in the
pulse rate over time, though it remained noticeably slower compared to other colonies.
Interestingly, individual males maintained consistent pulse rates in their calls throughout their
lives, preserving a unique vocal signature. Yet, the overall pulse rate within the Año Nuevo
population was shifting. One possible explanation for this change was the influx of males from
other colonies. By the early 1970s, nearly 43 percent of the males on Año Nuevo had migrated
from southern rookeries where faster pulse rates were the norm. This led Le Boeuf and his
collaborator, Lewis Petrinovich, to propose that these dialects likely developed due to prolonged
isolation after the seals recolonized different breeding sites. The early settlers of Año Nuevo may
have, by chance, had slower pulse rates, setting the tone for the colony’s vocalization patterns. Conversely, colonies that initially attracted seals with faster pulse rates would have developed
faster vocal tempos.
As the population continued to grow and inter-island migration increased, it was expected that
the vocal differences across colonies would diminish, with all locations eventually converging
towards the average pulse rate of the original Isla Guadalupe population. In the decades that
followed, this hypothesis appeared to hold true, as the distinct geographical variations in vocal
patterns that were evident in 1969 gradually became less pronounced.
By the early 2010s, this phenomenon was further examined by researcher Caroline Casey, who
was studying the same northern elephant seal population on Año Nuevo Island. She observed
that the vocal patterns Le Boeuf had documented decades earlier were no longer present. Through more sophisticated statistical analyses of both historical and contemporary data, Casey
and Le Boeuf confirmed that while dialects had existed in the past, they had effectively
disappeared over time.
Despite the loss of these distinct dialects, modern male elephant seals display greater individual
diversity in their vocalizations, and their calls have evolved to become more complex. Unlike the
relatively simple drumming patterns of the past, which varied primarily in tempo, today’s calls
feature more intricate structures, including doublets and triplets.
This evolution in vocal complexity suggests that while the clear-cut dialects of the past may have
vanished, the communication system of northern elephant seals has continued to develop in
other meaningful ways. These findings highlight the fluidity of animal communication and
underscore the significant impact that population dynamics, migration, and social interactions
have on the evolution of vocal behaviors in nonhuman species.
