Almost 40 years ago, deep in the Pacific, a single voice called out a song unlike any other. The sound reverberated through the depths at 52 Hertz, puzzling those listening to this solo ringing out from the ocean’s symphony. The frequency was much higher than a blue whale or its cousin, the fin, leaving scientists to ponder the mystery of Whale 52.

The leviathan has been heard many times since, but never seen. Some suspect it might have some deformation that alters its voice. Others think it might simply exhibit a highly unusual vocalization — a tenor among baritones. But Marine biologist John Calambokidis of Cascadia Research Collective suggests another possibility: “The loneliest whale,” so named because there may be no one to respond to its unique call, may not be an anomaly, but a clue.

Calambokidis, who has spent more than 50 years studying cetaceans, suspects Whale 52 may be a hybrid: Part blue whale, part fin whale.

Such a creature, often called a flue whale, is growing more common as warming seas push blues into new breeding grounds, where they are increasingly likely to mate with their fin relatives. A survey of north Atlantic blues published last year found that fin whale DNA comprised as much as 3.5 percent of their genome, a striking figure given the two species diverged 8.35 million years ago. If Whale 52 is indeed a hybrid, its presence suggests genetic intermingling among Balaenoptera musculus, as blues are known among scientists, and Balaenoptera physalus has been occurring for decades, if not longer. The North Atlantic findings suggest it is accelerating.

Cetacean interbreeding has been documented before, notably among narwhals and belugas and between two species of pilot whales, combinations attributed largely to warming seas pushing these animals into new territory and closer proximity. But hybridization has been more closely studied among terrestrial creatures like the pizzly bears born of grizzlies and polar bears. It is scarcely understood in marine mammals, and little is known about what intermingling will mean for the genetics, behavior, and survival of the largest animal to have ever lived.

“Blue whales are still struggling to recover from centuries of whaling, with some populations remaining at less than 5 percent of their historical numbers,” Calambokidis said. While the number of confirmed hybrids remains low, continued habitat disruption could make them more common, eroding their genetic diversity and reducing the resilience of struggling populations.

Before the arrival of genomics 30 years ago, marine biologists identified hybrids primarily through morphology, or the study of physical traits. If an animal displayed the features of two species — the dappled skin of a narwhal and stout body of a beluga, for example — it might be labeled a hybrid based on external characteristics or skeletal measurements. Anecdotal evidence might also play a role: Historical whaling logs suggest blues and fins occasionally interbred, though such pairings went largely unconfirmed. But morphology can, at best, only reveal the first-generation offspring of two distinct species.

By analyzing DNA, marine biologists like Aimee Lang can now identify intermingling that occurred generations ago, uncovering a far more complex history than was previously understood. This new level of detail complicates the picture: Are flues becoming more common, or are researchers simply better equipped to find them? As scientists probe the genetic signatures of whales worldwide, they hope to distinguish whether hybridization is an emerging trend driven by climate change, or a long-standing, overlooked facet of cetacean evolution.

In any case, some marine biologists find the phenomenon worrisome because flues are largely incapable of reproducing. Although some females are fertile, males tend to be sterile. These hybrids represent a small fraction of the world’s blue whales — of which no more than 25,000 remain — but the lopsided population of the two species suggests they will increase. There are four times as many fins as blues worldwide, and an estimate of the waters around Iceland found 37,000 fins to 3,000 blues. 

“Three thousand is not a very high density of animals,” said Lang, who studies marine mammal genetics at the National Oceanic and Atmospheric Administration. “So you can imagine if a female blue is looking for a mate and she can’t find a blue whale but there’s fin whales all over the place, she’ll choose one of them.”

This has profound implications for conservation. If hybrids are not easily identifiable, it could lead to inaccurate estimates of the blue whale population and difficulty assessing the efficacy of conservation programs. More troubling, sterile animals cannot contribute to the survival of their species. Simply put, hybridization presents a threat to their long-term viability.

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“If it becomes frequent enough, hybrid genomes could eventually swamp out the true blue whale genomes,” Lang said. “It could be that hybrids are not as well adapted to the environment as a purebred blue or fin, meaning that whatever offspring are produced are evolutionary dead ends.”

This could have consequences for entire ecosystems. Each whale species plays a specific role in ensuring marine ecosystem health by, say, managing krill populations or providing essential nutrients like iron. Hybrids that don’t play the role evolution has assigned to them undermine this symbiotic relationship with the sea. “Those individuals and their offspring aren’t fully filling the ecological niche of either parent species,” Calambokidis said.

All of this adds to the uncertainty wrought by the upheavals already underway. Many marine ecosystems are experiencing regime shifts — abrupt and often irreversible changes in structure and function — driven by warming waters, acidification, and shifting prey distributions. These alterations are pushing some cetacean species into smaller, more isolated breeding pools.

There is reason for concern beyond blue whales. Rampant interbreeding among the 76 orcas of the genetically distinct and critically endangered Southern Resident killer whale population of the Pacific Northwest is cutting their lifespans nearly in half, by placing them at greater risk of harmful genetic traits, weakened immune systems, reduced fertility, and higher calf mortality. Tahlequah, the southern resident orca who became known around the world in 2018 for carrying her dead calf for 17 days, lost another one in January. The 370 or so North Atlantic right whales that still remain may face similar challenges.

Some level of cetacean interbreeding and hybridization may be inevitable as species adapt to climate change. Some of it may prove beneficial. The real concern is whether these changes will outpace whales’ ability to survive. Flue whales may be an anomaly, but their existence is a symptom of broader, anthropogenic disruptions. 

“There are examples of populations that are doing well, even though they have low genetic diversity, and there are examples where they aren’t doing well,” said Vania Rivera Leon, who researches population genetics at the Center for Coastal Studies in Provincetown, Massachusetts. “They might be all right under current conditions, but if and when the conditions shift more, that could flip.”

“The effect could be what we call a bottleneck,” she added. “A complete loss of genetic diversity.”

These changes often unfold too gradually for humans to perceive quickly. Unlike fish, which have rapid life cycles and clear population booms or crashes, whales live for decades, with overlapping generations that obscure immediate trends. There have only been about 30 whale generations since whaling largely ceased. To truly grasp how these pressures are shaping whale populations, researchers may need twice that long to uncover what is happening beneath the waves and what, if anything, Whale 52 might be saying about it.