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Marine Biologists Solved a Wildlife Mystery From the Walkway of the Golden Gate Bridge: How the Harbor Porpoise Mates
The dolphin-like marine mammals returned to San Francisco Bay in 2008 after a decades-long absence from the area. Scientists took the rare opportunity to closely observe their behavior, and what they uncovered might help at-risk porpoise populations
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Key takeaways: Harbor porpoises
- At the mouth of San Francisco Bay, harbor porpoises mate beneath the Golden Gate Bridge. Researchers call the narrow, deep strait the “funnel of love.”
- Scientists recorded the activity through videos and photos, then worked with experts around the world to document the species’ sexual behavior and anatomy.
After spending hundreds of hours observing harbor porpoises from the pedestrian walkway of the Golden Gate Bridge, William Keener learned to say “porpoise” in a dozen languages. In German, it’s schweinswal, or “pig whale”; French uses marsouin, “sea pig”; and his favorite, Norwegian, is nise, from the word for “sneeze.” With these terms, he could explain to passing tourists why he was looking down at the water with binoculars.
Keener, a research biologist at the Marine Mammal Center of Sausalito, California, began standing on the bridge to watch the animals’ behavior. He didn’t intend to become an expert in harbor porpoise mating rituals. But “once we realized that these guys are having sex beneath the Golden Gate Bridge, we focused on that,” he says.
Harbor porpoises and humans have been neighbors for thousands of years, living side-by-side along ocean coasts throughout the Northern Hemisphere. Still, despite their long proximity to humans, the dolphin-like marine mammals have been very good at keeping much of their behavior secret. Nobody really knew how they procreated.
Identifying sexual behavior is the first step toward understanding what elements are required for a species to breed. “Mating is key to the success of a population,” Keener says. When it comes to making sure the harbor porpoise stays common where it is common and survives where it is rare, “just understanding what you’re seeing can be crucial.”
Scientists across the world had seen harbor porpoise calves appear from late spring to early fall, depending on the region. They saw signs of a reproductive season—for instance, that a male harbor porpoise’s testes weigh more than his brain during the few months of each year that he is fertile. But experts had gathered no undisputed observations of the act itself.
In 2008, harbor porpoises returned to San Francisco Bay by the hundreds, offering a new opportunity for research on the species. For the previous six decades, they had been rare visitors to the bay, deterred by ocean pollution, a lack of fish to prey on and a World War II anti-submarine net across the Golden Gate, Keener says. He and some biologist friends were eager to learn how the returning porpoises would act in this urban marine environment.
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Watching harbor porpoise behavior isn’t easy. The animals typically keep a low profile, exposing their dorsal fins at the surface for only a second or two while breathing. They are sedate, especially compared to the enthusiasm of, say, bottlenose dolphins, which jump into the air on the simple occasion of taking a breath.
At first, the scientists used boats to observe the animals, but the shy harbor porpoises avoided them. Then, the team realized that the Golden Gate Bridge offered the perfect observation point. Its pedestrian walkway extends across the bay about 230 feet above the water, close enough for a clear view of the activity below. The harbor porpoises seemed unaware of the bridge traffic thundering above them, let alone a scientist or two peering over the side.
When Keener first saw the splash from a porpoise body landing on the water, he thought it was a feeding behavior. Perhaps the porpoise had chased a fish to the surface and followed it into the air. The scientists began paying particular attention to these splashes, using a telephoto lens to snap pictures of them. Eventually, in 2010, a photo revealed not prey fish escaping the hunt, but a harbor porpoise penis. The organ is about a third of the length of the male’s five- to six-foot-long body.
That’s when the scientists realized the splashes came from mating behavior.
Each day, the marine mammals swim in and out of San Francisco Bay with the tides. And the Golden Gate itself, the team soon realized, is a prime setting for porpoise sex. It’s the deepest part of the inlet, where males sometimes wait for passing females with whom they can attempt to mate. “We call it the funnel of love,” Keener says.
Over the next eight years, Keener and his team took 144 photos or videos of harbor porpoise mating activity in the bay. What the scientists observed was consistent. The male always approached the female from the left side, even crossing over if he started on her right. “To have an extreme sidedness, ‘lateralization’ as we call it, in mating is unusual,” Keener says. “When mammals copulate, it can be belly to belly, or more often, rear mount, but it’s still symmetrical, not off to a side.”
In almost all cases, the male rocketed up from below when the female surfaced to breathe, making a splash as he landed back in the water. The entire mating process took one second, maybe two.
Yet the scientists saw only two penetrations, each with the male’s body crosswise to the female’s, his belly pressed to her left side. The other 99 percent of attempts were apparently unsuccessful—and almost 20 percent of the time, the male displayed his penis but did not approach closely.
For the scientists who knew that porpoises can only become pregnant during a specific fertile season, it was even stranger that they saw this mating behavior all year round. Keener suggests that the porpoises’ unusual reproductive strategy—which relies on the males approaching with the right angle and speed—could simply require a lot of practice. The team’s description of harbor porpoise mating dynamics, published in 2018, was the first of its kind for any porpoise species.
Keener and his team reached out to other harbor porpoise researchers to find out whether they were seeing similar behavior. One of these experts was Cindy Elliser, founder and research director of the nonprofit Pacific Mammal Research, who studies harbor porpoises in Washington’s San Juan Islands. She visited the Golden Gate Bridge with the San Francisco researchers during a science conference in 2015.
As Keener and his colleagues went through their photos, Elliser gasped. She had similar images of harbor porpoises from Burrows Pass, a narrow channel between two of the San Juan Islands. Hers showed splashes but did not clearly show a penis. “They were mating and I didn’t know it,” Elliser says. “After that, everything clicked into place.”
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Magnus Wahlberg’s office at the Fjord&Baelt Center in Kerteminde, Denmark, where he is chief scientist, overlooks a pool with some of the few harbor porpoises in captivity worldwide. Wahlberg and his research team used underwater hydrophones to study the captive animals’ mating attempts, listening to the harbor porpoises’ echolocation clicks. If the male went silent as he approached a female, that would suggest he was trying to surprise her.
“There were indications that the males may not have said so much when they approached the females,” Wahlberg says. But perfectly placing the hydrophones to capture this intimate, two-second encounter is as much a matter of luck as skill, he adds. “The results are not solid yet.”
Using drones, Wahlberg and his team also studied wild harbor porpoises in their local region and confirmed Keener’s observations of sexual behavior. His and Elliser’s research groups were among a dozen across Europe and North America that recorded porpoise mating behavior following Keener’s paper and verified that the male always approached the female from the left.
To find out whether there was something about harbor porpoise anatomy that made them lefties, Keener turned to an expert in the genital anatomy of whales, dolphins and porpoises: Dara Orbach, a marine biologist at Texas A&M University, Corpus Christi.
Using organs from harbor porpoises that died after stranding, Orbach confirmed that not only does a harbor porpoise penis end in a thin, flexible tip, but that the tip always juts to the left. However, that might not be the only anatomical reason for the male’s single-sided approach.
It could also have something to do with the female’s unusual vagina, which has many folds, sudden turns and pockets. To successfully bypass these barriers and increase the chance of fertilization, the male would have to be on the left side, according to Orbach’s research.
Although the males appear to strive for an element of surprise, female harbor porpoises can exert some control over the mating process, and Orbach believes the vaginal wrinkles may support that. “Since I started studying this in 2010, I’ve thought that those vaginal folds play a role in sexual selection, because there is so much diversity across species,” she says. By twisting her body to accept or reject her suitors, potentially sending sperm down a dead end rather than to the egg, maybe a female porpoise can have a say about which male will impregnate her.
Orbach’s work with silicone models of bottlenose dolphin vaginas suggests that the wrinkles may improve the quality of sperm, compared with sperm collected during artificial insemination. The wrinkles also might squeegee the penis to prevent sperm-killing seawater from entering the uterus during copulation.
It’s a little bit of a reach, but Orbach says her dolphin vagina research could help if scientists ever figure out how to breed the vaquita, the world’s most endangered marine mammal. A close relative of the harbor porpoise, found only in the northernmost Gulf of California in Mexico, the vaquita is at the very edge of extinction. Scientists estimate that at least seven to ten individuals, but not considerably more, are all that’s left of the species.
Right now, it is impossible to capture vaquitas, because the animals die from the shock of it. This has prevented scientists from establishing vaquita breeding programs. But if they ever find a way to set up such a program, researchers could theoretically use silicone vaginas to collect high-quality vaquita sperm for breeding efforts via artificial insemination, Orbach says.
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Andrea Bonilla, head of science at the Sea Shepherd Conservation Society, points out that harbor porpoises are closely related to vaquitas, have similar anatomy and are known to produce viable offspring with other porpoise species. This would seem to make them a likely host species to grow a cloned vaquita fetus, a feat that would require detailed knowledge of all aspects of the harbor porpoise’s anatomy and reproduction. But right now, that’s not even close to being a priority.
Bonilla says that no other conservation measures are appropriate until the vaquita’s habitat is safe, especially from the gillnets that are the major threat to the species’ survival. Mexican authorities and researchers are working to conserve the vaquita in its native habitat before undertaking any invasive efforts to breed the animals, clone them or even move them into captivity.
But hypothetically, this knowledge of harbor porpoise mating behavior and anatomy could be helpful to saving the vaquita one day. “I was surprised at these new findings, learning that the mating process in harbor porpoises is superfast,” Bonilla says. Vaquita sex is still a mystery. “In many years of visual surveys of vaquitas, we haven’t seen anything similar, so far.”
Harbor porpoises are thriving from the California coast to Alaska, and globally, they are considered a species of least concern on the International Union for Conservation of Nature (IUCN) Red List. But this research may still benefit localized harbor porpoise populations that are in trouble, Keener says. The Baltic Sea harbor porpoise population, for instance, is listed as critically endangered by the IUCN. The status of Black Sea harbor porpoises is uncertain, Keener adds, because a key researcher is based in Kyiv, Ukraine, where the war has limited research.
Knowing that splashing is linked to mating can help scientists identify hot spots for the porpoise’s sexual activity, such as under the Golden Gate Bridge, that are important for the species’ survival. This data can be used to create marine protected areas or prompt other conservation measures.
In an upcoming scientific book, The Complex Behaviors of Marine Organisms, Keener, Elliser, Wahlberg, Orbach and others contributed a chapter on the harbor porpoise’s sexual behavior and anatomy that details these findings, bringing together the various aspects of what they’ve learned for the first time to inform scientists and conservationists.
Wahlberg notes that even though harbor porpoises are well-studied compared to other marine animals, “We still know so little about their lives.” But the observations from the Golden Gate highlight how the right evidence at the right time can connect the dots in unexpected ways. When Keener and his team explained a behavior that had eluded scientists around the world, “suddenly, it was obvious,” Wahlberg says. “Maybe that’s how science works.”