Marine Biologists Document Harbor Porpoise Mating Behavior From Golden Gate Bridge

Harbor porpoises returned to San Francisco Bay in 2008 after a six-decade absence, giving researchers at the Marine Mammal Center an unprecedented opportunity to study the species close to shore. Research biologist William Keener and his team initially used boats for observation, but the shy animals avoided them. They then realized the Golden Gate Bridge's pedestrian walkway, about 230 feet above the water, provided an ideal vantage point where the porpoises appeared undisturbed by human presence.

Over eight years, the team captured 144 photos and videos of mating activity in the deep strait beneath the bridge, an area they nicknamed the "funnel of love." The observations revealed a highly consistent pattern: males always approached females from the left side, even crossing over if they started on the right. The mating attempt lasts only one to two seconds, with the male rocketing up from below as the female surfaces to breathe.

Of the documented attempts, only two showed confirmed penetration, with the male's body positioned crosswise to the female's, belly pressed to her left side. In roughly 20 percent of attempts, the male displayed his penis but did not approach closely. Mating behavior was observed year-round, not only during the known fertile season, suggesting the difficult maneuver may require extensive practice.

The left-sided approach prompted investigation into anatomy. Marine biologist Dara Orbach at Texas A&M University–Corpus Christi examined stranded specimens and found the male's penis ends in a thin, flexible tip that always angles to the left. The female reproductive tract features numerous folds, sudden turns, and pockets, which Orbach suggests may allow females to exert control over fertilization by twisting their bodies to accept or reject a suitor.

Acoustic studies by Magnus Wahlberg at the Fjord&Baelt Center in Denmark used hydrophones with captive porpoises and drones with wild populations, confirming the left-sided approach. Researchers including Cindy Elliser of Pacific Mammal Research verified the behavior in Washington's San Juan Islands after reviewing the Golden Gate findings.

The findings were published in 2018 and have since been compiled in a chapter for the upcoming scientific book *The Complex Behaviors of Marine Organisms*. While harbor porpoises are globally listed as a species of least concern, localized populations such as those in the Baltic Sea are critically endangered. Identifying mating hotspots through surface splashing behavior can help designate marine protected areas for at-risk groups.

Knowledge of harbor porpoise reproduction may also inform future conservation efforts for the vaquita, a critically endangered close relative found only in the Gulf of California. Vaquitas die from capture shock, preventing captive breeding programs. Orbach suggests that if breeding ever becomes feasible, silicone models of porpoise reproductive tracts—developed through this research—could help collect high-quality sperm for artificial insemination.

Sea Shepherd Conservation Society's Andrea Bonilla notes that habitat protection, particularly from gillnets, remains the immediate priority for vaquita survival. No invasive breeding measures are currently planned. Researchers emphasize that despite the new data, much about harbor porpoise lives remains unknown, including why they congregate periodically in larger groups.