Marine Biologists Solved a Wildlife Mystery From the Walkway of the Golden Gate Bridge: How the Harbor Porpoise Mates

For many years, scientists had been trying to understand how harbor porpoises, a type of marine mammal closely related to dolphins, reproduced. This knowledge was crucial for maintaining the species’ common presence and survival, especially in the face of various environmental pressures. In 2008, a significant increase in the number of harbor porpoises in San Francisco Bay provided an opportunity for researchers to study their mating habits.

William Keener, a research biologist at the Marine Mammal Center of Sausalito, California, began observing the animals from the Golden Gate Bridge's pedestrian walkway. Despite initial difficulties in observing the shy animals, Keener and his team found that the bridge offered an ideal vantage point for studying the porpoises' behavior.

In 2010, the researchers captured the first photographic evidence of mating behavior - a porpoise penis emerging from the water during a splash when a female surfaced to breathe. This revelation led to the collection of 144 photos and videos of mating activity over the following eight years. The scientists observed a consistent pattern in the males' approach: they always approached the females from the left side, a phenomenon known as "lateralization" in the animal kingdom.

Interestingly, the males would sometimes demonstrate their penis without attempting to mate, suggesting a possible learning process involved in successful mating. The team's findings, published in 2018, marked the first comprehensive description of harbor porpoise mating dynamics for any porpoise species.