- $4 with GoPro cameras on them captured incredible up-close footage of their hunting habits.
- The videos surprised scientists, as the $4 used suction to feed and ate venomous sea snakes.
Video footage from GoPro cameras strapped to a pair of $4 reveals the ocean animals' hunting habits up close for the first time.
Scientists at the National Marine Mammal Foundation fixed the dolphins with their cameras and set them loose in the San Diego Bay. They captured hours of video and sounds that reveal a few secrets of dolphin life.
It turns out that the animals use suction to feed, swallow venomous sea snakes, and squeal in victory after a successful hunt.
One video, below, shows the dolphin's face as it tracks a fish, grabs it, and swims a victory lap. It's not just the footage that's incredible. The dolphin's creaky and echoing calls are equally revealing. The dolphins emitted sonar clicks as they searched for prey. As they approached a fish, the clicks sped up to become a buzz, punctuated with a squeal as they caught and swallowed their meal.
The research was led by $4, a prominent marine-mammal scientist who earned nicknames like "Dolphin Doctor" and "the father of marine mammal medicine," before he died in his San Diego home in July.
Ridgway helped found the US Navy's Marine Mammal Program more than 60 years ago. That's the program that trained the dolphins in this study. He also founded and led the National Marine Mammal Foundation, the nonprofit behind the new paper. He dedicated his entire career to understanding the behavior, physiology, and health of ocean mammals — especially bottlenose dolphins.
These videos are one of his final research efforts. For the first time, Ridgway and his team captured up-close video and sound of dolphins hunting and eating live fish. A $4 about the footage was published in PLOS ONE on Wednesday.
"Dr. Ridgway was very proud of these findings and was ecstatic to know that this culmination was going to be published in PLOS ONE," Brittany Jones, a scientist at the National Marine Mammal Foundation who worked with the study authors while they completed their paper, told Insider in an email.
"He was always eager and excited to review the video and audio of these fish-capture sessions and recently spoke of his appreciation and admiration for [co-authors] Dianna Samuelson Dibble, Mark Baird, the amazing animals, and the animal care staff that made this research possible," Jones said.
One dolphin's diet included venomous sea snakes
Shocking to the researchers, one dolphin ate eight venomous sea snakes — a behavior never observed before in dolphins.
The video below shows one of those sea-snake meals. After catching the snake, the dolphin jerks its head and emits a high-pitched "victory squeal."
Did you catch that? It went quickly.
That's a yellow-bellied sea snake, and it's highly venomous. Scientists assumed that's why they'd only $4 and releasing them, not consuming them — especially not consuming eight of them.
Ridgway and his co-authors couldn't believe their eyes at first. They searched for other fish that might look like a sea snake on camera, but they found no other explanation.
"I've read that other large vertebrates rarely prey on the $4. There are reports of leopard seals eating and then regurgitating them. This snake does have the potential to cause neurotoxicity after ingestion and its venom is considered fairly dangerous," Dr. Barb Linnehan, director of medicine at the National Marine Mammal Foundation, said in a statement emailed to Insider.
The dolphin showed no signs of illness after its sea-snake meals, the researchers reported.
"Perhaps because the snakes ingested were thought to be juveniles, they had a lower amount of venom present," Linnehan said.
Dolphins appear to be suction feeders
The sea-snake footage is also revealing because the dolphin caught its prey in the open ocean, indicating that it used suction to capture and swallow its food. Researchers previously assumed that bottlenose dolphins use a technique called ram feeding, where they capture prey simply by clamping their jaws around it.
In the videos from the dolphin with a camera on its side, however, the researchers could see the dolphin's lips opening, tongue withdrawing, and throat expanding. They think all these subtle movements increase the space in the dolphins' mouths and create negative pressure for suction.
"With years of experience in feeding dolphins, we had not noticed this lip motion," the researchers wrote in their paper. "Rather than seizing fish in a 'claptrap' of the toothy beak, dolphins appeared to mostly suck in fish."