Video cameras attached to mine-hunting US Navy dolphins have filmed them hunting and eating fish and, to the scientists’ surprise, swallowing venomous yellow-bellied sea snakes.
It is the first time video and sound has captured bottlenose dolphins feeding on live fish, from the bursts of sonar clicks used to pinpoint the prey to the victory squeals after grabbing a meal.
The footage showed that dolphins use suction to gulp down their prey, produced by expanding their throats and flaring their lips. Previously, researchers thought the main strategy was to rush at the fish, an approach called ram raiding.
All six of the dolphins studied are based in California and are trained by the navy to locate and mark mines. The animals go out swimming in the open ocean every day.
Dianna Samuelson Dibble, at the National Marine Mammal Foundation (NMMF) in San Diego and part of the study team said: “They clicked almost constantly at intervals of 20 to 50 milliseconds. On approaching prey, click intervals shortened into a terminal buzz and then a squeal. On contact with fish, buzzing and squealing was almost constant until after the fish was swallowed.”
The fish chases were strenuous and the dolphin’s heartbeat could be heard on the films, she said. The scientists were also able to see how the dolphins used their eyes to home in on the fish when they got close.
The most unexpected finding was the eating of yellow-bellied sea snakes. Dolphins had been seen playing “cat and mouse” games with snakes before, but never eating them. One dolphin was seen to consume eight.
“The dolphin clicked as it approached the snake and then sucked it in with a bit more head jerking as the flopping snake tail disappeared and the dolphin made a long squeal,” the scientists reported.
“This snake does have the potential to cause neurotoxicity after ingestion and its venom is considered fairly dangerous,” said Barb Linnehan, director of medicine at the NMMF. “Perhaps because the snakes ingested were thought to be juveniles they had a lower amount of venom present.”
The dolphin, which had been born in captivity, appeared unaffected by its serpent meal. “Perhaps the dolphin’s lack of experience in feeding with dolphin groups in the wild led to the consumption of this outlier prey,” the scientists said. Linnehan said leopard seals had been reported eating the snakes, but then regurgitating them.
The study, published in the journal PLOS One, observed the dolphins in the open Pacific Ocean during mine detection training, and in San Diego Bay and a sea water pool. Their prey included sand bass, top smelt, yellowfin croaker, California halibut and pipefish. The smelt would often jump out of the water when being pursued, leading to the dolphins swimming upside down to track the prey and snap it up as it landed back in the water.
Dr Brittany Jones, a scientist at the NMMF said: “These findings are an incredible addition to the literature providing detailed analyses during prey capture in the open ocean, which would be very difficult to achieve with wild dolphins.”
The lead author of the study, Dr Sam Ridgway, died recently at the age of 86. “His creative approach to partnering with Navy dolphins to better understand the specie’s behaviour, anatomy, health, sonar, and communication will continue to educate and inspire future scientists for generations,” said Jones.
