If you’re older, imagine square dancers. Younger? Try hip-hop break dancing. Either way, when partners dart into the center of the circle to do their moves and grooves, they are not unlike spinner dolphins feeding in ocean waters, according to new images recently captured by an Oregon State University marine ecologist. (Four links at the end of this story provide spectacular images of this feeding strategies. — Editor)
“Synchronized swimmers have nothing on spinner dolphins,” OSU’s Kelly Benoit-Bird said. “The degree of synchrony they display when feeding is incredible – especially considering that they’re doing it at night, several meters below the surface where they can’t see their prey or each other.”
Long been known for their teamwork in capturing prey, the synchronization exhibited by seven-foot mammals is even more complex than scientists have realized and likely evolved as a strategy to maximize their energy intake, said Benoit-Bird.
The study, by scientists at Oregon State University and the University of Hawaii, with Benoit-Bird as the lead author, utilized high-tech acoustics to make the discovery.
It found that spinner dolphins engage in a highly choreographed night-time “dance” to enclose their prey. Then they dart into the circle of confused fish in pairs, feed for 15 seconds, back out and let the next pair in line take their turn.
To match their 3,200-calorie-per-day diet, they need to eat at least 650 fish each night – plus enough extra to fuel the energy they burn during the hunt, perhaps another 200 to 300 fish.
“To make that work, they need to eat about a fish a minute,” Benoit-Bird said, “and we think that’s why they’ve developed this elaborately complex system of group predation. Dolphins can’t open their mouths like baleen whales and swallow large amounts of food at once. They have to target individual fish and it’s too difficult and energy-consuming to hunt solo.”
The study is important, scientists say, because it greatly expands knowledge of spinner dolphin behavior and opens up new fields of scientific inquiry into underwater ecosystems made possible by technological advancements in acoustical monitoring.
It was funded by the National Science Foundation and the Office of Naval Research, and the results were published last week in the journal, Acoustical Society of America.
Until now, much of the knowledge about spinner dolphin feeding has been anecdotal because they are primarily nocturnal in their feeding, Benoit-Bird pointed out. However, acoustical eavesdropping allowed the scientists to “view” the dolphins’ behavior without interrupting their routine with lights and underwater cameras.
In their study off the coast of Oahu, Hawaii, the scientists used sonar readings from a “multi-beam echo-sounder” to monitor groups of spinner dolphins. The animals’ systematic approach to feeding was eye-opening.
Initially a small group of about 20 dolphins would swim side-by-side in a straight line until finding concentrations of prey – in this case, 3-5 inch lanternfish. When they got to within five meters of their prey, they would pull into a tight circular formation and sequentially swim up and down vertically, in essence, doing “the wave” like fans at a sporting event, Benoit-Bird said.
“They were using their bodies like a plow,” she said. “We’re not sure if they were creating a pressure barrier or trying to confuse the prey. But the result among the lanternfish was chaos.”
As the lanternfish became concentrated, the dolphins tightened their circle and formed 10 pairs. The pairs at one o’clock and seven o’clock would move in, feed for 15 seconds, and retreat back to the circle. Then the pairs at two o’clock and eight o’clock would do likewise.
The feeding would last for about five minutes, during which time each dolphin got two opportunities to feed, and then the group rose as one to the surface to breathe, maintaining their circle. The dolphins would take one breath, Benoit-Bird said, and then dive down and begin the process anew.
“If one or two individual dolphins would break the circle or head to the surface to breathe, it breaks their whole system up,” Benoit-Bird said. “They never did. So then you have to ask: How do they communicate with each other, and how do they pass on that knowledge to their young?”
The researchers are still working on the latter puzzle, but their acoustical monitoring study found that much of what scientists had assumed about dolphin communication may, in fact, be wrong in this species.
In a companion article also published in Acoustical Society of America, the researchers describe how they used underwater hydrophones to listen to the dolphins during their feeding forays.
Dolphins are often vocal and their use of frequency-modulated whistles was thought by many to cue their coordinated behavior. But the researchers found they didn’t use those whistles at all while hunting prey – just during non-foraging times or when they were surfacing. Instead, they used a series of “clicks,” with the highest click rates taking place just prior to foraging.
“Whistles are omni-directional, like turning on a light bulb in a room,” Benoit-Bird said. “Clicks, on the other hand, are directional like a laser. We think it may be a strategy to communicate only within the group and not to other potential lanternfish predators. Tuna and billfish are looking for the same prey and they can hear the whistles but not the clicks because the frequencies are too high and so focused.
“If you’re lined up to eat this great smorgasbord, would you want to tell the tuna next door about it?”
~ by Mark Floyd and Ed Curtin
Descriptions of the four images are followed by the links. Just click!
• This top view of actual data from multi-beam sonar observations of dolphin foraging is shown at eight times real speed. The yellow dots show the position of each dolphin in this group of 20, while the purple background shows their prey. Each time the color purple becomes brighter, it represents a doubling in the numerical intensity of prey. This foraging occurs in four distinct phases, highlighting in the timeline at the top of the clip: 1) Wide line, where the dolphins find a good spot in the prey to begin; 2) Tight line, when the dolphins begin to herd the prey forward; 3) Circling, the separation of the herded prey from the rest of the prey; and 4) Inside circle, when dolphins move into the circle of food in pairs to eat. ftp://ftp.coas.oregonstate.edu/pub/MarkFloyd/benoit_bird_animation_1.mov
• This is a side view of data, also at 8X real speed. The blue dots show dolphins behind the center of the circle of prey, while the yellow dots are dolphins in front of the plane. The brightness of the color purple increases with the density of prey and the four distinct foraging stages (see above) are visible. From this observation, it is apparent how the dolphins cover nearly the entire extent of the prey layer, working together to force it into a dense patch – first in front of the line of dolphins, and then within the circle of dolphins. ftp://ftp.coas.oregonstate.edu/pub/MarkFloyd/benoit_bird_animation_2.mov
• This visualization shows a 3-D representation of multibeam sonar data of foraging dolphins. It becomes clear from this view how the dolphins work together to surround an entire slab of the layer of prey. They circle their prey, forming a cylinder of dense fish between them, before rising as one to breathe at the surface. ftp://ftp.coas.oregonstate.edu/pub/MarkFloyd/benoit_bird_animation_3.mov
• An underwater image of spinner dolphins is available at: http://oregonstate.edu/dept/ncs/photos/spinner_dolphins.JPG