Predators stalking victims are part of the stock-in-trade of wildlife TV programs; we're fascinated by these creatures and the way they dominate their environment. But as Terrie Williams explains, we know virtually nothing about the physiology of these majestic creatures as they pursue and devour their quarry, and even less about predators in the aquatic environment. This didn't deter Williams. Setting out in the early 1990s to monitor Weddell seals hunting in the seas around Antarctica, Williams teamed up with engineers and behavioural scientists to develop tracking equipment small enough not to disturb the animals, and begin recording the first physiological measurements on actively foraging seals (p. 973).

But it was tailoring the cameras, accelerometers and other equipment to the seals that took Randall Davis, William Hagey and Markus Horning the most time. Eventually, the team designed a small canister, that could be carried on a seal's back, to monitor their activity as they plumbed the depths. Mounting a near infrared illuminated camera on the seal's head, the team were ready to send the animals foraging in the icy waters.

But there was one more problem. Would the seals return with their $50,000 back pack? Williams and Lee Fuiman realised that the only way to get them back to the camp was if they had their own private breathing hole. The team headed across the ice shelf until they found an unbroken region of ice. Having drilled a seal-sized hole in the ice, Williams and her colleagues despatched the first seal on its metabolic mission. Fortunately, after a nerve-wracking wait, the pioneer bobbed back to the surface; the team were ready to see whether their patience had been rewarded.

Williams admits it was a `champagne' moment when she saw the first video data collected by the foraging seal; she could clearly see the fish the animals had caught. And by placing a respirometry dome over the ice hole as the animal surfaced, Williams could also record the amount of energy that the animal had used while swimming beneath the ice. Coupled with the accelerometer tracking the animal's fin strokes and temperature recordings, the team had successfully recorded some of the first physiological data while an animal foraged and digested its meal.

Knowing how much energy the animal used in a dive and the number of strokes taken, Williams and the team were able to calculate the energy used per stroke while the animal foraged. It was only 0.044 ml of oxygen per kg body mass per stroke, making seals considerably more efficient than terrestrial hunters. Also, Williams calculated that the animals took enough oxygen on board to sustain 1400 strokes during a dive, extending the duration by incorporating oxygen-conserving glides. And when Williams calculated the amount of energy that the animals used foraging and digesting, she discovered that they consumed 44% more energy digesting their hard won meal, than when they were simply hunting.

Williams is very excited to have finally begun scratching the surface of the remarkable physiological lengths that predators go to get a meal. But having tracked the animal's antics during daylight hours, she is now keen to find out how seals locate lunch during the long winter months of Antarctic darkness.

References

Williams, T. M., Fuiman, L. A., Horning, M. and Davis, R. W.(
2004
). The cost of foraging by a marine predator, the Weddell seal Leptonychotes weddellii: pricing by the stroke.
J. Exp. Biol.
207
,
973
-982.