Speed Sensing in a Harbour Seal

Extract

Many pinnipeds travel long distances between breeding and feeding grounds in some cases showing remarkable homing abilities (James & Dykes, 1978). Very little is known about the methods they use to plot their course but if they were aware of their rate of movement, this ability could conceivably assist them. The following procedure which was undertaken with a captive male harbour seal (Phoca vitulina Linnaeus) demonstrated that the animal was able to determine his swimming speed.

The seal was first trained to swim at any speed through ten hoops 48 cm in diameter submerged at equal intervals around the periphery of an 8 m oval tank filled with sea water to a depth of 1.2 m. Each trial consisted of one circuit of the tank, and always began at the same hoop. A screen preventing the seal from going through this start hoop was removed to indicate the onset of each trial. The seal was then trained to swim the 17 m course maintaining a speed of 6 km/h. The experimenter observed from a bridge suspended over the tank, and measured the animal's speed by listening to a series of tones. These tones were audible only to the experimenter and were timed such that if the seal were swimming at 6 km/h, he would reach each hoop at the same moment the experimenter heard the tone. A buzzer was sounded whenever the animal swam at less than the required speed, and a whistle was blown if he were moving too quickly.