Tuesday, April 5, 2011
April 5, 2011 : Breaching
A breach or a lunge is a leap out of the water also known as cresting. The distinction between the two is fairly arbitrary: cetacean researcher Hal Whitehead chooses to define a breach as any leap in which at least 40% of the animal's body clears the water, and a lunge as a leap with less than 40% clearance. Qualitatively, a breach is a genuine jump with an intent to clear the water, whereas a lunge is the result of a fast upward sloping swim, perhaps as a result of feeding, that has caused the whale to clear the surface of the water unintentionally.
Whales such as Sperm Whales perform a breach by travelling vertically upwards from depth, and heading straight out of the water. Others, such as the Humpback Whale, travel close to the surface and parallel to it, and then jerk upwards at full speed to perform a breach. In a typical breach, as performed by a Humpback or Right Whale, the whale clears the water at an angle of about 30° to the horizontal. Around 90% of the body clears the water before the whale turns to land on its back or side. "Belly flops" also occur but are less common. In order to achieve 90% clearance, a Humpback needs to leave the water at a speed of eight metres per second or 29 km/h. For a 36 ton animal this results in a momentum of 288 thousand newton-seconds.
Breaches are often carried out in series. The longest recorded sustained series was by a Humpback near the West Indies - totaling 130 leaps in less than 90 minutes. Repeated breaches tire the animal, so that less of the body clears the water each time.
The Right, Humpback and Sperm whales are the most prodigious jumpers. However the other baleen whales such as Fin, Blue, Minke and Sei whales also breach. Oceanic dolphins, including the Orca, are very common breachers and are in fact capable of lifting themselves completely out of the water very easily.
Many reasons have been suggested for breaching. Whales are more likely to breach when they are in groups, suggesting social reasons, such as an assertion of dominance, courting or warning of danger. Scientists have called this theory "honest signalling". The immense cloud of bubbles and underwater disturbance following a breach cannot be faked; neighbours then know a breach has taken place. A single breach costs a whale only about 0.075% of its total daily energy intake, but a long series of breaches may add up to a significant energy expenditure.
It is also possible that the loud "smack" upon re-entering is useful for stunning or scaring prey. Noisemaking is believed to be the reason for lobtailing. Others suggest that a breach allows the whale to breathe in air that is not close to the surface, which may aid breathing in rough seas. Another widely accepted possible reason is to dislodge parasites from the skin. While other whale experts think some of the behavior might simply be playful in nature.
Breaching has also been observed in the following sharks and rays: the great white shark, thresher shark, shortfin mako, longfin mako, spinner shark, blacktip reef shark, salmon shark, porbeagle shark, copper shark and basking shark as well as the manta ray.