dolphin

dolphin.
1. A large wooden pile, or collection of piles, serving as a mooring post for ships, or occasionally as a beacon.

2. Small brass guns carried in a ship and fitted with two lifting handles over the trunnions. They were used mainly as anti-personnel guns during the 15th, 16th, and early 17th centuries, much like a murderer or robinet.

3. The plaited rope strap (also known as dolphin of the mast) round the mast of a square-rigged ship to prevent nip between the lower yard and the mast and at the same time to secure the puddening round the mast which prevented the lower yard falling to the deck if the jeers and slings were both shot away.

4. A small light rowing boat of ancient times. It was from this name that arose the story recounted by Pliny of a boy going daily to school across the Lake of Lucerne on a dolphin.

5. Small toothed whales. There are 34 known species, which include killer whales or orcas (Orcinus orca). Many of the smaller species of dolphin have the habit of riding the bow waves of ships under way. They often associate with large schools of tuna or whale sharks, and this makes them vulnerable to being accidentally caught in purse seine nets set around fish shoals. It was estimated that in 2003 as many as 8,000 dolphins were accidentally killed in this way in the North Sea, and the British government is currently (2004) experimenting with nets fitted with escape grids.

Dolphins usually associate in extended family units, some of which can be quite large, and occasionally large numbers of them become stranded. Sometimes the cause of their deaths is evident from clear signs of injuries from fishing nets but often there are no obvious reasons for their stranding. Being top predators dolphins tend to accumulate high concentrations of pollutants such as heavy metals and PCBs, one of today's environmental issues, and these may disorient them, as may the powerful sonars used by naval vessels. However, recently dolphins stranded off Southern California have tested positive for domoic acid, a natural toxin produced by a red-tide diatom Pseudonitschia, so one cause may be the results of eutrophication, another environmental issue.

In orcas the pods are led, and the hunts are organized, by the senior females, whereas the males, recognizable by their tall dorsal fins, tend to move between the family units. Like all dolphins they are highly vocal, using sounds both to communicate within the pod and as sonar to find and track their prey. Each killer whale group tends to have an identifiably different dialect for communication; even more interesting is that the dialects of groups of orcas that specialize in hunting other marine mammals are distinct from those that specialize in hunting fish. Off southern Argentina a group of orcas has developed a unique hunting technique of beaching themselves to snatch cubs from a breeding colony of fur seals.

Dolphins, particularly the bottlenose or bluenose (Tursopis truncatus), are highly intelligent. For example, those kept by the Institute of Marine Mammals in Mississippi are trained to keep their pools clean by holding on to any litter until they see a trainer, and the litter is then traded for fish. They have a very sophisticated sonar system, which enables them to acquire detailed information about an object in the water, so some have been trained by the US Navy to detect mines, either floating in the water or lying on the bottom. They are taught not to touch the mines, but mark them with electronic floats, while cameras on their bodies transmit images to their handlers.

See also pelorus jack.

M. V. Angel