If a single whale or dolphin strands, it usually is a very sick (and exhausted) animal. Such an animal often has some infections (pneumonia is almost always one of them) and a lot of parasites (worms in the nasal passages are very common). Sometimes these animals can be rehabilitated, but often they are so sick they won't make it.
Some species of whales and dolphins occassionally strand in groups. A stranding of 2 or more animals is usually called a mass stranding. There are a number of theories that try to explain the occurrence of mass strandings. No theory can adequately explain all of them. In some cases it will be a combination of causes. The most common explanations are:
- deep water animals (the species that most often are the victim of mass strandings) can not "see" a sloping sandy beach properly with its sonar. They detect the beach only when they are almost stranded already and they will panic and run aground.
- W.H. Dudok van Heel (1962)
- Sound and Cetacea.
Neth. J. Sea Res. 1: 407-507
- whales and dolphins may be navigating by the earth's magnetic field. When the magnetic field is disturbed (this occurs at certain locations) the animals get lost and may run into a beach.
- M. Klinowska (1985)
- Cetacean live stranding sites relate to geomagnetic topography.
Aquatic Mammals 11(1): 27-32
- in some highly social species, it may be that when the the group leader is sick and washes ashore, the other members try to stay close and eventually strand with the group leader.
- F.D. Robson (?)
- The way of the whale: why they strand. (unpublished manuscript)
- when under severe stress or in panic, the animals may fall back to the behavior of their early ancestors and run to shore to find safety
- F.G. Wood (1979)
- The cetacean stranding phenomena: a hypothesis.
In: J.B. Geraci & D.J. St. Aubin
Biology of marine mammals: Insights through strandings.
Marine Mammal Commission report no: MMC-77/13: pp. 129-188