Differentiating species:
Despite the historic practice of lumping the entire Delpinus genus into a
single species, these widely distributed dolphins exhibit a wide variety of
size, shape and colour. Indeed over the past few decades over 20 distinct
species in the genus have been proposed. Scientists in California in the 1960s
concluded that there were two species - the long-beaked and short-beaked.
This analysis was essentially confirmed by a more in-depth genetic study in
the 1990s. This study also suggested that a third species (D. tropicalis,
common name usually Arabian Common Dolphin), characterized by an extremely
long and thin beak and found in the Red Sea and Indian Ocean, might be distinguished
from the long-beaked species. The current standard taxonomic works recognise
this as just a regional variety.
Distribution:
The Common Dolphin is widely distributed in temperate, sub-tropical and tropical
waters throughout the world in a band roughly spanning 40 degrees south to
50 degrees north. The variation in make-up described above from one population
to the next suggested little interaction between distinct groups The species
typically prefer enclose bodies of water such as the Red and Mediterranean
Seas. Deep off-shore waters and to a lesser extent over continental shelves
are preferred to shallow waters. Some populations may be present all year
round, others appear to move in a migratory pattern. Preferred surface water
temperature is 10-28 degrees Celsius. The total population is unknown but
numbers in the hundreds of thousands.
Behaviour:
Common Dolphins travel in groups of around 10-50 in number and frequently
gather into schools numbering 100 to 2000 individuals. These schools are generally
very active - groups often surface, jump and splash together. Typical behaviour
includes breaching, tail-slapping, chin-slapping and porpoising.
Common Dolphins have been seen to mix with other cetaceans such as other dolphins in the Yellowfish Tuna grounds of the eastern Pacific and also schools of Pilot Whales. An intriguing theory suggests that dolphins 'bow-riding' on very large whales was the origin of bow-riding on boats.
The gestation period is about eleven months and the calving period is between one and three years. Sexual maturation occurs at five years and longevity is twenty to twenty-five years. These figures are subject to large variation across different populations.
Conservation:
Common Dolphins face a mixture of threats due to human influence. Populations
have been hunted off the coast of Peru for use as food and shark bait. In
most other areas the Dolphins have not been hunted directly. Several thousand
individuals have been caught in industrial trawler nets throughout their range.
Common Dolphins were abudant in the western Mediterranean Sea until the 1960s
but occurrences there have tailed off rapidly there. The reasons are not well
understood but are believed to be due to extensive human activity in the area.
References:
Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution.
Society of Marine Mammalogy Special Publication Number 4. 231 pp.
National Audobon Society Guide to Marine Mammals of the World.
Encyclopedia of Marine Mammals Whales, Dolphins and Porpoises, Mark Carwardine,