All dolphins are essentially social animals, it has evolved to be a benefit for all the members of a group. The influence of the habitat on group size is quite easy to see among dolphins. Dolphin groups sizes vary quite dramatically. Older male Orca tend to lead quite solitary lives but this is the exception.

River dolphins such as the Indus and Ganges river dolphins, are among the least social of all the dolphins. River dolphins usually congregate in fairly small groups, from 6 to 12 in number often living an almost solitary existence. Researchers expect that the individuals in these small groups may well know and recognize each other. This is because they inhabit shallow rivers in which their prey is distributed more or less evenly.

Coastal dolphins, such as the tucuxi, benefit in the same way as river dolphins from the varied topography of their habitat, but, at the same time, predators are more numerous here, and the dolphins' own prey tends to be more scattered. To protect themselves while they range further in search of prey, these dolphins tend to form social groups, typically 5-20 individuals.

Other species such as the oceanic Pantropical Spotted Dolphin, Heaviside's Dolphin and Spinner Dolphin travel in vast crowds, sometimes thousands in number. It is extremely unlikely that every member of the group is familiar with every other, as this would require more social interaction than found in humans. They live in the open, often featureless, waters quite unlike any terrestrial habitat. The threat from predators, such as sharks, is much greater and while the dolphin's prey tends to be concentrated it is widely distributed. For these dolphins the benefits of group living are even greater and, as a result, they typically congregate in schools of a 100 or more. However there is no doubt that such large packs can act as a single cohesive unit - observations show that if an unexpected disturbance such as a shark approach from the flank or from beneath the group occurs, the group moves in near unison to avoid the threat. This means that the dolphins must not only be aware of their next-door neighbours but also other individuals near by - in a similar manner to which humans perform "Mexican waves". Similarly by co-operating with each other they can split up and search vast areas of the ocean for prey.

Bottlenose Dolphins normally live in groups called pods, containing up to 12 animals. These are long-term social units. Typically, a group of females and their young live together in a pod, and juveniles in a mixed pod. Several of these pods can join together to form larger groups of one hundred dolphins or more. Males live mostly alone or in groups of 2-3 and join the pods for short periods of time

The fundamental social group is that of mother and calf. Calves are seldom weaned before they are about four months old and may continue to suckle for many years. This establishes a bond that is often maintained even after the calf matures. Males, on the other hand, are often peripatetic - they move from place to place, spending short periods of time with a number of different groups of females and young. There is a clear social status within dolphins as have been noted with studies of the bottlenose dolphins. Like wolves in a pack, size is usually the key factor determining rank, but an individual's status appears quite fluid, changing with health, reproductive ability and the presence of other group members. And like their canine counterparts, these dolphins also exhibit various kinds of behaviour to assert their dominance over rivals, including biting, ramming, jaw clapping, tail slaps and body checks. The play behaviour of dolphins probably has more to do with learning social skills and acquiring the necessary agility to pursue prey than social status.
Dolphins are predators however, and they also show aggressive behaviors. This includes fights among males for rank and access to females, as well as aggressions towards sharks and other smaller species of dolphins.

The bottlenose dolphin is commonly known for its friendly character and curiousity. It is not uncommon for a diver to be investigated by a group of them. Occasionally, dolphins have rescued an injured diver by raising them to the surface, a behaviour they also show towards injured members of their own species.

Complex play

Dolphins are known to engage in complex play behavior which includes such things as producing stable underwater toroidal air core vortex rings or "bubble rings".The two main methods of bubble ring production are rapid puffing of a burst of air into the water and allowing it to rise to the surface, forming a ring and the other is performed by the dolphin swimming repeatedly in a circle and then stopping to inject air in to the helical vortex currents thus formed. The dolphin will often then examine its creation visually and with sonar. They also appear to enjoy biting the vortex-rings they've created so that they burst into many separate normal bubbles and then rise quickly to the surface. Certain whales are also known to produce bubble rings.

Case Study - Spinner Dolphins

The flexibility of spinner dolphins' social groups shed further light on the relationship between group size and habitat. It has been shown not only how they adapt their behaviour to suit the habitats they exploit, but also that they co-operate with another cetacean species, the spotted dolphin. During the day, groups of about 20 spinner dolphins - often of the same sex and age - swim slowly into bays and inlets to rest. And although dolphins do not sleep in the accepted sense they do become relatively inactive and less alert. By staying close to the shore at this time they are less at risk from predators. Spinner dolphins do not feed inshore, however, and as evening draws in they prepare to go hunting. This is when they perform the remarkable aerial displays after which they are named. When they head offshore they join other groups to form a hunting school many hundreds strong. These large groups co-ordinate their efforts to help find the schooling midwater fish on which they feed.

When spinners are far out in the Pacific they can not return to the security of the coast to rest. Here, large schools of spinners have been seen congregating with spotted dolphins. As the latter rest by night and feed by day it has been suggested that this example of inter-species sociability offers a measure of protection against predators - as by day or night there will always be dolphins on alert for signs of danger.