In the past, bottlenose dolphin groups have been referred to as pods — social groups of unchanging composition. More recently, long-term studies of bottlenose dolphins have now shown that their group composition changes.
Bottlenose dolphin communities around the world are described as “fission-fusion” societies. This means that individuals associate in groups dynamically: they merge or split within the same aggregation several times per day. It has been seen that some societies live in large mixed-sex groups with strong associations within and between the sexes.
Bottlenose dolphin females form alliances primarily to obtain food resources, and their association with males seem to be mainly linked to a reproductive goal.
Bottlenose dolphin males in Shark Bay, Australia, have been observed to form groups to socialize and obtain access to females by two different strategies. One strategy involves the formation of a small and stable alliance (two to three males), where males cooperate to control individual females in reproductive condition. Then, teams of two or more of this alliance cooperate to attack other alliances or defend against them, forming second-order alliances. A second strategy involves the formation of flexible alliances within a stable large second-order alliance called “superalliance”. Here, individual males frequently switch their alliance partners within the superalliance.
Coastal bottlenose dolphins are primarily found in groups of 2 to 15 individuals. The associations of the animals are fluid, often repeated but not constant. Solitary coastal animals are observed in various regions of the world.
In general, group size tends to increase with water depth and openness of habitat. This may correlate with foraging strategies and protection.
Some group members establish strong social bonds.
In the wild, group composition and structure are based largely on age, sex, reproductive condition, family relationships, and association histories.
For example, the Sarasota, Florida resident dolphin community shows patterns of association. Basic group types include nursery groups (mothers and their most recent offspring), juveniles (both males and females), and adult males (alone or in pairs).
Mother-calf bonds are long-lasting; a calf typically stays with its mother 3 to 6 years.
Associations between males and females are short-lived.
A female may return to its mother or female relatives to raise its own calves, comprising a multi-generational group.
Adult male pair bonds are strong and long-lasting. Male pairs often engage in a number of cooperative behaviors.
Researchers have documented 20-year pair bonds.
Social hierarchies exist within bottlenose dolphin groups.
Bottlenose dolphins show aggression and establish and maintain dominance through posturing, biting, chasing, jaw clapping, smacking their tails on the water, emitting bubble clouds from their blowholes, and impact through body slamming in the most severe cases.
Dolphins often show aggression by raking — scratching one another with their teeth, leaving superficial lacerations that soon heal. Traces of light parallel stripes remain on the dolphin's skin. These marks have been seen in virtually all dolphin species.
Biting refers to the forceful contact between one dolphin's teeth and another dolphin's body, usually resulting in long, thin, parallel, or nearly parallel tooth rake marks on the skin. In a study done on some 285 bottlenose dolphins living around Shark Bay, Australia, 83% of the observed population has tooth rake marks. These rake marks are classified as either:
New (broken skin)
Obvious (white rake lines that are clearly visible)
Faint (faint evidence of rakes)
Inter-sexual competition and conflict are common causes of aggressive behaviors between group members.
Cycling females were significantly more likely to have new tooth rakes compared to non-cycling females or females that were pregnant.
Adult males were significantly more likely to have tooth rake marks than adult females, with the same tendency for juveniles. Body coverage of tooth rakes was significantly more extensive for adult males than for adult females.
Calves, both male and female, were observed with far fewer rake marks than any age class.
Aggressive (agonistic) interactions between females were extremely rare but females received aggression more often, with 84% coming from juvenile and adult males.
Other than 4 observed instances, adult females were not observed acting aggressively towards juveniles or adults of either sex, suggesting that they are highly tolerant. Of the 4 observed instances of adult female aggression, all were directed at their dependent offspring.
Male calves were significantly more aggressive than female calves, and calves had higher rates of agonistic interactions than their mothers.
Rake mark healing times.
Bottlenose dolphins continue to acquire both temporary marks and permanent scars throughout their lives; however, wounds tend to heal rapidly.
A layer of degenerating cells replaces the open wound and protects the underlying new growth of tissue.
Tooth rake wounds penetrate the skin and often harm the underlying blubber layer; scars of this nature disappear after 5 to 20 months.
Since tooth rakes tend not to damage deep tissue, re-pigmentation eventually occurs, causing such scars to become indistinguishable over time.
Daily Activity Cycles
Bottlenose dolphins are active to some degree both day and night.
Observations indicate that dolphins undergo daily cycles of activity, which include feeding, socializing, traveling, and resting.
Social behavior comprises a major portion of bottlenose dolphins' daily activities.
Feeding usually peaks in the early morning and late afternoon.
Daily activity cycles are influenced by both environmental factors (habitat, time of year, time of day) and physiological factors.
Dolphins frequently ride on the bow waves or the stern wakes of boats. This activity is probably adapted from the natural behavior of riding ocean swells, the wakes of large whales, or a mother dolphin's slip stream (hydrodynamic wake).
Dolphins have been seen jumping as high as 4.9 m (16 ft) from the surface of the water and landing on their backs, bellies, or sides in a behavior called a breach.
Both young and old dolphins chase one another, carry objects around, toss seaweed to each other, and use objects to solicit interaction. Such activity may be practice for catching food.
Protection & Care
Large adult males often roam the periphery of a group, and may afford some protection against predators.
Researchers have observed scouting behavior in bottlenose dolphins. An individual may investigate novel objects or unfamiliar territories and "report" back to the group.
Bottlenose dolphins may aid ill or injured dolphins. They may stand by and vocalize, or they may physically support the animal at the surface so it can breathe.
Interaction with Other Species
Bottlenose dolphins have been seen with groups of toothed whales such as pilot whales (Globicephala macrorhynchus), Risso's dolphins (Grampus griseus), spotted dolphins (Stenella attenuata), and rough-toothed dolphins (Stenella bradanensis). Increased protection from predators and more efficient hunting of fish are two of the likely benefits that both species gain by forming these unusual partnerships.
Bottlenose dolphins have been seen riding the pressure waves of gray whales (Eschrichtius robustus), humpback whales (Megaptera novaeangliae), and right whales (Eubalaena spp.).
Researchers have observed bottlenose dolphins chasing and displacing other species of dolphins from prime bow-riding spots in waves.
Dolphins respond to sharks with tolerance, avoidance, and aggression. Tiger sharks (Galeocerdo cuvier) elicit the strongest responses from dolphins. Researchers have observed dolphins attacking, and sometimes killing, sharks in the wild.
Some dolphins may approach divers, swimmers, or boaters.