Summary: Bottlenose Dolphins live in fission-fusion societies which means they have some consistent groupings but also other more temporary associations. The association patterns of dolphins are very complex and dynamic, but there are certain general rules to social behavior. Females tend to group with other females with the same reproductive condition, ie pregnant, with young calves, or with older calves. Females also tend to cluster with their maternal relatives such as their mother or sister. Males form long term strong bonds with an other unrelated dolphin of the same age. The male pairs form consortships to isolate receptive females and both mate with her. Bottlenose dolphins have overlap promiscuity and mate when the home range of a male pair and female group with a receptive female coincide. Fathers do not know their paternity or help raise the offspring in any way. Paternal uncertainty helps protect infants from infanticide.

 

Individual Recognition: Bottlenose dolphins are able to individually recognize and identify other dolphins even after years of separation. Individual recognition was proved in one experiment that played signature whistles of familiar and foreign dolphins underwater to infants and adults. Adults responded only to recordings of kin or close associates. Infants also showed individual recognition because they responded only to the call of their mother and were able to distinguish between it and other female whistles that they familiar with.

These findings are consistent with the theory that bottlenose dolphins have concepts of each other as individuals, track the history of their relationships, and that individual specific social relationships are important. This recognition pattern may have developed because dolphins interact with many different individuals, some frequently, but others very rarely. Signature whistles help in recognition which allows dolphins to maintain such long term individual associations (Sayigh 1999).

Female/Female: Wild females can be solitary or live in bands. Bands are usually base on relatedness or reproductive condition. Up to three generations of the same matriline have been seen associating together. Pregnant females and females with young calves will swim together as well. These groups are termed nursery groups and are often consist of the related individuals due to synchronized cycling (Reynolds 2000). Among a group of captive females a stable dominance hierarchy was observed based on age (Samuels 1997).

 

Male/Male: Male dolphins have complex complicated social networks. They maintain strong stable bonds with one to two other males, although some solitary males exist. These pairs or triples share a home range in which they live, hunt, and consort females together. The first-order bonds are formed before maturity between unrelated males of similar ages. The pairs sometimes cooperate in second-order alliances with other pairs to fend off other alliances of alliances (second-order). A larger instable superalliance existed between the individuals, pairs, and second-order alliances. Overall, home range overlap plays a large role in the frequency of secondary and tertiary(super) alliances (Connor 1999).

Male/Female Interactions and Mating:

Gender is the main determiner of dominance, with adult males always dominant over females. Females, however can be dominant over immature males. Some females attack new males entering their groups, and sometimes win agonistic encounters with males. All forms of agonistic behavior varied seasonally, with peaks in the spring and fall probably due to the seasonality of reproduction and or hormone levels (Samuels 1997).

Males are slightly more robust, especially in the axial locomotor muscles, which give them increased swimming speed. This could help them heard females during the breeding season. Such moderate sexual dimorphism is characteristic of systems where males compete for dispersed females. In order to heard very large groups of females, the males would have to be much larger, but since females are scattered rather than in large groups, the degree of sexual dimorphism is much smaller. The smaller degree of dimorphism could also explain why it is necessary for two or three males form consortships (see cooperation) to single out receptive females (Tolley 1995).

Most mating takes place in a 4 month breeding season which varies in timing and length between habitats. Dolphins living in more polar, colder waters had more highly synchronized mating seasons (Urian 1996). Copulation itself lasts only a few seconds, but is preceded by petting, rubbing, or floating with lazy eyes like a log. Females do have a bit of mate choice and have polyestrus cycling and spontaneous ovulation which can let females choose who fathers their child and mate with more than one male. Spontaneous ovulation means they can come into estrus, if not pregnant or nursing, if there is a male nearby they desire to mate with. The timing of estrus and number of cycles per year is variable with some cycling 6 times and others none. Mate choice is exerted because females want to be assured of good paternal genes since they invest highly in the young for several years.

Females form opportunistic groups to protect young females from males seeking to reproduce (Mann 2000). Disilvestro likens male groups to roving gangs who fight with other male groups for mates. The male groups ally with other groups to deter other alliances. Males do not successfully mate until they are about 20, much older than females who start breeding between the ages of 5 and 10 (Disilvestro 1998). Male pairs attempt to single out one female who is in estrus, but the other females in her group may try to deter them by rubbing up against and stroking the males with their fins, diverting the males attention so that the female could escape. This behavior shows a male group-female courtship attempt and that dolphins have a sense of self because they understand and practice deception (Bower 2001).

Offspring:

Births are moderately seasonal; most calves are born in spring and summer when thermodynamic costs are lowest. This timetable also assures lactating females enough food when the demands on their bodies are the greatest. Females tend to travel in large groups when the infants are less than three months old, the age when infants are the most vulnerable, to protect against predation and other dolphins (Mann 2000). Females have offspring usually every 3 to 5 years and gestation lasts 12 months. Infants can swim very soon after birth but take longer to learn how to breath through their blowholes efficiently so spend more time at the surface. They nurse for two years on average. The fission-fusion societies allow mothers with young infants to choose whom they associate with and whom they try to avoid. Mothers often leave their infants alone or in the care of others called allomothers (Mann 1998). Offspring remain with their mothers for 3 to 6 years, usually even after their mother has another infant (Disilvestro 1998). After separation from their mothers, juveniles form very active groups and spend much of their time socializing and playing, determining long lasting dominance relationships (Reynolds 2002). Females reach sexual maturity between the ages of 5 and 10 and males reach sexual maturity after 10 to 15 years, but do not successfully breed till they are even older (Disilverstro 2002, Reynolds 2002).

 

Infanticide:

Infanticide itself has never be observed in dolphins, although there is evidence that suggests it occurs. The autopsies of 20 stranded infants revealed that 9 died by blunt force trauma. In 6 of the abused calves, no external signs of trauma were present. All had patterns of lesions across their heads and necks suggesting blunt-force injuries from many directions. The nature of the injuries is not consistent with injuries due to boats, sharks, or natural injury due to rough surfaces. The injuries are compatible with reported descriptions of infanticide. The sexual selection hypothesis is the strongest explanation if this behavior, although it could be a result of rough play or sexual frustration (Milius 1998). If preformed by males during certain times in the breeding season, a female will come into heat again if she looses her calf (Dunn 2002). Infanticide is more advantageous in the beginning of the breeding season because a female who looses her calf late into the season will not be receptive till the following breeding season. Males may peacefully associate with a new mother to increase the chances of monopolizing her if the infant were to die (Mann 2000). Female promiscuity gives the infants some security because of paternal uncertainty. A male would not commit infanticide if there was a chance the calf was his own. Males attempt to stay with and monopolize the females they copulate with (Mann 2000).