page_title: Dusky dolphin
section_title: Social behaviour and reproduction
hierarchical_section_title: Dusky dolphin / Ecology and behaviour / Social behaviour and reproduction
caption_attribution_description: English: dusky dolphins named "Caligo" and "Umber". Deutsch: Zwei Schwarzdelfine (Dusky Dolphins) namens "Caligo" und "Umbra"
context_page_description: The dusky dolphin is a dolphin found in coastal waters in the Southern Hemisphere. Its specific epithet is Latin for "dark" or "dim". It is very closely genetically related to the Pacific white-sided dolphin, but current scientific consensus holds they are distinct species. The dolphin's range is patchy, with major populations around South America, southwestern Africa, New Zealand, and various oceanic islands, with some sightings around southern Australia and Tasmania. The dusky dolphin prefers cool currents and inshore waters, but can also be found offshore. It feeds on a variety of fish and squid species and has flexible hunting tactics. The dusky dolphin is known for its remarkable acrobatics, having a number of aerial behaviours. The status of the dolphin is unknown, but it has been commonly caught in gill nets.
context_section_description: Dusky dolphins live in a fission-fusion society, with most group size increases occurring during foraging and decreases in group sizes occurring during resting and traveling. In the Golfo San José off the Valdes Peninsula, dolphins commonly switch between small traveling groups and large sociosexual groups, and encounter a variety of associates. Studies of dolphins off Kaikoura, New Zealand, showed the dolphins normally live in large groups that split into smaller subgroups. These subgroups are composed of mating adults (mating groups), mothers with calves (nursery groups) and nonbreeding adults. Dusky dolphins have a promiscuous mating system in which both males and females mate with multiple partners. Mating groups are generally made of around 10 males and a single female. These mating groups can be found in both shallow and deep water but more often gather near shore.
  In the mating groups, the males pursue a female in high-speed chases. Females seem to prefer males with great speed and agility rather than size, strength, or aggression. Females may extend the chase as long as possible so only the best male remains. Females may try to evade males that are not vigorous or lack social skill. Males also may form alliances to catch females. Unlike male bottlenose dolphins, male dusky dolphins cannot monopolise females. The time when female dusky dolphins first reproduce varies between regions. New Zealand dolphins first reproduce at about seven to years, with possibly six to seven years for Argentine dolphins. A study of dusky dolphins off the coast of Peru showed the reproductive cycle lasts around 28.6 months, with mother dolphins pregnant for 12.9 months, lactating for a further 12 months, and resting for 3.7 months before the cycle begins again. During copulation, females tend to be on the top. As with all species where females mate with multiple partners, male dusky dolphins have large testicles for sperm competition. Dusky dolphins sometimes engage in sexual behaviour for reasons other than reproduction, perhaps for greeting, communication, or strengthening social bonds. Homosexual behaviour between males has been observed. Dolphins having sex for social reasons tend to be more relaxed.
  Females with calves tend to gather in nursery groups in shallow water. Nursery groups likely provide mothers and calves more time to rest, which is important for both. While the behaviours of nursery groups vary by month, resting is the predominant behaviour during most months. The formation of nursery groups in shallow waters also allows members to hunt prey species that inhabit in these waters. Both adults and calves have been observed to chase and catch fish, and the adults may be teaching the calves how to hunt. In contrast to shallower waters, hunting in deep water at night may be too dangerous for calves.  Calves are particularly vulnerable to predators like killer whales and use of shallow water by nursery groups may be a way to avoid predation. Nursery groups tend to avoid mating groups. Adult males in these groups will aggressively herd and chase females. They can separate calves from their mothers and harass them, as well. Calves may also become even more vulnerable to predators as they become exhausted and disoriented. Mother dolphins may look after calves that are not their own.
caption: Dusky dolphin performing a flip
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop", "Image DateTime": "2011:03:24 15:00:43", "Image ExifOffset": "150", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "710", "EXIF ExifImageLength": "555"}