Dusky Dolphin Compilation

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• published: 29 Jan 2013
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These are dusky dolphin nursery groups (moms and babies) close to the shore of Kaikoura, New Zealand in December 2010. Brief Overview of Social Behavior and Reproduction 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 socio-sexual groups and encounter a variety of associates. Studies of dolphins off Kaikoura, New Zealand show that dolphins normally live in large groups that split into smaller sub-groups. These sub-groups are composed of mating adults (mating groups), mothers with calves (nursery groups) and non-breeding 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 ten males and a single female. These mating groups can be found in both shallow and deep water but more often gather near shore. Two dolphins jumping In the mating groups, the males pursue the 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. It is also possible that males may form alliances to catch females. Unlike male bottlenose dolphins, male dusky dolphins can't monopolize females. The time when female dusky dolphins first reproduce varies between regions. New Zealand dolphins first reproduce at about 7--8 years with possibly 6--7 years for Argentine dolphins. A study of dusky dolphins off the coast of Peru showed that the reproductive cycle lasts around 28.6 months with mother dolphins becoming pregnant for 12.9 months, lactate for a further 12 months and rest 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 behavior for reasons other than reproduction, perhaps for greeting, communication or strengthening social bonds. Homosexual behavior 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 give mothers and calves more time to rest, which is important for both. While the behaviors of nursery groups vary by month, resting is the predominant behavior during most months. The formation of nursery groups in shallow waters also allow 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 disorientated. Mother dolphins may look after calves that are not their own.