Female reproductive strategies and mother-calf relationships of common dolphins (Delphinus delphis) in the Hauraki Gulf, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University, Albany, New Zealand
This study reviewed the habitat use, social organisation and behaviour of common dolphin groups categorised by their youngest member, as well as the behaviour of common dolphin calves of different age classes. Most newborn calves were sighted over late spring and early summer, and in water temperatures warmer than for other groups. The water depths at which common dolphin groups were found did not vary according to the age class of their youngest member. However, only groups containing newborns or infants were sighted under 20 metres of water depth. A high percentage of groups encountered contained calves (82.5%). suggesting the importance of the Hauraki Gulf for common dolphins' reproduction. Groups of common dolphins containing calves were found to be of larger size than any other group type. All groups proved to be similarly involved in different behavioural states and showed comparable patterns of association with other species, although groups including newborns were significantly less likely to be seen associated with whales and birds. Groups of common dolphins as a whole did not show a differential reaction to the boat according to the age class of their youngest member. However, mothers and their young calves kept greater distances to the boat than mothers and older calves did. suggesting that the boat may be perceived as a threat during the newborn period. Most of the behaviours that characterise mother-calf relationships varied according to calf age class. The occurrence, frequency, duration and distance of separations increased with older aged calves. A similar increase was found in the time spent without the mother, in the occurrence of association with non-mother dolphins, in the time spent in 'echelon position', and in both mothers' and calves' dive time. Mothers also had longer dive times than calves. Older aged calves tend to present the lowest proportion of synchronous breaths. These changes are likely to represent a gradual increase in calves' independence. Results from this study have extended our knowledge of common dolphins' reproductive ecology, demonstrated that studies of mother-calf relationships in pelagic species of dolphins can he achieved, and allowed future research needs to he identified and management recommendations to be made.