Social organisation, behaviour and population parameters of Bottlenose Dolphins in Doubtful Sound, Fiordland

Abstract

Bottlenose dolphins in Doubtful Sound represent a unique opportunity for a relatively longterm study of wild, known individuals near the southern extent of their worldwide range, and unaffected by incidental mortality. In order to gain a detailed picture of their society, I studied their association patterns with each other, their behaviour and their population parameters.

Association data shows that their social structure is broadly similar to that of populations in tropical environments. The closest associations are primarily between those of the same sex. Male pairs and triplets form very stable relationships that may last several years. A core group of males exists, the members of which form highly associated pairs and triplets which interchange partners on a scale of months. In contrast, females are related to each other more loosely through a series of "chain" links. Associations between males and females depend partly on female reproductive condition, represented by the age of her calf, and partly on seasonal breeding cycles. There is evidence of allomaternal care-giving and of "learning by parenting" by female subadults.

Group behaviour states were measured via scan sampling. Dolphins were found to long-dive (behaviour which probably represents feeding at depth) in the early morning and late afternoon and to rest for a defined period between 1000 h and 1400 h. Socialising was observed most often in summer, corresponding to a peak in breeding.

Continuous focal animal follows, analysed via sequence analysis, were used to examine the relationships among objectively-defined behavioural events. Sequence analysis does not neatly categorise behavioural events into distinct behaviour states. This indicates that behavioural events, at least in Doubtful Sound, have several meanings depending on context.

Continuous focal animal follows were used to describe behaviour of young-of-the-year calves. Calves play most in the mornings while their mothers feed, and are accompanied by other calves and female subadults during this time. They join their mothers to rest for several hours in the middle of the day.

Average birth rate over the past seven years was 0.066 ± 0.039, approximately equalising average maximum mortality rate (0.079 ± SD 0.055). Calving is highly seasonal and was observed only during five months centred on summer. Average calving interval is three years (SD ± 1.15). Mortality rates of calves (approximately 20% mortality by year one, 43.8% mortality by year three) are similar to those observed in other populations.