Social Aspects of Demographic Stochasticity in an Endangered Population of Bottlenose Dolphins (Tursiops truncatus)

Abstract

Photography is one of the most widely used tools in conservation biology. In the analysis of social species, analyses of photographic identification data are used to infer the degree of association among uniquely marked individuals. The present study aimed to develop and assess the practicality of a new time-based method for defining associations among individuals, comparing results to the commonly used group membership method. The method was applied to archived photographs from long-term monitoring of the population of bottlenose dolphins of Doubtful Sound, New Zealand, to assess differences in seasonal association rates among individuals.

The time-based method produced analyses of association at finer scales than the group membership method, and produced greater precision in association indices. Importantly the method can be applied retrospectively to any dataset in which individuals, marine or terrestrial, are uniquely identified via time-stamped photographs. Applied to the long-term dataset, results indicate differences in association rates between summer and winter seasons. During summer months the degree of sociality was generally higher; larger mixed-sex groups and greater rates of association among individuals were observed. Sociality in this population is female orientated; the majority of top-scoring individuals in centrality analyses were female. Explorations of whether a mother’s position in the social network influences the survival of her calves were inconclusive. Who the mother is significantly affects calf survival, but why this is so remains unclear. The most important influence on calf survival is birth timing; those born during the months of February, March and April have much higher chances of survival than calves born outside of this period. This is in agreement with previous studies on this population, though further research is required in order to tease apart the relative importance of driving factors of calf survival in this endangered and isolated population.