Behavioral development of dusky dolphins

Abstract

This thesis examines the characteristics of dusky dolphin (Lagenorhynchus obscurus) nursery groups and ontogeny of dusky dolphin calves. Data were collected via boat-based group focal follows of nurseries from October 2006-May 2007. A total of 87 nursery groups were encountered. Data were analyzed according to age category (infant or yearling) and season (early or late). Nursery group membership was lowest in the early season and when yearlings were present. The average number of yearlings in a nursery group was less than that of infants. The predominant activity of calves was rest. Early infants rested the most, while travel seemed most important for late infants, and early yearlings were most likely to forage. With the exception of early infants, all calves were more likely than adults to interact with boats. When taking month into account, yearlings were more social in general than infants. Infants showed a positive trend in sociality, while yearling sociality remained relatively stable. Nursery groups are markedly segregated by calf age, and 80% of nursery groups contained calves of only one age group. Dusky dolphin calves show a similar trend in preference for position in relation to the mother as that in bottlenose dolphins (Tursiops sp.), with echelon swim decreasing with age. However, all calves appear to prefer echelon swim when nursery groups are traveling. Calves were more likely to swim independently in the late part of the season and while foraging or socializing, and were more likely to be in close proximity to their mothers while resting or traveling. Calves learned noisy leaps, followed by clean, coordinated, and acrobatic leaps, in that order. There was no clear relationship between behavioral state and types of leaps performed by calves. Early infants leapt less often than older calves, but leap frequency did not differ among the older calves. The overall pattern in the ontogeny of dusky dolphin leaps indicates that the physical development of leaps is learned individually, while the context in which the leaps are performed is learned from conspecifics. These results indicate that nursery groups represent an important environment for healthy physical and social development of calves.