The behaviour of humpback whales: an analysis of the social and environmental context variables affecting their behaviour on migration

Abstract

Humpback whales (Megaptera novaeangliae) undertake one of the longest migrations of any mammal. Despite the extent of these journeys and the large proportion of time whales spend migrating, relatively little is known about how these animals behave during this time. Whales may encounter anthropogenic disturbances during migration, particularly if their route overlaps with populated coastal areas, and the effect of these disturbances on the behaviour of whales has become the focus of many studies. However, to distinguish between responses to a disturbance and naturally occurring factors, a clear understanding of the natural behaviour of these animals is required. Therefore, the overall aim of this study was to determine which social and environmental factors influence the fine-scale behaviour of humpback whales while on migration. Data were collected during the southward migration in September and October of 2010 and 2011 off Peregian Beach on the east coast of Australia. Over 202 hours of behavioural data from 94 groups of whales were collected using a focal sampling methodology. Simultaneously, social and environmental context data on these groups were collected; including information on group composition, nearest other whale groups, weather conditions, water depth, and recordings of the animals’ acoustic environment. Prior to analyses of these data, an independent study was carried out to determine the reliability with which observers collected behavioural data. The results of this study were used to modify the details of the ethogram and combine the behavioural events into more readily identifiable categories.The behaviour of humpback whales on migration can broadly be divided into two categories; (1) surface-activity and (2) diving and movement. Humpback whales are one of the most surface-active of the baleen whales and preform a wide variety of aerial behaviours which are thought to play a role in communication. Energetic behaviours such as breaches, pectoral and fluke slapping are regularly observed on migration, and in this study were found to be highly influenced by the social context in which they occurred. These results were used to infer the potential functions of surface-active behaviours and indicate that both pectoral and fluke slapping may be used for close-range and within-group communication, while breaching is more likely used for communication between distant groups of whales. Unlike surface-active behaviours, environmental factors, specifically water depth and wind speed, were the most important in predicting the diving and movement behaviour of humpback whales. In deeper waters, the length of dives increased, and in windier weather conditions, swimming speeds decreased. Behavioural states are often used to describe the behaviour of marine mammals. They have the advantage of combining multiple response variables and may be a more biologically meaningful measure if examining the population level effects of disturbance. In the humpback whale literature, the definitions of behavioural states varies both in detail and content. This may result in the introduction of error into the data if subjective definitions are interpreted differently by observers. Therefore, the detailed behavioural event and movement data collected here were used to objectively identify the behavioural states of humpback whales and the activities that occur during their southward migration. Based on the results, four states were proposed: social, resting/milling, travel, fast travel. Groups were found to spend different proportions of their time in certain states depending on whether or not they contained multiple calves or adults.The results of this study provide detailed baseline data on the behaviour of east Australian humpback whales on migration and highlight the complexity of this behaviour. In addition, the need for studies examining the effect of disturbances to include data on the social and environmental context of animals in their analysis is emphasised. If social and environmental context data are omitted from these studies, observed changes in behaviour could be incorrectly attributed to a disturbance. Further, the use of an objective methodology to identify behavioural states will reduce the issues associated with subjectively defined states and allow studies to examine whether animals are more or less susceptible to anthropogenic disturbance depending on their activities. Future research should focus on determining how individual whales use surface-active behaviours and how other factors, such as the presence of vessels and the sex and age class of animals, influence whale behaviour in different contexts. These data will further contribute to our understanding of the natural factors that influence the behaviour of humpback whales while migrating. It will also provide valuable information to future studies about the social and environmental factors to consider when examining the effects of anthropogenic disturbance on specific aspects of whale behaviour.