Hidden Markov models reveal complexity in the diving behaviour of short-finned pilot whales

Nicola J Quick,Andrew J Read,Saana Isojunno,Douglas P Nowacek,Dina Sadykova,Matthew Bowers

doi:10.1038/srep45765

Abstract

Diving behaviour of short-finned pilot whales is often described by two states; deep foraging and shallow, non-foraging dives. However, this simple classification system ignores much of the variation that occurs during subsurface periods. We used multi-state hidden Markov models (HMM) to characterize states of diving behaviour and the transitions between states in short-finned pilot whales. We used three parameters (number of buzzes, maximum dive depth and duration) measured in 259 dives by digital acoustic recording tags (DTAGs) deployed on 20 individual whales off Cape Hatteras, North Carolina, USA. The HMM identified a four-state model as the best descriptor of diving behaviour. The state-dependent distributions for the diving parameters showed variation between states, indicative of different diving behaviours. Transition probabilities were considerably higher for state persistence than state switching, indicating that dive types occurred in bouts. Our results indicate that subsurface behaviour in short-finned pilot whales is more complex than a simple dichotomy of deep and shallow diving states, and labelling all subsurface behaviour as deep dives or shallow dives discounts a significant amount of important variation. We discuss potential drivers of these patterns, including variation in foraging success, prey availability and selection, bathymetry, physiological constraints and socially mediated behaviour.

Highlights

Diving behaviour of short-finned pilot whales is often described by two states; deep foraging and shallow, non-foraging dives
Classification of any repertoire of diving behaviour requires identification of objective criteria that allow an observer to discriminate among various dive types; several methods have been used to identify such categories in odontocetes, see ref. 1 for review
digital acoustic recording tags (DTAGs) record kinematic and depth measurements of subsurface behaviour, together with a synchronized acoustic record, providing a rich data series of variables that can be associated with diving behaviour

Summary

Introduction

Diving behaviour of short-finned pilot whales is often described by two states; deep foraging and shallow, non-foraging dives. This simple classification system ignores much of the variation that occurs during subsurface periods. The use of non-invasive digital acoustic recording tags (DTAGs), attached via suction cups[3], has provided detailed records of diving behaviour in a number of deep-diving cetaceans, including sperm whales Physeter macrocephalus[4], beaked whales[5,6,7], short-finned pilot whales Globicephala macrorhynchus[8,9] and long-finned pilot whales Globicephala melas[10,11]. Dives were further classified as foraging dives by other metrics including the presence of vocal behaviour (click trains and buzzes, associated, not exclusively, with foraging8,9) similar to those seen during foraging in other deep diving odontocete cetaceans, such as beaked whales and sperm whales[4,5], and short-finned pilot whales[8], or kinematic measures such as sprints[8]

Methods

Results

Conclusion