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Odontocete Cetaceans: Quantifying Behavioral Ecology and Response to Predators Using a Multi-Species Approach
Dr. Andrew Read | Duke University Marine Lab
The primary objective of this project was to improve the knowledge of the baseline behavioral ecology of odontocete cetaceans and, specifically, understanding how these animals respond to certain types of sound. The secondary objective of the project was to increase understanding of the baseline behavior of odontocete cetaceans and, in particular, to understand some of the drivers of variation observed in this baseline. The project took a multi-species approach to quantify behavioral ecology and predator response of odontocete cateceans and use non-invasive tagging methods to collect subsurface dive state data.
Playback experiments were conducted to determine how short-finned pilot whales and Risso’s dolphins (Grampus griseus) respond to the sounds of mammal-eating killer whales (Orcinus orca), some of which have similarities to certain military sonars. Specifically, this work addressed whether social structure influences the response to the sounds of these predators by comparing the behavioral responses of the two species, which exhibit contrasting patterns of social organization.
The baseline foraging and diving behavior of short-finned pilot whales also were described using two types of tag records. First, data streams were combined from short-term Digital Acoustic Tags (DTAGs) with long-term Satellite-Linked Time Depth Recorders (SLTDRs) to analyze long-term foraging patterns, evaluate diurnal variation in foraging behavior, and determine pattern and variation in foraging bouts. Second, multi-state hidden Markov models were used to classify short-finned pilot whale diving behavior using DTAG data. This approach enabled classifying dives objectively into behavioral states and to objectively determine transitions between states. Animals were tagged from a variety of small Rigid-Hull Inflatable vessels in variable sea states using a carbon-fiber pole to attach the tag to the dorsal surface or fin of the whale. The tags were tracked using a VHF radio transmitter embedded the tag and they were programmed to release after a predetermined period of time. Data was then downloaded from the tags and converted pressure recordings using calibration information.
Short-finned pilot whales and Risso’s dolphins reacted strongly and divergently to biphonic calls of mammal-eating killer whales, but not to most other call types. Following exposure to biphonic calls, focal groups of both species demonstrated increased cohesion, but exhibited different vocal and movement responses. Pilot whales increased their call rate and approached the sound source, but Risso’s dolphins exhibited no change in their vocal behavior and moved in a rapid, directed manner away from the source. Thus, at least to a sub-set of mammal-eating killer whale calls, the two study species reacted in a manner that is consistent with their patterns of social organization. Pilot whales, which live in relatively permanent groups bound by strong social bonds, responded in a manner that built on their high levels of social cohesion. In contrast, Risso’s dolphins exhibited an exaggerated flight response and moved rapidly away from the sound source.
Short-finned pilot whales off Cape Hatteras demonstrated considerable variation in their baseline diving and foraging behavior. The whales dove to depths of more than 1200 meters and for periods lasting for up to 26 minutes. Mean duration of a foraging bout was 2.18 hours, with an average of 6.1 dives per bout. Foraging bouts longer than 4.5 hours required at least an hour of recovery time at the surface. No discernable pattern emerged in surface durations following dive bouts. Surprisingly, no diel pattern was observed in foraging depth or duration. The hidden Markov model analysis demonstrated that the diving behavior of short-finned pilot whales is much more complex than a simple dichotomy of deep and shallow diving states. Four separate diving states were identified that showed patterns of state persistence and switching among states. Predictions of state were based on the distribution of three readily observed variables: dive duration, maximum depth and number of foraging buzzes. Taken together, these baseline observations suggested that short-finned pilot whales are able to adapt their diving strategy on a dive by dive basis, switch effectively between different diving states, and do so while maintaining foraging efficiency and social cohesion.
The playback experiments described here facilitated identification of some of the key contextual factors of the behavioral responses to a threatening sound in two species of odontocetes. Short-finned pilot whales and Risso’s dolphins reacted to biphonic calls of mammal-eating killer whales in a manner that is consistent with our knowledge of their social organization. Furthermore, these biphonic calls share several characteristics with mid-frequency active sonars (MFAS). If odontocetes perceive the sounds of MFAS and the sounds of predators in a similar manner, or even if they merely respond to the two sound types in the same way, we can infer much about the nature and likely magnitude of the potential risks of MFAS by understanding the anti-predator response of each species. An important conclusion resulting from this study is that there is considerable merit from pursuing this line of reasoning: that is, constructing a formal conceptual model of the response of odontocetes to potential threats and using interacting factors such as habitat, social structure, and body size as predictors of response. A matrix of these predictive factors can be used to predict the behavioral response of odontocetes to any threat, including MFAS, at least in a coarse manner.
In addition, the project’s work on the diving and foraging behavior of short-finned pilot whales has generated an important baseline for future studies of disturbance with this species. The characterization of baseline diving and foraging behavior will benefit future behavioral response and playback studies, because one can now predict the probability that an animal will stop foraging, or switch from one diving state to another, under baseline conditions. This baseline information can then be used to predict the likelihood that a change will occur following exposure to a particular stimulus. Short-finned pilot whales do exhibit a considerable degree of inter- and intra-individual variation in their diving and foraging behavior, but this study was able to capture and describe important sources of this variation in its analysis.