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Acoustic species recognition in delphinids

Periodic Reporting for period 1 - ASRD (Acoustic species recognition in delphinids)

Reporting period: 2016-07-01 to 2018-06-30

Species recognition is a major function of animal calls and many animals use just one or a few calls to accomplish this goal. In animals that produce many different calls, it is often unclear whether all of these carry information about species identity. Dolphins produce whistles that are used for social communication and could be expected to carry information about species identity. However, every species produces many different kinds of whistles and the whistles of different species often sound similar to one another. Because of this, it is not clear whether dolphins are using whistles to communicate species identity and if they are, what characteristics of whistles carry this information.

These are interesting questions in the light of understanding dolphin communication, but they also have applied aspects. Dolphins spend a large amount of time under water where they cannot be seen and many species are found at great distances offshore or in harsh environments where it is difficult to find and study them. Many dolphins are very vocal and so to overcome these challenges, researchers use passive acoustic methods to study them. Many underwater acoustic recordings are made without associated visual observations and so it is important to be able to identify species based on their calls. This has proven to be difficult because each species produces so many different kinds of whistles and because different species can sound very similar to one another. Understanding whether and how dolphins are using whistles to communicate species identity will help scientist to develop more accurate tools for identifying their sounds to species in acoustic recordings.

The overall objective of this work was to investigate whether species information is carried in dolphin whistles using three approaches: 1) by comparing the whistle repertoires of six different dolphin species, 2) by playing whistles of different species to bottlenose dolphins and observing behavioural reactions, and 3) by playing whistles of different species to bottlenose dolphins and looking at changes in heart rate. Conclusions from these work packages are that dolphins share many whistle-types across species but also produce whistle-types that are unique to individual species. These unique whistle-types may carry species specific information. Dolphins exhibited behavioural reactions to whistles produced by different species but no significant changes in heart rate. This shows that dolphins recognize and react to differences in whistles produced by different species and that these reactions are not just driven by the autonomic nervous system.
The objectives of this project were met through completion of four Work Packages. In Work Package 1, I analysed whistle contours from six dolphin species recorded in the tropical Pacific Ocean to quantify dolphin whistle repertoire diversity for the first time. I compared whistle these repertoires between species and determined that many whistle contours are shared between species but whistles that are unique to individual species also exist. Dolphins may be using these unique whistle-types to communicate species identity.

In Work Package 2, I conducted playback experiments with bottlenose dolphins to investigate whether they perceive species differences in whistles. The playbacks consisted of whistles produced by bottlenose dolphins, spotted dolphins and melon-headed whales. I conducted playback experiments with four independent groups of dolphins and conducted four playback sessions with each group. During the playbacks the dolphins were allowed to swim freely in their pools. I made underwater video and audio recordings of the dolphins before, during and after the playback. I analysed these recordings to investigate any behavioural reactions that occurred in response to playback of whistles produced by different species. Statistical comparisons showed that swim speed increased significantly when the species producing the playback whistle changed. This demonstrated that bottlenose dolphins perceive differences in the whistles produced by different species and react to those changes.

In Work Package 3, I examined a physiological response to whistles produced by different dolphin species by measuring heart rate during playbacks to bottlenose dolphins. The playback sequences used in this Work Package were the same as those that I used in Work Package 2. During playbacks, I recorded heart beats from the focal dolphin. I then calculated average heart rate in 2-second bins and compared the heart rate bins immediately before the first whistle in the playback, the first whistle after the species changed and the last whistle in the playback to the four successive heart rate bins in each case. I found no significant difference in heart rate. Heart rate is controlled by the autonomic nervous system and so my study shows no autonomic orienting or defense response to whistles produced by unknown individuals, regardless of species.

In Work Package 4, I developed four educational videos about dolphin communication for primary school children. These videos are multiple episodes of a news show called Dolphin Speak. In each episode the primary school-aged hosts introduce a topic and interview marine mammal scientists to answer questions about that topic. I developed study guides to accompany the videos. The videos and study guides are posted on the project website. I also presented results from my study at two international scientific conferences, at the University of St Andrews School of Biology Away Day and at a National Institute for Mathematical and Biological Synthesis (NIMBioS) Investigative Workshop on Bio-acoustic Structure. I was involved in several outreach activities, including presenting three talks on to students at local primary schools and running a booth on dolphin communication at the St Andrews Science Exploration day. I am currently preparing three manuscripts describing the results of this project for publication in peer reviewed journals.
Man-made noise in the ocean can have negative impacts on dolphins, and passive acoustic methods are being used to study and mitigate this. One of the challenges in analysing acoustic data is identifying sounds in these recordings to species. Development of tools to accomplish this task is an active field of research, but accurate identification of dolphin whistles to species has proven to be a challenge. Most methods for identifying whistles to species are based on measurements taken from many whistles in a recording. My results show that dolphins do react to whistles produced by different species and that individual species produce whistles that are unique to them. This knowledge can be used to improve our ability to identify whistles to species. Using species-specific whistle-types to identify species rather than using measurements taken from many whistles may lead to more successful analysis tools. The ability to more accurately identify species in recordings will provide more complete answers to questions about how man-made sounds affect dolphin species and provide the capability to mitigate our impacts on dolphins more effectively. Many end-users will benefit from the results of this research, including scientists developing tools for acoustic species identification. These tools in turn benefit organizations that need to monitor and mitigate effects of anthropogenic activities on marine life.