To Main Menu

To Article

To Local Menu

New Insights into the Sociality of Narrow-Ridged Finless Porpoises

31 Oct 2024

Summary of our research

Dr. Tomoyoshi Terada, the first author of this paper and a former PhD student at Graduate School of Bioresources, Mie University (currently a JSPS postdoctoral researcher at the Graduate School of Arts and Sciences, the University of Tokyo), collaborated with researchers from Mie University: Professor Tadamichi Morisaka (corresponding author), Professor Motoi Yoshioka, PhD student Genfu Yagi (currently an Assistant Professor at the same graduate school), Technical Assistant Ms. Ikuko Kanda (Cetacean Research Center), and Mr. Kiyohiro Ogawa (Fisheries Research Laboratory). They conducted research on narrow-ridged finless porpoises (NRFPs) in Ise Bay using drone observation and acoustic recordings. NRFPs, commonly found in Japan's coastal waters, were previously thought to be solitary animals. However, this study revealed that, in addition to their solitary aspect, they also exhibit social behavior, frequently forming groups of 2-3 individuals (group-living). These findings highlight the importance of the underwater acoustic environment for the endangered NRFP and suggest that anthropogenic underwater noise may have a greater impact on these animals than previously thought.

Understanding the social organization of animals and how they interact with others is not only crucial for comprehending their ecology but also for estimating the impact of human activities on the species. Furthermore, comparing the social organizations of various animals can provide insights into how human societies have evolved. Traditionally, the social organization of animals, including cetaceans, has been studied by identifying individuals using natural markings, such as scars or coloration on their body (in cetaceans, primarily dorsal fins), and observing them over long periods. However, NRFPs lack dorsal fins, making individual identification extremely difficult. Despite living in coastal areas close to human activity, studies of sociality on NRFPs have been relatively limited. In this study, we focused on the behavior of NRFPs, the distances between individuals, and the range within which they can hear the conspecific sounds (active space) to determine whether their social organization is solitary or group-living.

We conducted a study on NRFPs inhabiting Ise Bay, focusing on the coastal areas of Tsu, Ise and Toba city. If we assume that the social organization of this species is solitary, we would expect individuals to maintain a certain distance from each other and to refrain from engaging social behavior with others. On the other hand, if we assume a group-living social organization, individuals would likely stay close to one another and frequently engage in social behavior to maintain their relationships. To determine which assumption is more accurate, we conducted drone-based behavioral observations and measured the inter-individual distances. Additionally, we recorded the sounds of NRFPs from boats to estimate their active space of their sounds. The distribution of inter-individual distances showed a bimodal pattern, with peaks at distances much closer than the estimated active space (averaging 6 meters) and at approximately the active space (around 70 meters). Frequent social interactions were recorded among groups of 2-3 individuals at close range, while more than half of the individuals were observed alone. These findings suggest that NRFPs may have a social organization that incorporates both solitary and group-living characteristics.

When we think of dolphins, we often imagine them living in large groups. However, some species, like NRFPs, live either alone or in small groups of 2-3 individuals. By comparing the social organization across such species, we hope to gain insights into how animal societies, including the complex social structures of humans, have evolved. As human activities in coastal areas increase, NRFPs, now classified as an endangered species, face growing challenges. The results of this study highlight that the underwater acoustic environment is more important for NRFPs than previously thought. The findings suggest that measures to prevent the increase of anthropogenic noise, such as that from ships and construction, in their habitats are crucial for the conservation of this species.

This research was published online in the international journal Behavioural Processes on September 14, 2024.
Bimodal distribution of inter-individual distance in free-ranging narrow-ridged finless porpoises

https://doi.org/10.1016/j.beproc.2024.105102

dolphin

Researcher information

Postdoctoral Researcher (JSPS), Terada

Terada Tomoyoshi
 Postdoctoral Researcher (JSPS), The University of Tokyo, Graduate School of Arts and Sciences

Specialized area:

Cetology

Current research field:

Evolutional factor of vocal imitation in cetaceans

Professor, Morisaka

Morisaka Tadamichi

 Professor, Mie University, Graduate School of Bioresources/Cetacean Research Center

Specialized area:

Bioacoustics, Ethology, Cetology

Current research field:

Description of interesting behaviors in cetaceans, Development of the elementary techniques for cetacean conservation, and Basic study for cetacean communication and society

Professor, Yoshioka

Yoshioka Motoi

Professor (Vise President), Mie University

Specialized area:

Marine Mammalogy, Reproductive Physiology

Current research field:

Understanding of the mechanism of cetacean reproductive physiology, Understanding of the life history of narrow-ridged finless porpoises in Ise Bay

Ph.D. student, Yagi

Yagi Genfu

Assistant Professor, Mie University, Graduate School of Bioresources

Specialized area:

Cetology

Current research field:

Development of noninvasive age estimation methods and life history studies for Indo-Pacific bottlenose dolphins

Assistant Technical Staff, Kanda

Kanda Ikuko
 Assistant Technical Staff, Mie University, Cetacean Research Center

Specialized area:

Cetology

Current research field:

Establishment of the monitoring system for the conservation of narrow-ridged finless porpoises in Ise Bay

Ogawa Kiyohiro
 Technical specialist, Mie University, Graduate Schoolo of Bioresources, Fisheries Research Laboratory