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The porpoise love knows no boundaries


Those of us who have seen harbour porpoises in the wild know how mysterious they are. You see them right there and then, gone! It can be very frustrating, but that’s what makes them so fascinating.


Because is so difficult to observe them at sea for long periods, key to learn about their behaviour, porpoise researchers depend on technological advances. For example, before a study published in 2018, we did not know they spend a lot of time at hearing distance from other porpoises – we know this because researchers are able to put a recording device on the porpoise’s back and record for up to 40 hours (1). And thanks to affordable drones, we know porpoises can engage in cooperative hunting when targeting a school of fish (2).


What about interactions with other species? Based on direct observations at sea or on stranded animals, we know those interactions do not always end well for the porpoise (see Catch me, if you can!) but that’s not always the case.


They are relatively rare (probably because of how difficult it is to see them), there are reports of “friendly” interactions between harbour porpoises and individuals or groups of other species. For example, a juvenile harbour porpoise was seen travelling with a group of bottlenose dolphins (Tursiops truncatus) in the Marmara Sea, spending much of its time in the echelon position – that’s where calves usually stay when travelling with their mothers (3).


Bottlenose and porpoise.  a-b) A juvenile harbour porpoise (indicated with white arrow) swimming with bottlenose dolphins, c-d) bow-riding with the observation boat –from Tonay et al 2017.



This case is rather surprising as in other areas bottlenose dolphins are responsible for dozens of harbour porpoise deaths every year (4).  


On the west coasts of the US and Canada, harbour porpoises share the same area with Dall’s porpoises (Phocoenoides dalli), although there seems to be a temporal separation in their presence. Although the two species have never been seen in mixed groups, there are confirmed hybrids of these two porpoise species in the area, which seem to occur at relatively high levels (5). By 2004, there were at least 20 cases, of which nine were confirmed via genetic analyses. In all cases where DNA tests were done, the mother was a Dall’s porpoise and the father a harbour porpoise, which explains why hybrids are observed in Dall’s but not in harbour porpoise groups (5).


The only known case of long-term interspecific interaction between an individual of another species and a harbour porpoise is the interaction between Kylie, a solitary female short-beaked common dolphin (Delphinus delphis) and harbour porpoises in the Firth of Clyde, Scotland (6, 7).


Kylie’s home. Area within the Firth of Clyde in west Scotland, where Kylie spent about 20 years of her life and interacted with harbour porpoises regularly.



Kylie lived in the Firth of Clyde since at least the early 2000s to 2022. During this time, she moved between different areas in the west of Scotland, so she received several names over the years, including Kylie, Donna, and Colin. Her sex was unknown until 2019, when it was confirmed via underwater images taken by a local diver. She lived a solitary life since she was first seen in the area, except when she hanged out with a harbour porpoise (6, 7).


Opportunistic observations made by many locals suggest Kylie spent time with a harbour porpoise regularly (David Nairn, personal communication). Ryan et al. (2017) reported two observations of Kylie travelling with a harbour porpoise, that were made in two different areas of the Firth of Clyde, four years apart. The authors had access to underwater photographs of the porpoise, and thanks to the colouration it was possible to identify the porpoise, which was the same in both cases (7).


During my PhD, I was lucky to study the acoustic interaction between Kylie and harbour porpoises the Fairlie/Hunterston Channel, in the Firth of Clyde. David Nairn, Director of Clyde Porpoise CIC, generously shared data he collected in 2016 and 2017, so we could investigate what Kylie was doing, acoustically, when hanging out with a porpoise (6).


Travelling together. Solitary common dolphin Kylie travelling together with a harbour porpoise (Top: photo by D. Nairn, 2018. Bottom left: photo by P. Nichols, 2017. Bottom right: photo by G. Patterson, 2009). All pictures were taken off Cumbrae, in the Fairlie/Hunterston Channel, in the Firth of Clyde, West Scotland



The first thing we discovered, though, based on photographs taken by David and others, that Kylie spent time with different porpoises, although with just one at a time.

The second thing we discovered, was that Kylie was doing some “weird” sounds. Before explaining these sounds, though, let’s talk about the sounds common dolphins and harbour porpoises typically produce.


Harbour porpoises only produce clicks. These are short sounds (100 microseconds) that have all the energy concentrated above 100 kHz. That’s the pitch of the sound – for context, humans can only hear up to 20 kHz. These clicks are also “polycyclic”, that means that it has many peaks. Common dolphins, on the other hand, can produce different types sounds, including whistles and clicks. However, common dolphin clicks are “oligocyclic”, with few peaks, and have energy in different frequencies, mostly below 40 kHz, although they are vocally flexible (8).




Kylie and porpoise clicks comparison. Left: Typical low-frequency click produced by Kylie. Right: typical click produced by harbour porpoises. Top panels: waveform (amplitude is normalised using the clipping level of the hydrophone as the maximum value). Central panels: normalised power spectral density. Bottom panels: Smoothed Pseudo Wigner-Ville Distribution. From Cosentino et al., 2021.



Back to what Kylie was doing. We did not record any whistles or other non-click sounds during these interactions. Kylie only emitted clicks – weird clicks.


What we saw is that although she emitted “typical” clicks often, she also did something we had not seen before. When producing a series of consecutive clicks (what we usually call a “click train”, Kylie would start with a typical low frequency, oligocyclic click and slowly but surely generating a polycyclic click “moving” the energy towards higher frequencies. Porpoise frequencies.


Click evolution. Click train emitted by Kylie when she was seen swimming with a porpoise. The top shows the waveform – how the clicks move from oligocyclic to polycyclic. The middle panels, show how the energy moves from low-frequency to high-frequencies. Porpoise frequencies.


If we now look at the previous figure, adding the new clicks Kylie made, we can see how similar they are to typical porpoise clicks! Not perfect, but very similar.



Kylie and porpoise clicks comparison. Left: Typical low-frequency click produced by Kylie. Middle: typical click produced by harbour porpoises. Right: High-frequency click produced by Kylie (recorded when seen alone). Top panels: waveform (amplitude is normalised using the clipping level of the hydrophone as the maximum value). Central panels: normalised power spectral density. Bottom panels: Smoothed Pseudo Wigner-Ville Distribution. From Cosentino et al., 2021.


We think what Kylie is doing is some sort of imitation. It does not necessarily mean she communicates with porpoises, but our results can be seen as evidence of attempts to reproduce porpoise sounds.


Kylie has been surrounded by porpoises for over 20 years, it is not crazy to think she could have learned to do these clicks from porpoises.


Given she has been seen with different porpoises over the years and that the porpoises came to her, to the buoys she frequented, suggests the interest is mutual. In fact, we have been given a photograph that confirms this. I just could not believe my eyes when I saw it!


Kylie the solitary dolphin with a harbour porpoise calf

Kylie and porpoise calf. Courtesy of David Ferguson


The picture was taken by David Ferguson and shared with David Nairn. It shows Kylie with a porpoise calf! David Ferguson saw a mother porpoise bringing the calf to Kylie. They played for a while and then it went back to its mom. How cute is that??


Unfortunately, David Nairn has told me that there has been no confirmed sightings of Kylie since February 2021, when the Valaris drill ship emergency occurred at Hunterston Quay, just meters from Kylie’s home buoy. During the emergency, 6 tugs boats spent almost a week operating to push and stop the drill ships running aground. The prolonged noise and other disturbance would have been immense and thought she has either left the site for good or been killed by propeller/thrusters from the vessels involved. A Clyde-wide search was made for her but could not be found and no strandings reports were received.


Here is link to news report and a video David took of the incident during the storm.



In early 2023, I was contacted by Daiane G. Anzolin, a Marine Mammal Observer (MMO) who works towards protecting marine mammals from dangerous noise sources. An MMO works on board platforms or vessels that can emit very high noise levels, such as pile driving during construction of marine wind turbines or vessels looking for oil and gas. These are required to have an MMO onboard to make sure there are no marine mammals within a certain range before they engage in noise-producing activities.

Common dolphin and porpoise calf. Courtesy of Luis Alejandro Lezama (instagram: @negrokistani).


In July 2022, during a geophysical survey in Ireland, a pod of at least 50 short-beaked common dolphins was observed swimming around the vessel. The group was divided in subgroups of approximately 10 individuals and were observed continuously for at least 40 minutes. Onboard, there were MMOs, including Berenice Gomes – who Daiane introduced me to. The sighting was so exciting, that many other crew members joined the MMOs, to observe the pod. Luis Alejandro, one of the geophysicists onboard, after checking the photos he took from the dolphins looked for the MMOs as something unusual had been captured. When the MMOs analysed the photo, they realised it was a calf of harbour porpoise, swimming along an adult of a common dolphin! Bernice was kind enough to share the image with me.


These are once-in-a-lifetime encounters and am not jealous at all! :)


Once again, our understanding of rare events come by being at the right place and the right time, and by sharing our knowledge with colleagues. I am happy to have been the recipient of all of these images and acoustic data and that I can share our discoveries with you all!


References

1)    Sørensen, P. M., Wisniewska, D. M., Jensen, F. H., Johnson, M. P., Teilmann, J., & Madsen, P. T. (2018). Click communication in wild harbour porpoises (Phocoena phocoena). Scientific Reports, 8(1), 1–7. https://doi.org/10.1038/s41598-018-28022-8


2)    Ortiz, S. T., Stedt, J., Midtiby, H. S., Egemose, H. D., & Wahlberg, M. (2021). Group hunting in harbour porpoises (Phocoena phocoena). Canadian Journal of Zoology, 99(6), 511–520. https://doi.org/10.1139/cjz-2020-0289


3)    Tonay, A. M., Dede, A., & Öztürk, A. A. (2017). An unusual interaction between bottlenose dolphins (Tursiops truncatus) and a harbour porpoise (Phocoena phocoena). Journal of the Black Sea/Mediterranean Environment, 23(3), 222–228.


4)    Ross, H. M., & Wilson, B. (1996). Violent interactions between Bottlenose dolphins and Harbour porpoises. Biological Sciences, 263(1368), 283–286.


5)    Willis, P. M., Crespi, B. J., Dill, L. M., Baird, R. W., & Hanson, M. B. (2004). Natural hybridization between Dall’s porpoises (Phocoenoides dalli) and harbour porpoises (Phocoena phocoena). Canadian Journal of Zoology, 82(5), 828–834.


6)    Cosentino, M., Nairn, D., Coscarella, M., Jackson, J. C., & Windmill, J. F. C. (2022). I beg your pardon? Acoustic behaviour of a wild solitary common dolphin who interacts with harbour porpoises. Bioacoustics, 31(5), 517–534. https://doi.org/10.1080/09524622.2021.1982005


7)     Ryan, C., Macleod, G., Dinsdale, C., & Cook, S. (2017). Long-term association between a solitary common dolphin (Delphinus delphis delphis) and a harbour porpoise (Phocoena phocoena). Aquatic Mammals, 43(1), 113–115. https://doi.org/10.1578/AM.43.1.2017.113


8)    Roch, M. A., Klinck, H., Baumann-Pickering, S., Mellinger, D. K., Qui, S., Soldevilla, M. S., & Hildebrand, J. A. (2011). Classification of echolocation clicks from odontocetes in the Southern California Bight. The Journal of the Acoustical Society of America, 129(1), 467–475. https://doi.org/10.1121/1.3514383

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