Ultrasonic Vocalization of Weddell Seals

Weddell seals produce ultrasonic vocalizations

The Journal of the Acoustical Society of America (JASA)
https://doi.org/10.1121/10.0002867

Authors: Paul A. Cziko, Lisa M. Munger, Nicholas R. Santos, and John M. Terhune

In this episode, we interview Jack Terhune of the Department of Biological Sciences at the University of New Brunswick.

Read more from The Journal of the Acoustical Society of America (JASA).

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Music Credit: Min 2019 by minwbu from Pixabay. https://pixabay.com/?utm_source=link-attribution&utm_medium=referral&utm_campaign=music&utm_content=1022 

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Ambri Phillips  00:06

Welcome to Across Acoustics, the official podcast of the Acoustical Society of America’s Publications office. On this podcast, we will highlight research from our four publications, The Journal of the Acoustical Society of America, also known as JASA, JASA Express Letters, Proceedings of Meetings on Acoustics, also known as POMA, and Acoustics Today. I'm your host, Ambri Phillips, Business Administrator for the ASA.

Joining me today is Jack Terhune. of the Department of Biological Sciences at the University of New Brunswick. We'll be discussing his article “Weddell seals produce ultrasonic vocalizations”, which appeared in the December 2020 issue of JASA. Thank you for taking the time to speak with us today. Jack, how are you doing? 

Jack Terhune  00:56

Oh, fine. Thank you.

Ambri Phillips  00:58

Um, can you give us a bit of background on yourself?

Jack Terhune  01:02

About 50 years ago, I started working with a project at the masters level, looking to try to determine if seals echolocate, and have the same abilities that dolphins and bats have. The question was, how do they find their food in wintertime when they're diving deep below the ocean surface where it's very dark. And at that time echolocation in dolphins and porpoises was just beginning to get underway. And so we wondered if seals did the same thing. So as part of that study I began by studying the hearing abilities of the seals basically, the same as humans have a regular hearing test. We just trained the animal to respond to sound and it tells us when it can hear it and what it can't hear. And at the same time, we made some high frequency recordings on sea ice in amongst the harp seal breeding herd., We were trying to see if we could actually hear any echolocation type signals. And the long and short of it is that the seals do not have the echolocation abilities to the same degree that dolphins and porpoises do. But they hear very well underwater and many have a very wide variety of vocalizations.

Ambri Phillips  02:31

Okay, and what can you tell us about Weddell sells?

Jack Terhune  02:35

They live around Antarctica. They tend to inhabit the really heavy ice areas such

 as thick pack ice or land fast ice. And during the winter, the sea ice will freeze up to 200 to 300 kilometers out from the coast. So that will be a completely solid sheet of ice. By the end of the winter, it's about two meters thick. And what happens in the summer, the sea ice, basically most of it melts away and begins to reform in May. And as it reforms, some seals will move into that area. And they maintain breathing holes through the ice really from as soon as it starts to develop and such and so they will maintain breathing holes all winter long. And then in the spring, in the Austral spring and October, the females will come up onto the ice and have their pups. The pups are nursed for four to six weeks. While this is going on, dominant males will establish underwater territories under the tgroup where the females are and they defend their territories very vigorously by using very high amplitude vocalizations, particularly trills, and other call types. And if an intruder male comes in and is not scared away by the shouting, so to speak, they will they physically beat up on one another. It's not to the extent of elephant seals. So the seals live in this very heavy ice pack ice for most of the year. We think that during the winter, when there's a little bit of light in the sky, although most of them are just south of the polar circle. There's some twilight in the middle of the day. We think that they spend a lot of their time feeding at that time, and we infer this because they're vocally quite quiet, and so forth, but during the night, and particularly at the equivalent of dusk and dawn, they are vocally quite active. And so they seem to maintain a constant communication among individuals throughout the season. They're quite a large seal. They're close to three meters long, and a couple of 100 kilos. One nice attribute about their behavior is that because they have evolved in an area where there are no land based predators, they're not afraid of humans. And so you can work around them quite easily. And they will return to very normal sort of behaviors within 10 or 20 minutes after a helicopter is flown away, So they're really they're nice animals to study. If you go right up to them and approach them, then they will fully defend themselves. But if you just back off a couple of meters, they'll not pay attention to you. 

Ambri Phillips  06:12

Before the study, what was known about the underwater vocalization of what else hills?

Jack Terhune  06:18

The underwater vocalizations were first recorded by some researchers at McMurdo Sound in the late 1960s, and then again more thoroughly in the 1970s. And it was determined that time that they have a very wide or large vocabulary. They make a whole series of calls and many of them sound very much like bird whistlesand chirps. But there's also a lot of whistles and low frequency calls grunts and groans and chugs. The male's produce trills, which start off at a very high frequency, higher than we previously believed before this recent work, they will drop from around 20 kilohertz the upper range of human hearing, right down to about 60 Hertz, a span of about 14 octaves a very wide range, and these trills can last for over a minute. Mostly they're 15 to 30 seconds, but some of them are running between one and two minutes long of continuous sound exposure and sound production. They are produced with the mouth and nostrils closed underwater. And from some on-ice observations, we believe that they are produced in the larynx, the voice box of most mammals, and that the sound sort of projects forward out of the front of the throat. We don't know how they make this, the sound  is source driven. In addition to the large number of different call types, Weddell seals exhibit geographic variation in their calls, something equivalent to dialects in humans. Groups that are as close as five or 600 kilometers apart, will often have distinctive calls and that suggests that the both the males and the females are really staying close to home. They remain in the area where they were born. Some of the calls have a very high amplitude, they're really able to effectively shout underwater, and under perfect conditions of deep water, no intervening islands. in theory, one seal can hear another seal at a range of 30 kilometers or more. Now, most of the communication ranges are lower than that, because of being blocked by islands and icebergs, etc., but they really can produce a call that's about 120 decibels above their hearing threshold. That's a little louder than humans can shout. The high amplitudes and the fact that sound conducts very well through water results in these very long, long distance ranges. Both males and females make calls. We know that for sure because we've seen adult females that are up on the ice, and sometimes they'll dream and they'll call while they're dreaming. If you do run up and approach a female too close, she'll make some defensive calls as well, again with a mouth and nostrils closed. In addition, if you really get close to the seal they will try to bite you, but that's just what any animal would defend itself though so we just have to keep a bit of a distance. Some of the males in particular will make sequences of calls. They can make, say, a low frequency roar, and then a mid-frequency roar and then a trill and then a series of chirps. And individual males will tend to do that, in in a sequence, as I said, it may be a way that the seals can identify themselves as a specific individual, while producing call types that are common to most of the species in that area.

Ambri Phillips  10:57

Can you describe how ultrasonic underwater vocalization works?

Jack Terhune  11:03

This is the recent work that was done by Paul Cziko and his colleagues, Lisa Munger and Nick Santos. They made a series of recordings, McMurdo near McMurdo Station, and they had the establish the McMurdo Oceanography Observation unit, which was a submerged device, and it recorded oceanographic features and sound and was able to take pictures within up to a couple of 100 meters visibility. Starting in 2017, they recorded continuously or close to continuously for about close to two years. And as part of that study, they picked up a lot of Weddell seal calls when the seals were in the area. And they sort of brought me in into the study at that time to help  workout what was going on. The prior to that they've been really very little work in the high frequency range, simply because of limitations of the recorders. Most of the field recorders that I've used, were actually built for humans recording music on etc. And so they stopped at 20 kiloHertz. And we'd had some indication that some of the calls were as high as 20 kiloHertz. And then with Paul's Czilo’s results, we really confirmed that there were a number of different call types at very high frequencies, up to as high as 50 kiloHertz. And so that's kind of important because, we believe, from other studies, that the seals can probably hear up to 60 to maybe 70 kiloHertz is likely their upper frequency range. And the fact that they're making calls at 50 kiloHertz, suggests that they must be able to hear them as well. , Many of the ultrasonic calls had the same frequency shift patterns as the lower frequency calls that we've heard before. So there are trills that start off at a high frequency and then drop down over time to lower ones. There are also chirps and whistles as well as constant frequency sort of tones, virtually pure tones that go on for five or six seconds,. We'd heard those types of calls that the lower frequencies. And this recent work then shows that indeed, the animals are producing similar calls, in terms of their frequency shifts at these very high frequencies. Also, this work has demonstrated that for the previously low frequency or sonic calls that were recorded, we're only picking up a part of the overall signals. There were a number of call types in which the call started off at an ultrasonic frequency, and then dropped right down to well below one or two kiloHertz, so a complete frequency shift. And some of the complex sequences of calls that we've previously detected, we now believe will have had ultrasonic components that we didn't hear before. So there's a greater complexity in the in the calls. One important observation as well associated with the ultrasonic calls, is that in water high frequency sound is absorbed very rapidly as it moves through the medium while the low frequency sound is not absorbed. So when we're listening to seals at a distance, we really pick up the low frequency components of their calls .It appears likely that they will be behaviorally associate with calls directed to animals in the immediate vicinity. Whereas the really low frequency calls, such as the loud, really high amplitude trills, they're more of a broadcast call. I think the males are probably sending two signals. One is to attract females. They're advertising that they've established a territory. Second thing is to probably repel other males. 

We don't know exactly why they make the calls at such a high amplitude. But it could well be that if I can shout louder than you can, I'm probably bigger than you are. So, if you're another male stay away. But if you're a female, I'm very fit and would make a very good mate. We've also found that with the high frequency calls the proportion of the different call usage changes over the season. In the Paul Cziko study, we found that there's a different pattern of utilization of calls between recordings that were made in the middle of winter with 24 hours of no sunlight, and calls made in the late spring when it was 24 hours of sunlight inside. So this suggests that some of the call types will have a different behavioral function. But in general, aside from the trills that only the males make, we really haven't been able to establish the behavioral function of the different call types. We don't know why they have such a wide range of call types. And so there's some reason for them to be very diverse. There's probably well over 100 Call types, when you take into geographical variation into account. And so they they've got this very wide diversity of calls, which was expanded to these higher frequencies. And with a few exceptions, it's not been possible to observe the animals while they're calling because we're up on top of the ice recording. And the seals are under the ice making the calls. Now their underwater calls are loud enough that humans on the ice can hear them coming up through the ice, which is kind of spooky, in a way. And we also believe that the females that are on the ice likely hear the calls as well. So there's some sort of one way communication between the males that are defending the underwater territory, and the females that are up on the ice and nursing their pups. The pups are weaned probably four to five to six weeks after they're born. And at that time, the females come into estrus and will mate. So the males and female groups that are formed, it's worthwhile for the male to establish his territory because there's a very high probability that the females will mate with that particular male. Otherwise, they wouldn't bother because it's a lot of work calling and staying underwater most of the time.

Ambri Phillips  18:41

They have their own little language there. Okay, what was learned from the study that was not discovered from previous studies on Weddell seals?

Jack Terhune  18:52

Well, the complexity and the numbers of high frequency ultrasonic call types. That was new. And in a way, it was a bit unexpected. In the late 1970s according to the McMurdo Sound researcher at the time, she did some short recordings at high frequencies, and didn't actually pick up any sounds. And so, she had proposed that all of the calls were in a sonic range, and from work with other species, particularly the harp seals from the northern hemisphere, who are almost as loquacious as the Weddell seals, and they have about 30 or 40 call types. Their call types did not suggest anything that would be a high frequency. And so I think the number and diversity of the higher frequency calls policy call recorded was a bit unexpected. We've also worked out because the McMurdo oceanographic observatory that Paul was using, did have a camera. And there were a few occasions when a seal swam by and vocalized at the same time. He was able then to determine first of all, that it was a seal that was vocalizing. And he noted that when the seal was chirping it was moving its head with every chirp. So somehow it was probably using some overall movements of the neck and throat, to generate air or to pass air across the vocal cords. So certainly a definite indication that it was a Weddell seal that was calling. It was possible to estimate the distance that the seal was and then determine the source level. And what we found was that the high frequency calls were at a lower amplitude than the lower frequency calls. So they can generate these really high pitches, but not at the high amplitude of the lower of the lower frequencies. And that may give us some indication of how they're actually producing the sounds. We really don't know how they're able to achieve it. I mean, how can an animal with a mouth and nostrils closed, just using air in the trachea and the throat, produce a continuous pure tone whistle for over 60 seconds? Wow, there is yet work to be done.

Ambri Phillips  21:46

So where can we expect more studies about ultrasonic underwater vocalization?

Jack Terhune  21:52

Yes, and that's really due in part to the advent of technology. As I mentioned, in the 70s, 80s, and 90s, when I was doing most of my recordings, the equipment that I had was limited to an upper frequency of 20 kiloHertz. Now, it's very common with digitization, for higher frequency sampling. It's just technically possible. People are recording sounds now as high as 200 kiloHertz over long periods of time. And so that's really going to allow new researchers to really use the full frequency spread spectrum. Not only for the Weddell seals, but for the seals and particularly the dolphins and porpoises in particular. Another advent of modern technology is the ability to record for extremely long periods of time. As they were in the late 60s, when I started recording harp seal calls on the sea ice, we had a portable tape recorder. It was reel to reel, and a tape would last for 15 minutes, and you'd have to change it. By the 1990s, digital audio tapes came along and they would record for two hours at a time, which was nice. Now today's scientists are putting down devices that are literally recording for a year at a time continuously. This is leading to massive amounts of data. terabytes, literally, but also to the challenge of what you do with that many hours of recording. So many researchers now are developing an automated call recognizers. And they're able then to apply the recognizer so they run the recording and analyze the whole thing. The recognizer will tell when it thinks it hears a particular call type. That's going to be I think, a real boon because it will allow people to gather massive amounts of information.  In 2002, to one of my students at another Antarctic base did 24-hour recordings manually. He was able, because of logistics, to do two days a month. Nowadays people can put it out, come back six months later and have that entire time filled with frequencies that cover the entire range of the of the animals calls and such. And so with the ability to develop automatic recognizers, it's possible to actually analyze all that data

Ambri Phillips  25:00

Well thank you for taking the time to educate us on these wonderful seals. We look forward to seeing more of your research.

Here are some clips of the vocalization of the Weddell seals. Check out the JASA article to learn more,

25:15

Ultrasonic vocalizations sequences of the Weddell seal from McMurdo Sound, Antarctica, recorded in 2017 and 2018. Modified audio file audio data reduced to one quarter of original frequencies, presented at normal speed, normally inaudible, ultrasonic seal vocalizations are audible.

Ambri Phillips  26:35

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