Acoustic signature reveals blue whales tune life history transitions to oceanographic conditions: Podcast Transcript

William K. Oestreich (credit: Taylor Kubota)

In this podcast for Functional Ecology, Assistant Editor, Frank Harris, sits down with William K. Oestreich—a postdoctoral fellow at Monterey Bay Aquarium Research Institute, USA—to discuss his recently published paper ‘Acoustic signature reveals blue whales tune life history transitions to oceanographic conditions’.

Will’s paper suggests that blue whales use flexible cues, perhaps including individual sensing of food availability and social information from other individuals (blue whale songs are audible over hundreds of kilometres) to match timing of feeding and migration with ecosystem processes. This flexibility could be key to survival of this endangered population in an era of rapid global change

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Frank: Today, I’m delighted to welcome William Oestreich to the podcast. He is a postdoctoral fellow at Monterey Bay Aquarium Research Institute, USA. He earned his PhD in Biology from Stanford University, as well as MS and BS degrees in Environmental Engineering from Northwestern University. He also previously held positions at the Woods Hole Oceanographic Institution and the Natural Resources Defense Council. He has authored over 20 papers, and, At MBARI, he is investigating how social information and oceanographic drivers influence the behaviour of both predator (blue whales) and prey (krill) groups in a patchy and dynamic marine ecosystem.

Today, on top of finding out more about William, we are going to be discussing his 2022 Functional Ecology research article—Acoustic signature reveals blue whales tune life history transitions to oceanographic conditions. Hello Will, how are you?

Will: Hello, thanks for having me on. 

Frank: We’re really, really excited to be chatting about blue whale songs—which is probably not what people conventionally associate with blue whales—but before we get into all that I wanted to start with introductions. Could you please tell us a little bit about yourself, what your research interests are, and how did you come to study blue whales?

Will: I think the introduction you read off there speaks well to my background on paper (in terms of what’s on the CV) and how I ended up here. I have a background in oceanographic research, which is what I was doing between my masters and my PhD. In my studies, I’ve been looking to understand both physical and biological variability in the open ocean. And that sort of background and my long standing general interest in animal behaviour made me really want to put those two things together. There are animals that are migrating or moving nomadically through these open ocean ecosystems, so I wanted to explore how they were responding to the incredibly dynamic and variable habitat that they live in. And then looking forward, what does that mean for their ability to adapt to long term changes in the ecosystems they’re inhabiting? So that’s kind of the mindset that I brought to my PhD program in Biology.

I was originally intending to study this in a migratory fishes context because I wanted to have that connection to fisheries management, but I met one of my co-authors on this study, Dr. John Ryan, at MBARI early in my PhD and he really turned me on to the amazing world of bioacoustics. I especially was thinking about how we have such a challenging time observing the behaviour of animals continuously through time in oceanic ecosystems. It made me understand just how valuable of a tool bioacoustics can be, and I really bought into that during my PhD and took a hard left turn into whale bioacoustics which was totally unexpected. Eventually I was able to bring it back around to oceanographic variability and animal behaviour. Ultimately, I feel incredibly fortunate and privileged to have had the opportunity to study such a fascinating and frankly charismatic creature.

Frank: So before our pre-recording chat, I had always assumed that people who studied whales were converted by the conservation aspect, what with whaling being one of the first truly effective global protests. But it sounds like that that wasn’t your journey. Perhaps you could mention what your favourite study organism is?

Will: I’m leaning into a theme of just loving the bioacoustics world. Honestly, I’ve really become obsessed with batsthey’re a little bit more accessible in a lot of ways than blue whales! You know, you can see them in your neighborhood at sundown or sunrise. And, due to my interest in acoustics and using acoustics to study animal behaviour, I ended up getting really fascinated with bats. I’m really just a fan of, without really being an active participant in, bat research. I started recording using an ultrasonic microphone in the backyard, but this was more as a hobbyist and following along the amazing research that people are doing around the world on those animals. I just really appreciate the opportunity to use these tools because you can, to some degree, see the animals that you’re recording. This is true even though you can’t hear the animals that you’re recording, given that it’s above our human hearing range. There’s just something about that mix of factors that really draws me into bats, but it also has a lot of connections to the work that I actually do on bioacoustics in the ocean. So that’s kind of been a newfound favorite organism for me since getting into the bioacoustics. 

To your question about the journey into working on whales, I think that the background that you outlined of what one might expect is one that I hear a lot—you know, that expectation. To be honest, there’s no harm in that that approach to things, but it’s not how I came to it. I grew up in the middle of the country, not really anywhere near the ocean. I think some of the fascination with all of these sorts of questions comes from a place of feeling like these ecosystems are very alien to me. However, once you start to study these ecosystems, I am constantly reminded in just how many ways they really are similar to the ways that we think about our own lives and our own behaviour. However, as we discussed in the paper and maybe we’ll get into a little bit later, there are some really notable differences about just what it means to be living in a liquid medium and what that means for how you make behavioural decisions. 

Frank: We’ll certainly get on to that! Being able to study the behaviour of of an organism through sound seems like alchemy. You take something ostensibly unidimensional and are able infer and gain so much information. We will get on to the paper soon; however, just to set the tone for the podcast—a quick sound pun—we’re going to be playing a recording of something for the listeners. 

Will: What you’ve just heard is a recording from the underwater microphone (or hydrophone) that’s positioned in outer Monterey Bay here in the Central California coast, and that was a record recording of a blue whale song! This has been sped up by 10 times to bring it more into middle of our human hearing range. The frequencies or pitches that these animals are producing are extremely low frequency which are right at the lower limit of our hearing range. Without really nice headphones or speakers they can be really hard or impossible to hear. Speeding it up helps us to hear them more clearly, but I think that it also it helps us to better hear the actual structure of the song. These songs play out over minutes, and, occasionally, when these animals go into really long bouts of song, can play out over hours. So we’ve brought that a little bit more into a human time reference frame to get a sense of the structure. 

Frank: Fascinating. I was wondering if you could talk about the differences between sound and song (i.e., what differentiates sound and song), and, what was that song indicating?

Will: There’s a lot of different terminology that gets used in the bioacoustics world, particularly in animal behaviour and bioacoustics. And there’s not always—and I’m guilty of this myself—consistency in the terminology we use. However, generally speaking, I would say that the song really is a subset of sounds that blue whales can make. Around the world blue whales in different populations make different types of sounds, but, here in the northeast Pacific Ocean, the population that we were studying makes really 2 main categories of sounds. These are different call types which have been creatively labelled with letters: A, B, C, and D. Those AB and C calls are what we have just heard in the recording in that repeated pattern. They take these three different call types and they string them together in these long repeated patterns that we refer to as ‘song’. The D calls are a separate more variable, less stereotyped and patterned sound that these animals make in different behavioural settings. In addition to this, with specific mention to blue whales, the songs that they produce are thought to come only from males. This is based on relatively limited samples—it’s often challenging to know what sex the animal that you’re observing or listening to is—but there is some pretty compelling evidence that this is a male behaviour, and to be honest the function is not fully understood. Because it’s thought to be a male specific behaviour, the leading theories on why these animals are singing is: 1) as a secondary sex characteristic meant to attract a mate; or 2) perhaps it’s a territorial behaviour that these animals are using as display to other males. There really is some uncertainty over exactly what these sounds mean and that ended up being a really big part of this paper. We thought to ourselves, we’re hearing the sounds of these animals, we’re analyzing and doing statistics on their patterns through time, but, from a behavioural standpoint, what are we really measuring? That was a big part of this work.

Frank: So let’s let’s dig into it! Could you, in plain terms, explain the novelty of the paper and what it contributes to our understanding of life history transitions? 

Will: Sure thing. Before getting into that, maybe I’ll give a bit of useful background, which comes from a closely related paper that we published a couple of years ago, which really dug into what I was getting at in the last answer about the behavioural context of song. The study that we published in Functional Ecology in 2022 uses, as the primary data source, six plus years of continuous audio recording at the Monterey Bay Aquarium Research Institute’s underwater hydrophone. Prior to that, in this previous paper, we really wanted to get a sense of the behavioural context for those songs. Therefore, to study this, we combined this regional population-level understanding of when blue whales are singing (from the hydrophone) with a very individual-level understanding of when animals are singing. We did this by placing instruments directly on the backs of blue whales—attached via suction cups—and listening to the sounds that they made. We monitored this alongside other elements of their behaviour: their dive patterns; when they opened their mouths to feed; how they moved in two dimensions; and were they in foraging mode here in Monterey Bay or were they passing through on their migration to their breeding grounds?

So what we found in that paper is that there actually is a pretty clear seasonal cycle in the time of day that these blue whales sing. That came from the hydrophone which was looking at this population- level view. We were hearing song, during the summer and early fall, almost entirely during the night time. This was from a really broad sampling range of 10,000 plus square kilometers with which we could hear them. What we noticed is that as the amount of song these animals are producing peaks—in late October/early November—the 24 hour patterns of their song started to change. It wasn’t this really strong night time bias. We started to hear song more throughout the day. This jumped out of the page at us when we first started performing these analyses. The peak in song lined up with a total shift in when they were singing. We thought that there must be some behavioural correlate to explain this. We asked ourselves, what else is going on in the behaviour of these animals that might drive that shift in when they were singing. That’s where the suction cup attachments helped us to unpack the puzzle. 

What we found is that the whalesin this feeding modeare up in the bay during the summer and fall packing on as many pounds as they can by feeding intensively on dense swarms of krill (they spend pretty much all day diving and feeding on these dense aggregations of krill). What we found is that during the night appears to be when they stop feeding and spend much more time singing. So, there’s a really clear day/night delineation of feeding in the day and singing at night. 

We also found something very interesting with the whales that had finished their feeding season. These whales enter this transitionary phase where they migrate south for the winter, heading to the waters off of Central America, where they will give birth, rear their young, and feed very sparingly. As they started transitioning to that southward migration, that’s when we started to hear them spreading their songs more evenly throughout the day and night. So, we’ve got these really robust results at the individual level. We were then able to turn back to our population-level metrics from the hydrophone and realize that what we were hearing is the population switching from feeding season to migratory and breeding season. 

So, with that information in hand, I am really interested in the variability of oceanic ecosystems and how animals respond and adapt to that variability. We naturally thought “ok, we can hear the population switching to migration now. How flexible are they in that switch depending on what’s going on in the environment?” In any given year we have a really strong seasonal cycle in the ecosystem here off the coast of California. We have really intense northwesterly winds that happen in the spring and summer that cause upwelling—the offshore movement of surface water which is replaced by deep, cool, nutrient rich waters—which really just inject nutrients into the base of the ecosystem. This means an incredible bloom of life throughout the food web. Blue whales are here in the summer and fall to take advantage of the incredible density of krill that the upwelling supports. 

So, what we chose to investigate in the functional ecology paper was an investigation of the varying seasonality throughout years of this ecosystem. We wanted to know when upwelling was occurring and how intense that upwelling was? Then, in response to that, were blue whales being very fixed or flexible in their migrations? And if they were being flexible, in what ways were they being flexible. What we found is that this ecosystem does vary quite a bit year-to-year in its seasonality, and blue whales DO respond accordingly in WHEN they choose to migrate. In years where we have much more intense, stronger upwelling—which peaks much later in the year—blue whales stay feeding longer and we hear this migratory transition through their songs later in the year. Whereas years when the upwelling occurs primarily earlier—and is somewhat weaker—we hear these animals transitioning to their migration earlier in the year. This may seem like an obvious thing (‘when the feeding is better, you stay longer and migrate later’), but one of the things that I think is really interesting about this result is that it suggests that these animals are able to sense how much food is available to them over an enormous and extremely patchy potential foraging arena, if you will. 

Really, the area that these animals are feeding in is from the US-Mexico border, all the way up to southern British Columbia, and, anywhere from a few to perhaps 100 miles offshore. This is an enormous potential area in which they’re searching for these incredibly dense swarms of krill that are really the only food patches that are suitable for their body size and feeding mode (lunge feeding and filtering). Therefore, it did strike me as a somewhat unexpected and pretty remarkable result—these animals are in some way sensing that environmental seasonality and responding accordingly, even though the food source they’re looking for is really like a needle in a haystack. These animals appear to be able to do some sort of sensing that’s incredibly flexible in its response to the dynamic environment that they’re living in. 

I’ll just wrap up this answer to note that this is very different from their strategy when they’re migrating northwards into their feeding grounds. In this study, we were exploring how blue whales migrate away from their feeding grounds (their southward breeding migration). However, when they’re coming back in the spring from their breeding grounds, previous work from one of my co-authors, Briana Abrahms, showed that they’re actually extremely consistent in showing up to the foraging grounds. I think that really is highly suggestive that these animals are queuing on something in the environment during the feeding season that allows them to be flexible on when they leave to breed. Whereas, when they’re showing up, they are coming with no information, right? So it makes sense to play the odds, if you will—be consistent when you come and track long term averages rather than the individual conditions of a given year. However, when they’re deciding when to leave, they’re just working with so much more information. They’ve been feeding in this ecosystem and they have a sense of what the foraging conditions are like in this year. Perhaps they’re asking themselves “is this a good year? Is it a bad year for krill?” 

As we speculate in the paper, they’re also hearing each other and the song patterns that other animals are producing. So, if we can hear whether animals are feeding or migrating, it would suggest that blue whales can also hear this daily pattern as well. Perhaps they have a sense of what other animals are doing. So let’s say you’re a blue whale and you’re struggling to find krill. You might think this is not a good feeding year, but all of the song that you’re hearing for hundreds of kilometers around you is the feeding pattern of song. It might be really useful information to an individual inferring “ok, well, feeding isn’t good specifically where I am, but it’s not time to migrate yet there, there still is food to be found in this ecosystem.” That’s getting more into the speculative parts of the paper and some of the things that we’re following up on now. I’ll pause there. That that was a lot! 

Frank: I mean, that’s absolutely fascinating and incredible. I think one thing I wanted to ask about, especially with relation to sound, is how do you pull back from anthropomorphising? There must be a very grey area in there where you’re trying not to speculate too much. So perhaps you can tell me about the evidence behind these speculations?

Will: It’s a great question and I think it kind of gets at a long standing question in social behaviour in animal groups which is “why share information?” There’s this desire as humans to say “they’re sharing information about whether they’re feeding or migrating, and that’s the function of song.” It definitely helps them have a more optimal timing of migration. However, from an evolutionary standpoint, is there really an incentive for an animal to be sharing this information? That’s something that we’ve thought about a lot. What I just said is a potentially interesting and charismatic interpretation of these results. But what would be the benefit of that to individuals, or even to the population? I think this is something we’re really digging into now, primarily from a computational and more theoretical perspective. For this, I’ve been working closely with my collaborator, Stephanie Dodson. She’s a mathematician at Colby College and we’ve been digging into this exact question on whether there are alternative explanations for this transitional flexibility. How useful is that information actually to these animals? Does it give them more information than they can gain in their own exploratory movements throughout the foraging habitat? 

One potentially interesting explanation comes back to what I was talking about earlier regarding what is thought to be the function of song—of it perhaps being a mating or territorial display. There could be this idea that this information on whether animals are feeding or migrating could be a secondary outcome of what really is not the intended use of song. Males might be singing for some totally different reason; however, when they’re producing that song it provides secondary information to other individuals. This could be an unintentional sharing of information that individuals can ‘eavesdrop’ on. They have evolved to sing these songs for a reasonthey’re extremely loud, low frequency sounds that can be heard over hundreds of kilometers in the ocean. So it really is tempting to lean into the idea that they are sharing information. But even if they’re not intentionally sharing information, they’re still putting information into the environment which could be useful. The other part is that because it is so incredibly useful to us as researchers, it really is tempting to infer that it would be useful to the animals themselves in terms of making decisions. 

This is the follow-up work that we’re working on in my postdoc at MBARI. We are thinking about the feasibility of those explanations. What are some of the potential evolutionary drivers of dynamics like that? We also want to look at what role krill behaviour plays in this whole puzzle, which is something we weren’t able to get into as much in the Functional Ecology paper.

Frank: Now ecologists always hate it when I ask them to do this, but I want you to get your crystal ball out and describe your hopes are for the future and where the research should be directed next. Perhaps you can tell me what changes you hope your work will precipitate? And perhaps just something on the future of blue whales in general—are we seeing a bounce back from previous very high threat levels?

Will: I can take this answer in a couple of different lanes. First, I’ll zoom way out and go beyond blue whales and focus on animal migration, as well as behavioural and movement ecology. I think one of the things I learned, with my journey toward and background on working in bioacoustics, is just this really newfound appreciation I have for how valuable a tool that is for movement and behavioural ecology. It’s really opened our perspectives on oceanic ecosystems where we’ve historically struggled to ask and answer these sorts of questions. So that’s really become a central part of how I think about my research now, and I hope it can be influential for how others think about their own research. This approach can be so valuable for connecting levels of biological organization in behavioural and movement ecology, especially when there’s this classical idea in ecology that what we’re doing is trying to connect patterns across scale. 

So much of the research on behaviour, especially as it relates to migration, really is about individual-level behaviour. And there really has been this emerging field of collective behaviour and collective migration studies. I think to really understand dynamics in that field, we need this cross-scale inquiry. We need this individual-level understanding of how animals make decisions but also how that plays out in similar and sometimes confoundingly different ways when you start to scale up to groups and populations of animals. For these types of studies, these bioacoustic tools really are incredibly valuable. They inherently give us that perspective, especially in the ocean, where sound travels so far and fast. They give us a more regional population-level view on behaviour. Of course, they at times can obscure individual-level pattern. That’s why I mentioned that tagging-based study to give individual-level context for population-level patterns. I guess I’m really excited about the potential for synthesis across those individual, group, and population levels and how that can improve our understanding of really what’s driving the patterns that we observe regarding how animals make decisions, as well as how those things change depending on the scale of observation. 

Specific to blue whales, one of the things that comes out of this study for me is a real appreciation for how adaptable these animals are to oceanographic variability. I think there are very valid and real concerns from a conservation perspective and with regards to viability of the population for a lot of large whale species that were hunted nearly to extinction. I am certainly not suggesting that they’re fine and that they’ll figure it out. There’s especially threats to a specialist predator like the blue whale that only feeds on one prey item and doesn’t have a lot of flexibility in terms of foraging strategy or preferred prey. However, I will say that the results of this study are encouraging in some senses because maybe they can’t switch what they’re eating, but they can switch what time of year they are foraging and migrating. 

Frank: Fantastic. So just quickly to wrap up, I’d like to ask if you’d like to give any shout outs to anyone or anything that’s helped you along the way?

Will: Yeah, absolutely. Every one of the co-authors on this Functional Ecology paper deserves a massive shout out. I mentioned Briana Abrahms and her work on the memory-based northward migration of blue whales. Her work was really a big inspiration for this study and she was extremely helpful in helping to understand the patterns we were observing. My co-author, Megan McKenna, is an expert in bioacoustics who I always learn a lot from. Larry Crowder, one of my PhD advisors, has just a great holistic understanding of behaviour and how that relates to resource management and conservation. Jeremy Goldbogen runs the lab that has done all of this tagging work which is just an outstanding and really remarkable dataset for providing a perspective on that individual-level context. And finally, John Ryan, who I’ve mentioned several times. He’s my postdoc advisor and really has been the driving force behind this incredible long term passive acoustic monitoring project. Lots of folks to thank! 

Frank: That’s great. I’d like to let everyone know that the paper and the plain language summary will available in the description of this episode. Finally, I’d just like to thank Will for his time. I hope everyone’s found this episode as fascinating as I have. Thank you Will. 

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