Dan P. Costa
Director Institute of Marine Science
Department of Ecology & Evolutionary Biology
Let’s start with your background. Your work focuses on the foraging ecology and reproductive biology of marine mammals and seabirds. In particular, your research focuses on the movement patterns of these species and how they may adapt to climate change. What initially got you interested in the field of marine biology?
Everything about these animals – how they're adapted to their environment, how specific or general they are – is going to give us insights into how they'll be able to respond to climate change.
That's an easy one. When I was taking high school biology, I learned how to SCUBA dive and so I got very interested in marine biology. I grew up in Los Angeles so the ocean was accessible, about an hour away from where I lived. As an undergraduate, I took a class on vertebrate biology and the way that course was taught was very question-oriented in how animals are adapted to the environment. I got very excited in terms of how animals are adapted to where they live and do what they do. That’s been a theme that has stuck with me; ever since I was in my junior year at UCLA, I sort of pivoted to vertebrate biology, marine vertebrates, and how they make a living. That focus has gone through various iterations from understanding how they deal with water balance and whether or not they drink seawater, to how they acquire food and resources. When you start asking questions about foraging ecology, food consumption, and energetics, you start to think about how animals find their food, their movement patterns, and what's behind the movements. Once you start dealing with those sorts of questions, of course, how can you not study climate change? Everything about these animals – how they're adapted to their environment, how specific or general they are – is going to give us insights into how they'll be able to respond to climate change.
What species of seals have you worked with over the course of your career?
Over the years I’ve worked on almost all of the Antarctic pinnipeds including leopard seals, Weddell seals, elephant seals, Antarctic fur seals, and crabeater seals. The one I haven't seen and haven't worked on is the Ross seal but I have a project with the South African scientists at the end of this year that hopefully will work with Ross seals.
An interesting example in Antarctica is the work we've been doing on both crabeater seals and Weddell seals. Crabeater seals are definitely krill specialists, whereas Weddell seals feed a whole variety of different fish. If you look at the foraging patterns of crabeater seals, they have very specific habitat requirements. Weddell seals are the southernmost mammal other than people. They are very much more flexible in terms of the kinds of habitats they can be found in and the kinds of things that they do. I suspect that Weddell seals will be more plastic in their response to climate change than crabeater seals.
You’ve studied marine mammals and seabirds in a variety of locations around the world, including California, Australia, and Antarctica. Are you studying the same kinds of questions at all of these different locations, or do you have specific interests unique to each habitat?
It’s all variations on a theme. One of the things I do here at Santa Cruz is drive about 30 minutes up the coast to the Año Nuevo State Reserve where there's an elephant seal colony. We have access on a day-to-day basis to free-ranging elephant seals. A lot of the methods and techniques we develop with our local elephant seal colonies are new technologies, new ways of making measurements, and we test our equipment on the local elephant seal colonies. This gives us the ability to troubleshoot things before we apply them in a remote place like Antarctica. I’ve also worked in the Galapagos and Australia, again looking at the same sorts of questions in foraging ecology and energetics of reproduction. That allows me to take a broad brushstroke and look across pinnipeds, and even seabirds, to compare and contrast how these animals have adapted to these different habitats. For example, I’ve worked with fur seals in the Galapagos, Antarctica and the Bering Sea; as well as sea lions from the Equator to the polar regions. You can ask the question: How different is the fur seal in the Galapagos versus the fur seal in Antarctica? What are the things that make a fur seal a fur seal, and what are the things that help it adapt to the local environment?
For example, one very strong theme in my work is body size. Fur seals and sea lions are the smallest pinnipeds, while elephant seals are the largest. So why do you need to be big? One of the advantages of being big is that it gives you greater endurance in terms of fasting ability. Elephant seals go to sea, feed for nine months, and accrue all the resources they need. They then fast on shore for the 26-day period where the seal pup gets all the milk and energy from the mother, while she is fasting. In contrast, fur seals and sea lions are a lot smaller. They don't have anywhere near that endurance, so they have to go and feed locally; they’ll spend up to a week at sea and come back and feed the pups, so it takes them longer to wean the pups. But the advantage of small body sizes is that your absolute food requirements are less even though you don't have the endurance. So an elephant seal needs more food, but because it's bigger, it can dive deeper and can forage over a much broader area of the ocean. So there are these trade-offs. There are certain habitats and certain places where small body size is advantageous and other patterns where large body sizes are advantageous. That also fits with diving ability: the bigger you are, you can use your oxygen at a slower rate and stay underwater longer. If you can dive longer you should be able to dive deeper. Along this theme I recently (January 2023) was able to travel to Kazakhstan to work on Caspian seals, one of the smallest true seals. Again we employed the same suite of measurements. However, in this case to look at one of the smallest seals.
You can also compare feeding habits--whether you're feeding on the continental shelf, feeding out in the open ocean, or what kinds of food you feed on. Comparing northern elephant seals, which are from the north Pacific to southern elephant seals, which are in the Southern Ocean, they're very similar: the same genus, different species. Their reproductive patterns and where they feed in the water column are very similar. However, they differ in habitat. Northern elephant seals are found at a lower latitude than southern elephant seals. Interestingly, southern elephant seals have a higher latitude distribution than northern elephant seals and a lot of that probably has to do with geography. It also might have to do with the fact that there are more land predators in the northern hemisphere, in the form of polar bears.
Much of your research has focused on elephant seals in the Antarctic. Consequently, what have you learned about their physiology that makes them so adept at diving? How far/deep do the seals typically travel in this region? How have they adapted to diving with a finite amount of oxygen, how have they adapted to thermoregulate at such cold temperatures?
Take fur seals and sea lions. Fur seals are smaller than sea lions. and as you get larger the reliance on fur gets less and the reliance on blubber gets more profound. The largest fur seal is the Australian fur seal and it's almost the same size as the smallest sea lion, which is the Galapagos sea lion. Fur is a much more effective insulator than blubber because it has air in it. But, the problem with fur is that it compresses. One of the general ideas that was published years ago by a paleoecologist was that for deeper diving, it's better to have blubber because your blubber layer doesn't compress. so when you get really small, fur is a much better insulator. You need a lot more blubber than fur to get the same amount of insulation; if you get down to the size of a 25-kilogram Galapagos fur seal, if you put blubber on it, it would be a blubber ball. The smallest seal is the ring seal and it is a little blubber ball; it's a very plump little animal. The trade-offs with fur versus blubber are related to body size. When you get larger you need less insulation because you have a smaller surface area to volume ratio; being bigger gives you more thermal capacity and you don’t need as thick a blubber layer.
The other aspect is the diving environment. In the Galapagos the surface layer of the ocean is warm, but when you get below the thermocline it gets quite cold. Animals in the region have to be able to thermoregulate going from warm surface waters to extremely cold water at depth, and we are just now starting to look at that. There has been a lot of work that speculated on what animals do, and I have a student that's been working with elephant seals to measure the thermal gradient in terms of how good the blubber layer is at insulating. When the animal dives into colder water, the blubber layer allows the peripheral shell to cool; then, when the animal resurfaces they are gaining heat from the environment because they are now colder than the environment. So, that's some of the work we're doing on thermoregulation.
There must be a larger thermal gradient in the northern latitudes than in the Antarctic.
Exactly. Just last year we published a review article that made the point that in the polar regions the water column is much more uniform. And so, for a Weddell seal, it's almost always in -2°C seawater and maybe a little bit warmer than that. Even though it's really, really cold in Antarctica, the animal is adapting to a much more stable environment, whereas if you go to the tropics the surface can be really warm, and depending on how deep you dive you can get into some really cold water. In the regions of the planet where the water column is highly stratified and where there's a strong thermocline, one could argue that it's more challenging in terms of an animal having to go from warmer water to colder water.
In numbers, how deep do these seals typically travel? I know it will be different for each species of seal, but do you see a difference between northern elephant seals versus elephant seals in the Southern Ocean?
For southern elephant seals the deepest dive is 2400 meters and for northern elephant seals the deepest dive is about 1750 meters. However, most of their dives are between 500 and 600 meters and it varies over the course of the day. During the daytime, they dive deeper and at night time they dive shallower. This is because they are following vertically migrating prey. The deepest diving sea lion is the New Zealand sea lion. It dives somewhere around 500 meters. Seals are typically better divers than sea lions but it also varies between species. Ironically, sea lions aren’t particularly good divers, even though the paradigm is that bigger is better for diving. This was something we recently found out working on southern sea lions along with other people working on northern sea lions; they tend to feed on the continental shelf and they don't tend to feed very deep. This gets into the interaction between body size and what you feed on and the advantages of body size with overall energy balance. Sea lion diving depth probably has less to do with diving ability. For elephant seals, it's definitely all about diving ability; bigger is better. They take advantage of their large body size to be able to dive deeper.
Have you noticed any patterns throughout your career in terms of seals adapting to changes in habitat due to climate change?
The common thread that everybody's seeing is that glaciers are retreating and fur seals and elephant seals are moving further south in their distribution.
I’ve had a long window of time working in McMurdo Sound in the Ross Sea. I can tell you that certainly in McMurdo things have changed since my first trip there in the 1978-1979 season, as well as in the Antarctic Peninsula since my first trip in 1998. The common thread that everybody's seeing is that glaciers are retreating and fur seals and elephant seals are moving further south in their distribution. It is too early to tell whether or not the crabeater seal population is decreasing, but Luis Huckstadt and I are going to Antarctica in June 2022 to work with crabeater seals. We put out tracking devices on crabeater seals in 2001, 2002, and 2007 and Luis used those data to develop habitat models and then he modeled how the habitat will change.1 We've made some predictions in terms of how crabeater seal habitat has changed and we’ll continue putting tracking devices out on crabeater seals over the next two years as well as doing some aerial surveys with drones. We hope to identify how crabeater seal distribution and movement patterns have changed in response to changes in climate. The Antarctic Peninsula is the area in Antarctica with the most rapidly changing climate.
Can you talk to me a little bit more about the tagging process? How do you attach it to the seal and how long does it typically stay put?
We have to work on a stable platform. More recently, I was working on leopard seals on the Antarctic Peninsula; that was the last Antarctic seal to cross off my list other than Ross seals because the leopard seals are a little more dangerous to study. We use a variety of tags for tracking seals and sea lions. Most tags have satellite tracking abilities and are able to record the diving behavior and location of seals. Many of the tags have GPS sensors attached to them that transmit the location via the Argos satellite system whenever the seal surfaces. A lot of the tags we put on seals in Antarctica aren’t meant to be recovered. So, all of the data has to be transmitted back to us via satellite. We glue that tag to their head, because in order for the transmitter to communicate with satellites, the tag needs to be out of the water. If you put it further back on the body it probably wouldn't come out of the water. We glue it to their head with five minute epoxy then it will fall off within a year when the seals molt.
I’m guessing CTDs are pretty standard. I know technology is always advancing. Is there any new technology or new sensors that have recently come out that you're excited about?
One of my students is working on a project trying to address where these animals sleep within the water column. The sensor we are using is essentially the same type of sensor that measures REM sleep in humans. We glue these little sensors on the head that produce an electroencephalogram that allows you to measure REM sleep, or slow wave sleep. It’s a data logger that a colleague in Switzerland has designed, and has been used on albatrosses, frigate birds, and a variety of other animals. We’re the first ones to put it in an underwater housing to measure the sleep of a free-ranging marine mammal. The results of this work have recently been accepted in Science.
How long do seals typically sleep for?
We’ve put the sensor on for about a week and this is just in the earliest phases of this work; it's really cutting-edge. We’ve been able to put together a sleep record from an elephant seal that's drifting asleep at depth. We measured it going through a REM cycle before the seal woke up and surfaced.
Another newer technology is using acoustic tags that can measure heart rate. We’ve combined this with accelerometers to record heart rate along with swim velocity and depth. In one case, we recorded the dive pattern of a seal after we played a killer whale vocalization. When the seal began to surface, we played the whale vocalization and the seal proceeded to dive and its heart rate declined.
That's interesting. I would think if I was near a killer whale, my heart rate would skyrocket. Is that something that seals have adapted to lower their heart rates so that they’re less detectable or more energy efficient?
In order to dive longer, these animals shut down their metabolism. When this happens, their heart rate declines because the heart is a pretty energetically active organ. When the heart rate slows down it reduces blood flow to the peripheral tissues; this is a stress response, a fear response. The animal realizes there's danger and that it needs to dive longer so it lowers its metabolism, drops its heart rate, shunts the blood that goes to the lungs and brain, and is able to dive longer. Elephant seals typically dive for about 20 minutes. The longest dive we’ve ever recorded is 119 minutes. We think that the animal was scared and something freaked it out, but we don't know because all we have is the recording.
One more example of an interesting sensor involves work we’re doing with our Japanese sensor. We’ve attached an accelerometer to a seal’s mouth; it records when the animal opens and closes its mouth. We’re able to record feeding events and look at where they happen within the water column.
The latest thing is we just deployed this sensor along with one that measures oxygen content in the water. In one instance, we found that seals were feeding off fish within the oxygen minimum zone, essentially an area in the water that has little oxygen. We believe the fish are more lethargic in this area, so this would be a perfect place for a seal to feed.
You've been to Antarctica over 20 times and you mentioned your first opportunity was during your postdoc. How has working in McMurdo or any of the other stations changed over the years?
Let me first say that I really enjoy the field seasons where you’re with four or five other people and it's just a few of you out in your own field camp and you're self sufficient. Even though you have to cook and clean the dishes it's much more intimate. I've only been to McMurdo over four different years; I was there for one season in 1978-79 and then from 2011 to 2013 I was there, multiple times. It's really different at McMurdo because it's such a big place and there are a lot of restrictions. It makes sense because it can be a very dangerous place but it's a different experience to be in McMurdo with all of the hustle and bustle. It's convenient because it's a nice place to work.
Probably the biggest thing that has changed is communication. When I was in McMurdo we would have MARSgrams, which was a form of a telegram. If you wanted to make a phone call you had to go to Scott base because they were the only ones that had a true phone. Otherwise, you put in for a ham patch with the ham radio operator that would call somebody in the US. Some people still do this because ham operators love connecting with people in Antarctica. So you'd be talking to somebody at home, your parents or whatever going “Hi, things are really good over here. How are things with you?, over” and you do that maybe once every couple of weeks, and you would get mail. You would get really excited when a letter would come in. Now, even in the most remote field camps you have satellite phones and often e-mail. In some cases you have internet access. It was quite something for my postdoc who was in the field on our recent leopard seal project. She recently had her second child and she was able to use the satellite phone to talk with her family for 20 minutes every day! That’s not something you were able to do thirty years ago.
You've been a member of the SCAR Expert Group on Birds and Animals, EG-BAMM, for over 30 years. How did you originally get involved with it and what's been your role?
The other thing I find so exciting about SCAR – it is one of the few places I can go to where I will talk to a glaciologist, a physical oceanographer, and a phytoplankton biologist in the same place.
I was invited to participate in EG-BAMM because I was a seal researcher working in the Antarctic. Earlier on the group was slightly different; it was an expert group on birds and seals. Later when SCAR reorganized we realized that we should include cetaceans and so became an expert group on birds and marine mammals. That has been one of the more fun aspects of SCAR because there's this community of colleagues who work on marine mammals and seabirds. SCAR really brings us together. All the Antarctic tracking retrospective analysis work I mentioned was hatched at the Barcelona SCAR Biology Symposium in 2013. It took us almost 10 years to make it happen because we had to bring in data from all the researchers, then we had to harmonize all the data in the same format. We then managed to get a postdoc, Ryan Rosslinger, to do the analysis and then we had group meetings where we worked through it. To me, that was a phenomenal result that only happened because of the Expert Group, and because of SCAR. It is one of the proudest things that I've done. It's not only good science, but it shows the goodwill and the collaborative nature of our group. It's a great example of what SCAR is capable of.2
The other thing I find so exciting about SCAR – it is one of the few places I can go to where I will talk to a glaciologist, a physical oceanographer, and a phytoplankton biologist in the same place. It's ironic, I know more about the Antarctic than I know about the north Pacific because SCAR is just such a focused group.
1 Huckstadt, L. A., A. Pinones, D. M. Palacios, B. I. McDonald, M. S. Dinniman, E. E. Hofmann, J. M. Burns, D. E. Crocker, and D. P. Costa. 2020. Projected shifts in the foraging habitat of crabeater seals along the Antarctic Peninsula (vol 56, pg 213, 2020). Nature Climate Change 10:791-791. https://doi.org/10.1038/s41558-020-0745-9
2 Hindell, M. A., R. R. Reisinger, Y. Ropert-Coudert, L. A. Huckstadt, P. N. Trathan, H. Bornemann, J. B. Charrassin, S. L. Chown, D. P. Costa, B. Danis, M. A. Lea, D. Thompson, L. G. Torres, A. P. Van de Putte, R. Alderman, V. Andrews-Goff, B. Arthur, G. Ballard, J. Bengtson, M. N. Bester, A. S. Blix, L. Boehme, C. A. Bost, P. Boveng, J. Cleeland, R. Constantine, S. Corney, R. J. M. Crawford, L. Dalla Rosa, P. J. N. de Bruyn, K. Delord, S. Descamps, M. Double, L. Emmerson, M. Fedak, A. Friedlaender, N. Gales, M. E. Goebel, K. T. Goetz, C. Guinet, S. D. Goldsworthy, R. Harcourt, J. T. Hinke, K. Jerosch, A. Kato, K. R. Kerry, R. Kirkwood, G. L. Kooyman, K. M. Kovacs, K. Lawton, A. D. Lowther, C. Lydersen, P. O. Lyver, A. B. Makhado, M. E. I. Marquez, B. I. McDonald, C. R. McMahon, M. Muelbert, D. Nachtsheim, K. W. Nicholls, E. S. Nordoy, S. Olmastroni, R. A. Phillips, P. Pistorius, J. Plotz, K. Putz, N. Ratcliffe, P. G. Ryan, M. Santos, C. Southwell, I. Staniland, A. Takahashi, A. Tarroux, W. Trivelpiece, E. Wakefield, H. Weimerskirch, B. Wienecke, J. C. Xavier, S. Wotherspoon, I. D. Jonsen, and B. Raymond. 2020. Tracking of marine predators to protect Southern Ocean ecosystems. Nature 580:87-92. https://doi.org/10.1038/s41586-020-2126-y