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In this episode of The Highlights, we're joined by Mary Caswell (Cassie) Stoddard, a professor in the Department of Ecology and Evolutionary Biology (EEB). We discuss her career in sensory ecology and color vision in birds, her field experiments in the Rocky Mountains of Colorado, and the science of nonspectral colors.
Brains, Black Holes, and Beyond (B Cubed) is a collaborative project between The Daily Princetonian and Princeton Insights. The show releases 3 episodes monthly: one longer episode as part of the Insights partnership, and two shorter episodes independently created by the 'Prince.' This show is produced by Senna Aldoubosh '25 under the 147th Board of the 'Prince.' Insights producers are Crystal Lee, Addie Minerva, and Thiago Tarraf Varella. This show is a reimagined version of the show formerly produced as Princeton Insights: The Highlights under the 145th Board of the 'Prince.'
Please direct pitches and questions to podcast@dailyprincetonian.com, and any corrections to corrections@dailyprincetonian.com.
Thiago 0:00
Hi everyone. My name is Thiago. I'm a graduate student at Princeton University and I'm your host. The highlights is a sister podcast to Princeton Insights in collaboration with the Daily Princetonian. Insights is a newsletter written by Princeton undergrad, grad students and postdocs who write about the most exciting groundbreaking research being conducted here at Princeton, in the form of short, fun and easy to read reviews. We cover a range of topics including psychology, neuroscience, biology, computer science and physics to name a few. Make sure to check out our website at insights.princeton.edu. Right now, I introduce my fellow grad student Jerome Ali, say hi, Jerome.
Jarome 0:36
Hi, everyone. I’m really excited to be here to chat with Professor Mary Cassie Stoddard about her paper on the visual capabilities of wild hummingbirds. I wrote about this in a recent article for Princeton insights, so do check that out on the Princeton insights website. I'm thrilled to introduce Cassie, who also happens to be my advisor to the podcast. Cassie Stoddard is an associate professor in the Department of Ecology and Evolutionary Biology. She joined the Princeton faculty in 2016, following a postdoc at Harvard, and her PhD work at the University of Cambridge, the Stoddard lab investigates animal coloration and color vision, especially in birds. I think we're in for a real treat today. Welcome, Cassie.
Cassie Stoddard
Thanks for having me.
Thiago 1:21
Nice to meet you Cassie. I've noticed that your lab studies sensory information in birds? Is that something you're always interested in? Or is it something that showed up later in your career?
Cassie Stoddard 1:33
You know, I was always interested in birds. Or at least I was interested in birds from a young age, my grandma and my mom taught me to identify birds when I was growing up. So I was always very curious about the natural world and always curious about birds. But I'd say that my interest in sensory ecology and sensory systems didn't really kick off until college, I was a freshman, I was able to take this really cool freshman seminar course that allowed us to conduct research in the collections, kind of the behind the scenes collections of a natural history museum. And I gravitated toward the ornithology collections. And I started working on a project on bird coloration and color vision. And it was then that I really started to learn that birds could see colors that that humans can't. And that idea that you could quantify and measure and tap into a sensory world of other animals, even when it's beyond the sensory experience of humans, really excited me and captured my imagination. So I've been very lucky, I think with, with birds, to be able to continue that kind of line of research.
Thiago 2:49
That's really exciting. So,
Jarome 2:52
Cassie, you started this Associate Professor position at Princeton in the past few years? Could you maybe tell us a little bit about how the transition went from postdoctoral researcher to faculty? And and maybe also tell us what you like best about being a professor?
Cassie Stoddard 3:09
Sure. Well, I've really loved building a lab, here at Princeton, I think just getting to work with so many talented students and postdocs on a daily basis is very exciting. And it's really a privilege. So I think that collaborative aspect of working in a lab and doing this research together has absolutely been the best part of being a professor. Of course, there are a lot of new things that you learn when you become a professor and and some of what you do changes along the way. So so when I was a postdoc, I was able to dive into a project for days and days and days at a time. And I found that really rewarding. And now I have other responsibilities that mean that sometimes I can't go off and really read or study or write code for days at a time, but I have other folks in the lab who who can do that. And that's been really great.
Thiago 4:03
It must be really hard being a professor.
Cassie Stoddard 4:07
Yeah, it's wonderful though, I think, you know, there's a lot of learning on the job. But it's it's very exciting to be able to do this research and also to teach I teach a sensory ecology class for undergrads. And I was able to develop that course when I arrived at Princeton, and it's been a blast really teaching that I've enjoyed so much getting to know the the Princeton undergrads and getting to share with them some of my favorite examples of cool animal behavior in the sensory sensory realm. And that's been been very rewarding.
Jarome 4:39
So having been a TA for the sensory ecology course, and this is something you mentioned already, birds can see colors that humans can't. Can you tell us why studying this aspect of good vision is important and exciting to you, Cassie?
Cassie Stoddard 4:53
Yeah, sure. So I guess to take a step back when we do think that birds can see a whole range of colors that humans can't. So we think that the birds out there see the world quite differently. And that's because we humans have three color cone types in our eyes. So our color cone types are sensitive to blue and green and red light. But birds have a fourth color cone type in their eyes, and it's sensitive to ultraviolet light. This fourth color cone type is interesting for two reasons. The first is that it extends the spectrum of visible colors for birds. So if you're a bird looking at the rainbow, you can see all the colors a human can see sort of ROYGBIV and the rainbow, but you can probably also see some ultraviolet tacked on on the end. The second reason this ultraviolet sensitive cone is cool is that it probably provides birds with an extra dimension of color perception relative to humans. And that's because this UV cone should allow birds to see a vast range of combination colors, like UV green, and UV red. So with this fourth color cone type that we learn, birds can not only see a greater range of colors, but they can see, you know, combination colors for which we humans completely lack of vocabulary. And so if you, you know, if you think about how important it is to understand how birds evolved, how to best protect and conserved bird biodiversity, we really have to understand their visual experience. And a big part of that is recognizing that we humans don't see everything that birds see. And so developing the kinds of imaging tools and cameras and computation tools for capturing, describing and analyzing bird color, even though we don't see it ourselves, is really important for getting a clear look at what the bird experience is like.
Thiago 6:58
Nice. You've mentioned that there's combinations of colors, is it like multiple cones will be activated? And then the combination will generate a different color? Is that how it works?
Cassie Stoddard 7:09
That's how we think it works. Yes. So in humans, all the colors we can see arise from the combination of stimulation of our red cone types, our blue cone types and our green cone types. Now it's really interesting when we're thinking about birds with their fourth color cone type is that they probably can see a lot of non spectral colors. And when we say non spectral, we're talking about a combination color that arises when the color cones are stimulated by light from widely separated parts of the color spectrum. So for humans, we really have one major type of non spectral color, and that's purple. Technically, purple is not in the rainbow. It arises when our blue or shortwave cone types and our red long wave cone types are stimulated at the same time, but not our green cones. So we humans have this, you know, experience of seeing purple, I'm looking at, you know, purple piece of paper right now it's it's a real color to us, but it's actually not a spectral color in the rainbow. birds can theoretically see a bunch more of these non spectral colors. So they have up to five different non spectral color types. They've got purple, they've got UV red, they have UV green, UV, yellow, and UV purple. And so that that became something really interesting to us that birds might be able to see and experience these non spectral colors, only one of which we really can, can visualize which is purple.
Thiago 8:52
When you mentioned that I was thinking about the mantis shrimp because I read somewhere that they have like a lot of cones. So does that mean that they are able to see way more colors than humans?
Cassie Stoddard 9:06
Okay, that's an excellent question. mantis shrimp are you know, they're famous in the color vision world and that's because they have 12 different photoreceptors that are used for color, you know, compared to four and birds and three in humans. So you might think then, that the mantis shrimp has this exceptional 12 dimensional color vision system and can therefore see an unthinkable number of colors, including tons of these non-spectral colors. Yes, you might think that and for many of us, we thought that for a long time, but interestingly, a study came out several years ago showing that mantis shrimp color vision isn't quite as refined as we expected. So in some color discrimination tasks where scientists expected the mantis shrimp To perform very well, they actually performed less well than then humans did. And so that's not to knock the mantis shrimp. I think the mantis shrimp still has a really cool color vision system. But what scientists think now is that they have an unconventional color vision system that's probably prioritizing, really efficient processing over really detailed color discrimination. So this mantis shrimp is probably able to look at a scene and almost scan the environment and compare colors that it might recognize and pray to, to sort of a, almost like a lookup table. And in that way, make quick efficient decisions about what an object might be, rather than having really refined color discrimination where it can distinguish between you know, a blue green color and slightly different blue green color. So all that is to say that we're learning a ton more about how mantis shrimp color vision really works. And it remains mysterious and exciting and strange. And so I think, you know, ask again in five years, and hopefully sensory ecologists will know a lot more about about the mantis shrimp.
Jarome 11:22
So far, we've heard you know, humans have three cone types birds have for the mantis shrimp has a bajillion. How variable are visual systems across the animal kingdom?
Cassie Stoddard 11:35
animal systems, color vision systems are quite variable. So some animals have one color cone types and have two most mammals have two color code types, we're kind of lucky as primates to have three color cone types. Birds and some other animals have four color cone types, butterflies can have up to nine color cone types, some of which they can kind of turn on and off for different behaviors. mantis shrimp have 12 so there you know, if you're looking at the number of color cone types or color photoreceptors. These do vary tremendously across the animal kingdom, the the wavelengths of light that animals are sensitive to also vary. So some animals with those, those photoreceptors can detect ultraviolet light, but not red light, for example. And and some animals have better long wave sensitive vision. So you can see plenty plenty well in the red and yellow part of the spectrum, but but not well in the UV. So I think that, you know, one thing that's been really fun as a sensory ecologist appreciate it, is that you could have five different animals in the same meadow. And they all are experiencing that Meadow in really different ways. They can see different colors in that in that meadow. And we now have, you know, technology cameras that we can use to go out and quantify those colors, and then model them simulate how they might look to these different animals. And I guess another thing I would say about the evolution of color vision in, in animals is, is that a lot of times people ask me, you know, why did birds win the lottery? How did they get to be so lucky with this fourth ultraviolet sensitive cone type. And if you look at the evolutionary history of color vision in vertebrates, it looks like the tetrapod ancestor, you know, some 300 million years ago, have these four color cone types, the condition that's been retained in modern birds, it looks like early in mammal evolution to those color cone types were lost. And in some primates, third color cone type was evolved revolv acquired Yes, through through duplication of one of the existing code types. So it's not really that the birds won the lottery at all, they've just hung on to this ancient color vision system. And it's rather that we, we humans have suffered from our early mammalian history that occurred during a time when mammals were mostly nocturnal and had no need for sophisticated color vision system. So we kind of have this piecemeal, basic color vision system compared to what what birds have today.
Jarome 14:29
So you started talking to us about non spectral colors, and we really want to talk to you about your 2020 paper that discusses wild birds having this ability to discriminate between non spectral colors. How do you go about trying to solve this problem of if birds were seeing non spectral colors and if they could tell the differences?
Cassie Stoddard 14:50
Yeah, we were very excited to test this out in hummingbirds and in wild hummingbirds. And that's because hummingbirds require Very little training. With respect to color vision experiments, they've evolved to be sensitive to color the colors of flowers. They feed often from feeders. And so we were able to capitalize on this by going to the Rocky Mountains in Colorado, we do this work at the Rocky Mountain Biological Laboratory, or RMBL, as it's affectionately known. And we set up some experiments right there in in an open meadow, so that we could test color discrimination in wild hummingbirds. To do this, we designed special LED light tubes that can display these bird visible colors we've been talking about. So these light tubes can produce a UV green color, or a UV red color. And we call these tubes, the tetra color tubes, because they can produce colors visible to a tetra chromatic animal, that's an animal, like a bird that has these four color cone types. In the past, it's been really hard to design light displays that can produce these complex combination colors. So for us, this was technical advance that opened up a lot of new possibilities, especially in the field.
Thiago 16:18
I have a question about this bulbs. When we humans look at this bulbs? Do they look like whatever is closer in this spectrum? Like green? Or do they look like a bulb that is off?
Cassie Stoddard 16:30
Yeah, that's a great question. So if I set up, let's say, have two tubes, and one tube is, is producing green light, and the other tube is producing UV green light, those two tubes will look identical to us, I can't tell the difference, they both look green. So the we can't detect the UV. If we have the UV light on, and it's next to one that's off, we really can't see that UV, if you really squint, the UV is probably bleeding a little bit into the into the blue wavelength. And you can you can tell me something tiny is on there. But we are really blind to that ultraviolet light. So if it's UV versus off, both tubes will look off. And so we're able to produce these colors and measure them and convince ourselves that Yep, we've we've made UV green. But in fact, it looks identical to a green color to us. And that was what was so just thrilling about doing these experiments is that to a human, you know, asking the birds to discriminate between UV green and green, you know, were like, these look exactly the same. But over the course of several hours, this population of birds could easily distinguish between these two colors that looked identical to us. And you know, we know from from past research that birds are sensitive to the UV, so we fully expected them to be able to make this this discrimination. But there was something really magical about seeing it with your own eyes. I mean, there's there's kind of knowing it, and then there's seeing it and the seeing it for me was was really exciting and gave me a real rush. So that's what's going on with these tetra color light tubes. We then set up two feeders, we have our two light tubes, and we have two feeders, one feeder contains sugar, sugar water, and the other contains just water. So we place the feeder next to the LED light tube. So you can imagine looking out into the meadow on a tripod. Each tripod is holding a feeder. And next to that feeder is a light tube, the experiments would proceed with a kind of color of the day a rewarded color of the day. So the tube next to the sugar water would display the rewarded color, let's say that's UV green, while the tube next to the water would display a different color. And we could say that color is green. Because we didn't want the birds just to memorize the spatial position of the feeders and the tubes. we swapped the position of the tubes and the feeders after a fixed interval. So if the birds wanted to find the sugar water, they had to learn to pay attention to where that rewarded color was because it was moving
Jarome 19:26
Cassie, is it possible that these birds were using smell to find the sugar versus the water?
Cassie Stoddard 19:32
Yes, we did consider that possibility because of course they might not be using color. They could be using smell or they could be copying each other and using some kind of social learning. And so to to make sure that that wasn't the case we did a series of control experiments. And in these controls we had the tetra color light tubes both tubes display identical colors. So one tube was To display green and the other would display green, but one of the tubes would be next to a feeder with sugar water, and one of the tubes would be next to a feeder with water. And we go about the experiment swapping the positions just as we would any other experiment. If the birds were using smell or some other cue and ignoring color, then they should be able to find the sugar water under this kind of control experiment. But they didn't, they performed no better than chance. When we were using identical colored lights, which suggested to us like phew, they're not using smell. They're not using social learning or if they are, they're doing it to a negligible sort of
Jarome 20:46
effect. So So in these control experiments, spirits were basically kind of guessing
Cassie Stoddard 20:50
basically, the number of birds go into feeder one versus feeder two was just a 5050 split. So we don't think that they were doing anything better than random chance when they no longer had an informative color signal. And these kinds of controls are really important for exactly the reason you you mentioned that there are many things that can be influencing the choices these birds are making in the field. And we tried to do our best to account for those by by doing and repeating this kind of control experiment.
Thiago 21:30
What are some of the challenges or funny stories? They do have fieldwork or working with wild animals? Well,
Cassie Stoddard 21:38
I think you You said it exactly. fieldwork is always unpredictable, especially up in the Rocky Mountains, where it can and has snowed in June. So sometimes we go up there, we don't know what to expect. We work for really long hours. And we have to be prepared to deal with finicky technology in the middle of a meadow. And so sometimes these LED light tubes would would break and we'd be out there with a soldering iron trying to fix them on the fly. We've had moose wander through our field site. We're very friendly with the marmots live nearby. So we've got marmots living in the shack where we store our equipment. So so every day is different, but extremely, extremely rewarding. And the field site is a is a very beautiful place. And I felt lucky to do the work there. And also lucky to work with a really great team on this on this project. We had many collaborators, some from Princeton, some from other universities, and we couldn't have done it without this team effort. That's cool.
Jarome 22:41
Cassie, I was wondering if this experiment sort of suggests that these birds are seeing non spectral colors? Is this definite proof that they're that that's what they're doing? Or is there more work to be done to kind of make sure that that's the case?
Cassie Stoddard 22:54
There's definitely more work to be done. I think that, you know, even though we've assumed for a long time that birds can discriminate a variety of these non spectral colors. Our results suggest that this is indeed the case. But it's a lot harder to answer the question, what what do these colors really look like? to birds? What? What is UV red to a bird? You know, does it? Does it look like red? Does it look like UV? Is it some sublime new color that has totally different sort of meaning to the birds? I think I think those are the kinds of questions that, you know, we may never be able to answer. But by measuring colors in the wild, we might get a better clue. So so in this study, we analyzed a very large data set of feather and flower colors to look for the prevalence of these non spectral colors like UV red, and we've seen that they are common in the natural world, but one future direction we'd like to explore is okay, well, how are they using these colors? Do they show up in flowers? Do they show up in plumage that males are using when they're trying to impress females? How you know how much variation in a population is there and the ability to learn about and discriminate these colors? are older individuals better at our color vision tests? Can hummingbirds discriminate different spatial patterns instead of just these different colors? So I think I think we have a lot more work to do to really figure out not just how birds are, are performing in these color vision experiments, but also using color in their natural habitats, to make good decisions about foraging and about navigating about courtship. And we're just starting to get going there. So I think that our you know, our experiments definitely suggest that these birds can see these colors but how they're doing it, what genes are involved, how you know, at what rates they learn about these colors, those are Totally wide open questions.
Thiago 25:01
That's very interesting. Is that something that by studying hummingbirds will be able to generalize to other animals? Like what other mysteries Do you think that hummingbirds might help us solve?
Cassie Stoddard 25:13
Well, I think that's a great question. We think that most diurnal birds that you know, are active during the day, like hummingbirds have the capacity to see these non spectral colors. So we do think that our results are likely to generalize to other other birds. But you know, before we can be too confident, I think it'll be really important to extend this work to the other Hummingbird and and non Hummingbird species out there. You know, there are over 300 species of hummingbirds. Are they all capable of doing this? Or only some hummingbirds doing this? That's one question. And then asking whether this is the case in other birds will be important as well? And of course, you know, it's not hard to then start imagining, well, you know, birds are living dinosaurs. What did the What did the non non avian dinosaurs of the past See, could they see these non spectral colors? Did they have these non spectral colors on their on their skin? So I think I think there's a lot that this kind of work might be able to, to sort of shed light on both looking into the evolutionary past, but also thinking about the present. And I'd say that, you know, what we're seeing in the Rocky Mountains with the broad tailed hummingbirds is that this is a fragile population that definitely has already been impacted by climate change. And if we can't figure out how they see what colors they care about, and how they're using those colors, I think we'll you know, we won't be able to make the best decisions in terms of conservation and protecting biodiversity. So that's something I'm really interested in, too is, how can you use this knowledge now to to make more informed decisions as we see these populations struggle?
Thiago 27:00
That's awesome. Yeah. I would have a lot of questions just on that. And but it was really fun talking to you. That's very interesting. I'll definitely look into how we can use this sort of knowledge to help the preservation of our wildlife. And thanks, thank you very much for coming here.
Cassie Stoddard 27:19
Oh, it's my pleasure. Thank you.
Jarome 27:21
Thank you so much, Cassie, for joining us.
Cassie Stoddard 27:22
Thank you so much for having me.
Thiago 27:25
This episode of the highlights was written by Thiago Tarraf Varella and Jarome Ali. It was produced by Isabel Rodriguez under 145th managing board of the Daily Princetonian. For more podcasts and other digital media from the Prince, visit www.dailyprincetonian.com. Many thanks to Professor Stoddard for speaking with us. To read more about her work, check out the Princeton insights article covering her research which can be found in the description of this episode. Thank you for listening and until next time,
Transcribed by https://otter.ai