Reconstructing Ancestral Primate Vision: An Interview with Breanne Boughan

By Annamaria Dall’Anese • PhD Anthropology

As part of her master’s degree, Breanne Boughan has completed a phylogenetic study of primate colour vision. She tells Anthropolitan readers about what the world may have looked like to the ancestral primate, what is special about human vision, and what wisdom she has gained thanks to her research topic.

A young Barbary macaque eats a small red fruit in Affenberg Salem, Germany. Macaques, like humans, are routinely trichromatic and able to differentiate red and green.

Q: What can you tell us about your research on colour vision in primates, which you did for your master’s dissertation?

A: Primates are very interesting in that they have four distinct types of vision, whereas other mammals only have two. And this can range from being completely colour blind to having full colour vision, like humans. The overall type of vision an individual has is determined by the number of colour pigments we can see. For humans, this is three (trichromatic), for most other mammals this is two (dichromatic), or one (monochromatic), whereas a large portion of monkeys have a special type of vision in which males and females have different numbers of these pigments – they can be dichromatic (all males and some females) or trichromatic (only some females).  I wanted to look at why colour vision, that is trichromatic vision, as humans have it, evolved the way that it did.

Q: More specifically, what were your research questions?

A: The research questions that I wanted to look at were: did colour vision, trichromatic vision, co-evolve with activity patterns? Do you have to be active during the day to have full colour vision? Or did trichromatic vision evolve in order to find foods, specifically fruits, which tend to be bright and red?

Q: What were your main findings?

A: There doesn’t seem to be correlated evolution between either of those. There is no correlated evolution between trichromatic vision and being active during the day, and there is no correlated evolution between trichromatic vision and eating fruits.

Q: Is colour vision correlated to anything in particular then?

A: It looks like it might be more correlated to eating leaves, perhaps being able to differentiate between young and new leaves. But being able to use field studies to figure this out can be difficult. For instance: how do you define leaves? This question can be difficult to ask when you have 299 taxa to look at.

Tree illustrating phylogenetic relationships between species. Courtesy of 10ktrees.nunn-lab.org

Q: This leads me to ask you about your research methods, because you didn’t go to the jungle to do your study…

A: The way I decided to look at it was to collect data on 299 taxa from the literature. I went through every single species and subspecies on that list and looked up their vision, their diet, and when they were active. Then I put this information into a huge excel sheet and I used phylogenetic methods to reconstruct the evolution of each trait and to test for correlation between different traits. The phylogenetics software I used came out in March 2017, so it’s definitely one of the newer methods to look at this type of stuff.

Q: What were the advantages of using this research method?

A: The main reason why I did a huge analysis like I did was so that I could look all the way across the evolutionary tree of primates, because within any observational study it is never gonna be feasible to do this.

Q: Now that you have looked at the whole tree, do you think there is any particular branch in the tree that is worth exploring in the field?

A: Yeah, new world monkeys, that is south American primates, are. If you wanna look at the costs and benefits of dichromatic versus trichromatic vision, they are the place to look. For example, there’s the howler monkey that has trichromatic vision like us, and then there’s the spider monkey that has polymorphic vision, and they both live in the same areas. So there’s researchers who have gone out and compared vision in these two species.

Q: Besides this, do you think there is any more room for future research?

A: Yes, definitely. There’s missing data. For instance, there’s a group of leaf-eating lemurs that haven’t even been tested for their vision. So without having that knowledge you can only make limited conclusions. Also, it would be helpful to sequence more primates, and sequence wild populations instead of captive populations. ’Cause most of the data we have at present comes from captive populations, ’cause it is easier to collect.

A wild howler monkey yawning with her young infant clinging to her back. Howlers are the only South/Central American monkeys with routine trichromacy. Taken in Chiriquí Province, Panama.

Q: Apart from lack of data, did you encounter any other problem in your research?

A: Trying to figure out how to code for diet is difficult. You either wanna look at what they eat most of – do they eat more fruit than anything else? or you can look at if they eat fruit at any point during the year. I ended up doing it both ways. Taking as a criterion their main food source, the data doesn’t make any sense: there is no evidence for correlated evolution, and you can’t even reconstruct the primate tree at all. But if you look at: ‘do they eat this, or do they not?’, you can reconstruct how the dietary adaptations may have evolved through the primate tree.

Q: In what ways is your research innovative?

A: The most innovative part is that, from my research, it looks like there might be a possibility for trichromatic vision at the root of the primate evolutionary tree. I wanted to take into account that there is a specific group of species, the tarsiers, that are really important when looking at evolution, because they are very close to the root. Knowing their evolutionary past is one of the ways we can reconstruct the tree accurately. Previous research said that the prototypical “first primates” were dichromatic, nocturnal, and ate insects, possibly fruits. Essentially, tarsiers are now dichromatic, but according to some findings it looks like in the past they may have had polymorphic trichromacy. My research shows that incorporating polymorphic vision as an ancestral state for tarsiers actually helps resolve the tree. I also did the same analysis for diurnality, because there’s growing evidence that tarsiers used to be diurnal. My research suggests that the first primates ate fruits, may have had trichromatic vision, and possibly were active during the day. It is gonna be a bit controversial, because most primatologists are agreed where our origins sit: dichromatic, nocturnal, and mostly insectivorous.

Q: Looking at primates as a whole, what’s particularly interesting about their colour vision?

A: I think the interesting part is that in apes colour blindness, as we find it in humans, is really rare. Humans as a species are trichromatic unless a genetic mutation causes the loss of a colour pigment gene. This “defect” (I don’t like that word, but ‘mutation’ isn’t exactly right) is more common in male humans than in female humans and more common than in all apes that have been tested for it. As an evolutionary anthropologist, I was interested in discovering why there may be more colour-blind males now than there were in the past. Is there some sort of advantage to being dichromatic? Some have actually hypothesised that it has to do with camouflage, that it is easier to see a camouflaged object if you are dichromatic versus trichromatic.

A female capuchin eats a cicada in Panama. Capuchins possess polymorphic trichromatism. As a female, she could potentially possess trichromatic vision unlike the males of her species which are forced to be dichromatic.

Q: Extending our discussion to animals other than primates, I would like to bring in Jessica Ullrich’s talk on interspecies art, which was part of our departmental seminar series. Tell us your impressions about her lecture.

A: When she started talking about interspecies art, the first thing I thought was: ‘but a dog doesn’t see the same thing as we do!’, but she actually addressed that later, I was pretty happy. And looking at interspecies art more broadly, I think it is fascinating. ’Cause birds see one more colour than we do. So their perception of the world will be different. Even as someone who isn’t colour blind you see colours differently from someone who is colour blind. This led me to realise that science must be horrible if you are colour blind. If you look at graphs in any scientific publication, I don’t know what they would look like to someone who is colour blind, but I can imagine that it would be really difficult to interpret them. So I made all of my figures in my thesis colour-blind friendly.

Q: How did you achieve that?

A: You can either use the colour-blind filter in Photoshop, or colour-blind simulators available online. Either way, you pop in the file, and tell the system how you want to view the picture: I wanna see it as someone who is blind to red, or blind to green, or who can’t see any colours, who is monochromatic. And then you yourself can inspect it: ‘Oh, I can’t tell these two colours apart, maybe I should pick a different colour for this.’ I tried to find the happy medium: to allow both trichromats and dichromats to distinguish between different shades was important to me as a personal mission.

Q: It seems like your research went much further than simply looking at evolution…

A: The big takeaway for me as a researcher is to realise personally that the capabilities of people are different, and that science doesn’t always make it easy if you are not in the norm. So, for me, learning what it’s like to be colour blind is actually a big part of this project. As a human who interacts with other humans – in addition to monkeys, of course.