Sex & Violence: Color Outside the Lines of the Natural World
The hot sun was beating down on my neck, my clothes were sticking to my skin, and my legs were starting to cramp as I knelt in the same position for what felt like a decade. In that moment, I was the living embodiment of a Peeping Tom, witnessing a private act of intimacy that I really had no right to witness. I felt like I was hiding in someone’s bedroom closet, with my camera set up and notes ready, attentively watching as two people…well, further contributed to the benefit of the species. Except, in reality, it was happening in one of the bushes on North Seymour Island in the Galapagos. And they weren’t really people, but the legendary Blue-footed boobies. Try not to laugh at that too much.
While I was slightly uncomfortable taking notes and even drawing this whole act unravel before me, I was equally fascinated by it. While the mating behaviors of Blue-footed boobies are quite the sight, I was captured by the beautiful blue coloration of their feet. As part of their mating behaviors, the males will perform a sort of dance to impress the females that could involve displaying their wings, arching their necks, presenting twigs or leaves as part of a nest, or, most importantly to me, showing off their blue feet. It’s the equivalent of a guy showing off his muscles at a beach where most girls are beyond uninterested.
A study was done to investigate the relationship between the intensity of the blue color in their feet, and how successful they are at mating in terms of how often the females respond to male courtship behavior and how much the females invest in the relationship. It was found that the more well-fed birds had more brightly-colored feet, meaning that the better the birds were at hunting for food, the brighter their colors were (4). It was thus determined that “carotenoid-pigmentation [in the feet] can reveal the immunological state of individuals” or how healthy they are in terms of nourishment (4).
“Look at the booby over there,” said Fausto, our tour guide, as I was watching two males waddle over to a female booby on Seymour Island. “One of these little guys is about to get the worst friend-zone experience of his life,” pointing to one of the males.
I let out a laugh. “Wait, what?”
“Yeah, you could say he’s the one with the nicer personality. Just look at him.”
I noticed that its feet were a significantly lighter blue in comparison to the other male, less bright, less vibrant. And I think he was aware of it too. While the more “handsome” male approach the female boldly, displaying its wings with confidence and doing a strong waddle towards her, the other male stayed back and kept its head down.
My heart broke a little. “Oh, that’s a little tragic.”
“Eh, it’s his fault. He should really step up his game, hit up the gym or something,” he said jokingly as he walked away. While I couldn’t help but feel sorry for the little guy, I also recognized that this is the survival of the fittest, and the sexiest, since physical indicators such as these can reveal how suitable a partner might be. Think of it like a quick guide to swipe left or right on Tinder for a date for animals.
In fact, sexual selection based upon bright coloration in physical features is actually a reoccurring phenomenon not only on the Galapagos, but all over the world. You might be wondering what the difference between natural and sexual selection is to begin with. Natural selection, as many of you may have heard from time to time, is the ability of an organism to successfully compete for resources in a natural environment based on their adaptive abilities. Through natural selection, evolution of traits in the whole population happens over a long period of time, from generation to generation (1). Meanwhile, sexual selection is when members of a species compete with one another to find a mate and pass on their genes. The “fitness” of a member of a species is actually dependent upon how often they can successfully pass on their genes. Males can compete with other males, females with other females, and the criteria for being a desirable mate can vary from species to species, usually some extravagant form of visual display on the male’s part (1). Male frigate birds on the Galapagos have inflated red pouches and male poeciliid fish around Central and South America have brightly colored patterns, all to attract the females and leave their legacy behind in the form offspring (3).
On the other hand, bright coloration on an organism can also indicate that you they might kill you, or in technical terms, indicate that they are aposematic. Aposematism can come in many different forms, from the color to odor of a particular species that demonstrate their undesirable and unprofitable nature to a predator (3). Say you’re just a typical predatory bird hunting away somewhere in North America. You’re tired, your stomach is grumbling, and you’re just about ready to give up. Suddenly, you come across a snake that seems like it is completely unaware of your presence. Hallelujah, time for a tasty snack, right? Wrong. You swoop in, and immediately notice that the snake has red, yellow, white, and black banding patterns all across its body. You stop mid-swoop and fly the other direction as fast as you can, because that is no ordinary snake. That is a coral snake, one of the most venomous ones out there, and you know this because your instincts kick in and tell you that there is something suspicious about the colors on that snack of yours. Most scientists agree that this is learned behavior, that enough members of a species have had unpleasant experiences with an aposematic species that they know that the costs of an attack on that species outweighs the benefits (3).
Therefore, considering the two roles that the physicality of color plays in the natural world, how can it be the object of desire in one case and an indicator of probable, most certain death in another?
And you thought you were getting mixed signals from your crush.
Despite the fact that they may seem like they’re on the opposite ends of the evolutionary adaptation spectrum, sexual selection and aposematism may have more in common than you think. Within species, there is directional sexual selection for aposematism. Fancy scientific vocabulary aside, when members of a species are looking for a certain kind of mate, and that species has signaling colors to indicate their dangerous nature to predators, they usually pick members that most strongly demonstrate those signaling colors. For example, a poisonous
frog species, Oophaga pumilio, maintains its aposematism because the females prefer males that have brighter and more brilliant colors since it confers a higher chance of survival for the offspring (2). Sounds familiar? It’s because female Blue-footed boobies make the same distinction, to pick brighter and more brilliant colors, and for similar reasons, too. Female poison frogs pick males with that kind of preference not only because predators will tend to avoid frogs with brighter colors since it is an honest indicator of higher toxicity, but also because brighter colors may indicate higher physical fitness. Therefore, aposematism is favored by both natural and sexual selection (2).
“But wait!” you might say. “How does this make any sense? Wouldn’t having brighter colors allow you to be more detectable by predators? Wouldn’t having camouflage make more sense?” Good question! I wondered the same exact thing when I witnessed this reoccurring theme of bright, colored physical traits on the Galapagos. It such a noticeable physical feature, that while it may be beneficial for sexual selection, it may as well be a neon sign on them saying, “Hey predators! Come eat me!”
There is another phenomenon in nature called “batesian mimicry” where organisms can adopt the coloration of aposematic organisms as to “mimic” aposematism while not actually being poisonous. A study was done to see the effects batesian mimicry on the predation of butterflies of Papilionidae and Pieridae (5). If they aren’t poisonous and have bright colors, you would think this adaptation is actually harmful to the species since it makes them more noticeable to predators. However, an interesting phenomenon was found. Non-mimetic female butterflies, or dull-colored females, were actually attacked more frequently by avian predators, such as birds. This may be because females are heavier since they carry eggs and are less agile, or that they spend more time around oviposition plants that makes them more susceptible to attacks. Considering these disadvantages all female butterflies have, the one advantage they had to protect themselves against predation was actually seeming more aposematic. It is thought that the association predators already have between bright colors and danger keeps them wary of female butterflies that have Batesian mimicry. Therefore, higher predation in some situations actually leads to having brighter colors in the natural world (5).
It’s weird to think about these things, isn’t it? A few mutations here and there and natural selection just decides, “Yeah, you know know what’s a great way not to get killed? Just have these strange colors all over your body and predators will think you’re dangerous.” Even weirder, members of the opposite sex in a species look at those strange colors and think, “Oh yeah, that’s hot.” However, it opens up the mind to ponder ever more questions. Consider a potential idea for a scientific study. A particular bird species has females that have a strong preference for
males with bright red feathers. However, the birds are also aware of a poisonous frog that is blue-colored and have learned from previous experiences that they are dangerous. What if the feathers of the male birds were manipulated so that they were similar hue of blue? What if the intensity of that blue was manipulated so it was either a more vibrant or duller blue? How would sexual selection function in these situations? How would natural selection function? It is interesting to see how natural and sexual selection are often at odds with each other (1).
There are so many areas of exploration that learning more only makes you aware of how much you don’t know, and how much curiosity you have to know more. The Galapagos may have been an unforgettable experience where I witnessed the wonders of nature, but I came to the realization that my curiosity didn’t have to end with that trip. I’ve just been condition to not open my eyes to the wonders that are present all around me back home. On one of our last excursions on the Galapagos, Fausto told me something that I probably won’t forget for a long time: “The most amazing thing about nature is how you don’t notice the amazing things that are right before your eyes.”
1 Arnold, Stevan J., and Michael J. Wade. (1984). "On the Measurement of Natural and Sexual Selection: Applications." Evolution 38.4, 720.
2 Maan, M. E., and M. E. Cummings. (2009). Sexual Dimorphism and Directional Sexual Selection on Aposematic Signals in a Poison Frog. Proceedings of the National Academy of Sciences 106.45, 19072-9077.
3 Mappes, J., N. Marples, and J. Endler. (2005). The Complex Business of Survival by Aposematism. Trends in Ecology & Evolution 20.11, 598-603.
4 Velando, Alberto, René Beamonte-Barrientos, and Roxana Torres. (2006). Pigment-based Skin Colour in the Blue-footed Booby: An Honest Signal of Current Condition Used by Females to Adjust Reproductive Investment. Oecologia 149.3, 535-42.
5 Ohsaki, Naota. (1995). Preferential Predation of Female Butterflies and the Evolution of Batesian Mimicry. Nature 378.6553, 173-75.