Monday, April 14, 2014

Ornaments and Opportunity

The Age of Acne
I’m sure we all remember those awkward middle school days of acne and bed head. You walked around school avoiding bullies, eyeing the crush of your dreams and trying for the life of you to figure out some way to talk to him or her without making yourself look more inept than you may have already felt . (If you’ve never experienced this, consider yourself a step above the normal human race.) However, I’m even more certain that we’re all grateful those middle school days are long behind us, with only a hidden yearbook picture as a dreaded reminder.

Imagine if you will though, that those acne spots and uncontrolled hair were all that mattered in your future for finding that special someone. And even now, whatever blemishes you may or may not have, will signify to a future mate the compatibility and long life you may (or may not) be able to have. Such a terrifying concept is, fortunately for us, just a  dream world for the human race as few people hold an unsightly stage of adolescents, or an unfortunate week of pimples, against someone they could see as a potential mate.

The animal world, on the other hand, is not so forgiving. Often features that animals consider attractive, such as a jungle foul’s cock comb or the yellow of a gold finch’s feathers, make all the difference in the world in being forever alone or happily mated in the animal kingdom. While many people struggling with acne are trying all sorts of treatments from ProActiv to home remedies, animals must deal with treatments of their own in order to maintain attractiveness whether they are conscious of this fact or not. 

Take Care of those Carotenoids
Male partridges are prized by their female counterparts for their red beaks and red accents around their eyes1.  Such attractive feature can be attributed to carotenoids. These pigments can act to either brighten an animal’s coloration or increase its immune response5.  Although carotenoids are highly valuable for their attracting tendencies, they can only be found in food and cannot be self made. Therefore, the allocation of carotenoids is very important in being able to attract a mate with one’s bright and bold ornamentation.

The Hamilton-Zuk hypothesis explains that these vibrant ornaments are an indication of an individual’s health 4. Females look for those males with big bold secondary sex characteristics because they can indicate his parasite load and overall genetic vigor. As an individual’s parasite load increases, his health decreases and, subsequently, so does the attractiveness of his secondary sex characteristics4.

Although testosterone is, by itself an immunosuppressant, it does act to increase an individual’s dietary carotenoid uptake1,2 . As testosterone increases so does the uptake of dietary carotenoids which can either be used to fight free radicals in the blood stream in an anti-oxidant capacity, which increases the cell-mediated immune response, or as pigmentation to make ornaments flashier and brighter. The more carotenoids used in an immune response to build T-cells, the less there will be for pigmentation of ornaments3.

A relatively recent study was performed on Alectoris rufa, also known as the red-legged partridge, in which scientists infected the birds with an intestinal parasite, coccidian and then tested the birds for their immune response and the effect that response had on secondary sex characteristics2. The study found that males with a redder beak and eye rings also had less parasites and higher carotenoid levels. This indicates that while testosterone provides information to potential mates about long-term health and potentially resistant genes passed to offspring, carotenoid levels indicate the individual’s current health and virility.

In summary, testosterone is a hormone that starts functioning at the onset of puberty to increase size and structure of prominent secondary sex characteristics. If an individual is infected by parasites from the beginning, and has nonresistant genes to such parasitic attacks, then that individual's ornamentation will be lacking in form and structure. However if an individual can afford high testosterone levels and attractive ornamentation he must have good genes. Furthermore, if an individual has attractive ornamentation that is also bright in coloration, then he must have an ample supply of carotenoids to spare. 

It's hard to cheat in the animal world. No amount of plumage serum or beak brightener can help turn an awkward pubescent partridge into a strapping specimen to mate with. It either has it or it hasn't. I'm grateful that I can safely say no braces and acne stage can be held against anyone in the big scheme of things. At least…I hope not.

1)   Blas, J., L. Perez-Rodriguez, G. R. Bortolotti, J. Vinuela, and T. A. Marchant. "Testosterone Increases Bioavailability of Carotenoids: Insights into the Honesty of Sexual Signaling." Proceedings of the National Academy of Sciences 103.49 (2006): 18633-8637.
2) Mougeot, Francois, Lorenzo Perez-Rodriguez, Nuria Sumozas, and Julien Terraube. "Parasites, Condition, Immune Responsiveness and Carotenoid-based Ornamentation in Male Red-legged Partridge Alectoris Rufa." Journal of Avian Biology 40 (2009): 67-74.
3) Peters, Anne. "Testosterone and Carotenoids: An Integrated View of Trade-offs between Immunity and Sexual Signalling." BioEssays 29.5 (2007): 427-30.
4) Shykoff, Jacqui A., and Alex Widmer. "Parasites and Carotenoid-based Signal Intensity: How General Should the Relationship Be?" Naturwissenschaften 83.3 (1996): 113-21.
5) Zuk, M. Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are. Orlando: Harcourt, 2007. pp 180-206.


  1. While the Hamilton-Zuk is a great way to predict the fitness of an individual, there are exceptions. In some cases, the parasite isn't common making the parasite more difficult to detect or the females are attractive too causing selection not to be associated with parasites. Despite knowing this, I like how you integrated diet into the discussion of selection. As humans, we often think about how diet affects our health. Those with the most money have access to more food and better quality of food. However, we seldom think about how the fruits and vegetables that we need are also needful for animals as well. In areas where there is an abundance, it makes selection less of an obstacle. On the contrary, in low income country, the animals do not have such easy access causing them to have compromised fitness as you so eloquently pointed out.

  2. Another problem with using the Hamilton-Zuk hypothesis is the handicap of immunocompetence (Folstad 1992). This is where the trait that attract females-like bright ornamentation- are secondary sexual characters. The expression of horns and antlers are dependent on testosterone, which acts as an immunosuppressant. So although the male has an advantage with more weapons (horns and antlers) the male also is more susceptible to parasites. This creates the paradox that although sexual selection favors reproduction by individuals more resistant to parasites, the involvement of testosterone in the signaling mechanism renders these individuals more vulnerable to the same parasites.

  3. When reading your blog and this section of the book I think about aging and how people in countries with less processed meats and more fruit age slower than we do as Americans. Our diets here for the most part consist of antibiotic loaded meats and overgrown vegetables and fruits which are probably grown under chemical conditions. I like to think that this could have something to do with how our body ages and handles the chemicals. Of course, that could be a long shot, but it is interesting to think about.

  4. I enjoyed how you linked the idea of our own beauty to the Hamilton-Zuk hypothesis. Not only could acne and hormones be a sign of sexual maturity...there are many other secondary sexual characteristics that we find more attractive in a potential mate. What I wonder is, does this make us appear more healthy, and therefore more attractive? Like say, does a bigger beard or larger breasts indicate higher parasite tolerance, or is that completely ridiculous? (Probably, but it's worth a thought...) And even if we are not so lucky to grow these characteristics, are humans so evolved that we can still be attractive to a partner without them? I think that the Hamilton-Zuk hypothesis is a pretty good explanation for the animal world, but as you mentioned, we have so many ways of hiding sickness and lack of (or too much) secondary characteristics for this hypothesis to completely hold true to humans.

  5. Like Kate said, it was interesting the way you connected human sexual selection to that of the animals in your example. In one of my classes last semester we discussed the traits that are favorable to humans, such as larger breasts and fair skin in women. Of course in high school and college every girl wants to be tan, but apparently lighter skin has a greater evolutionary benefit. Lighter skin is indicative of more vitamin D production, so if humans ever needed to move towards the poles, fair skin would provide a benefit that darker skin could not. Larger breasts also provide an evolutionary benefit: they're associated with higher production of estradiol and progesterone, both of which promote fertility. BUT I don't want to give men the excuse of sexualizing women (big boobs, small waist, big hips) "for evolution"!

  6. I think it is interesting that Caretenoids can be representative of the organism's current health as well as its genetic strength. I do believe however, that there is still a way to trick nature. Say a partridge who was genetically weaker, grew up in a region where the foods that are full of caratenoids are surprisingly for at least a few years. This partridge would be able to reproduce as successfully with a genetically stronger partridge simply because the weaker had so much more caratenoids in its system. In this way, lek participation seems to be much more definitive. A weaker bird that may have avoided infection, in order to participate in a lek, would necessarily expose himself to other birds who may be parasitized, eventually reducing the weaker bird's participation. And to Emily's comment: it is true.

  7. We heavily discussed the advantages and disadvantages of testosterone, but I love the addition of carotenoids as a secondary check mechanism. This further supports the Hamilton Zuk hypothesis and goes back to the car house paradox in which individuals who can afford a nice car can typically afford a nice house. I guess if you added carotenoids it would be the car, house, yacht hypothesis. Although we can theory craft scenarios in which the H-Z would not work, as mentioned before, I think it's a solid foundation to start when looking at sexual selection. Overall, it has proven effective in most scenarios.

  8. So you're saying that my fancy car isn't worth anything if I don't have the big house to bring the girls back too? Dang. It seems as if the "natural order" of the Hamilton-Zuk hypothesis does not work in the favor of those that can only own a car. It is a fortunate thing that we live in a society where we can "cheat" this hypothesis. Actually, it would be more accurate to say that we are fortunate to live in a a society where the H-Z hypothesis does not apply (since we lack a significant threat of parasites and we have more "free will" than animals.) Life would be more miserable if we had to go through it worrying about a heavy load of parasites. The misery only compounds when you add in the courtship we have to go through in order to find a mate.Think of building up muscles while dealing with several intestinal parasites; I'd think twice about lying on the bench. But, this is a great supplement to our previous studies on the H-Z hypothesis. I didn't know that Carotenoids played a role in this. Enlightening.


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