Continuing my investigation into stages of individual development, I’ve stumbled upon a study of the maturation of semi-wild mandrills (Mandrillus sphinx). Mandrills are one of the most visually striking species of Primates (check out this beastly male to the right), and exemplars of the power of Sexual Selection.
Sexual selection is a special subtype of Natural Selection, where the within-species competition here isn’t so much for survival (as in natural selection) but more specifically for reproduction. Sexual selection is believed to be responsible for many differences between the sexes: male primates often (but not always!) have much larger bodies and canine teeth than females, traits that can be beneficial when competing with other males for access to female mates. And/or females may prefer larger-bodied or -canined males for whatever reason. In accord with the power of female preferences, sexual selection is invoked to explain why males of many species are so wildly colored or ornamented.
So mandrills are perhaps the best example of sexual selection in primates. Males’ faces, butts and genitals are brightly colored, spanning the spectrum from blood-red to nearly bioluminescent blue. Conceivably, at some point in mandrills’ evolutionary history most males were drab-colored, but then who comes riding into town on a silver stallion but a mutant male who was more colossal and colorful than the rest, and females were like, “OMG did you see that variegated guy? I want him so bad,” and as a result, this male reproduced more, and the rest of the story writes itself. Coloration may actually communicate information to females about the health or dominance status of the male (e.g. Setchell 2004). I wish I had the time to investigate the physiological bases of how their hair and skin can produce such colors. To revolutionize the tattoo industry.
Mandrills are also remarkable in how much larger males are than females, in terms of canines (Plavcan and van Schaik 1992), molars (Scott et al. 2009) and body size (Wickings and Dixson 1992). And this brings me to my original thought.
The plot to the right tracks growth in body mass (in kilograms) of male and female mandrills (Wickings and Dixson 1992: 132, fig. 1). The male is the top line and the females the bottom one. The arrows indicate timing of sexual maturity. Holy crap, by the time males are sexually mature, they are about 3 times the body mass of females.
The union of the ~25 lb female with the seemingly-paint-splattered, 75 lb male must be a truly terrifying sight.
Plavcan, J., & van Schaik, C. (1992). Intrasexual competition and canine dimorphism in anthropoid primates American Journal of Physical Anthropology, 87 (4), 461-477 DOI: 10.1002/ajpa.1330870407
Scott JE, Schrein CM, & Kelley J (2009). Beyond Gorilla and Pongo: alternative models for evaluating variation and sexual dimorphism in fossil hominoid samples. American journal of physical anthropology, 140 (2), 253-64 PMID: 19358294
Setchell, J. (2005). Do Female Mandrills Prefer Brightly Colored Males? International Journal of Primatology, 26 (4), 715-735 DOI: 10.1007/s10764-005-5305-7
Wickings, E., & Dixson, A. (1992). Development from birth to sexual maturity in a semi-free-ranging colony of mandrills (Mandrillus sphinx) in Gabon Reproduction, 95 (1), 129-138 DOI: 10.1530/jrf.0.0950129
7 thoughts on “Variation 2: The female with the flamboyant and massive male”
What keeps me busy right now is the question whether or not it's usual for Primates to show such a big difference when it comes to the age of sexual maturisation between males and females and if this difference correlates with the general degree of sexual dimorphism within a certain species.This would make sense if sexual dimorphism is simply the result of a prolonged period of growth, but judging from the graph in your post I somehow doubt that this is completely true for mandrills. Also there should be differences in the age of sexual maturisation between male Orang-Utans which express secondary sexual characteristics and those who do not express them.I'm still trying to get a grasp of how developmental biology really "works", but it certainly generates some very intriguing questions.
See Figure 1 in the following paper for the "terrifying"…"union of the ~25 lb female with the seemingly-paint-splattered, 75 lb male."Setchell JM, Hutchard E. 2010. The hidden benefits of sex: Evidence of MHC-associated mate choice in primate societies. BioEssays, 32: 940-948.
@Eric, I'm also trying to figure out how this stuff works, myself. I probably should learn more before I keep on blabbing about it. I'm not sure what the relationship is between overall morphological dimorphism and the difference in age @ sexual maturity. It is pretty common for females to be fully grown and mature earlier than males, though. As for orangutans, I believe that both the flanged and unflanged male are fertile, and the unflanged actually sire a decent proportion of offspring.@Rich, I guess it's not so "terrifying" after all, but the size disparity is still pretty impressive. Thanks for the reference!
No, it's terrifying! At least to my human eyes.
I think you might've misunderstood what I tried to say about the differences between the two types of males of Orang-Utans.So let me try again:For the sake of simplicity, let's assume that the differences in the age of sexual maturisation are at least partially because of the development of secondary sexual characteristics. This means, the sex which becomes sexual mature at an higher age, does so because it takes more time for them to develop their addional sexual characteristics.Now, if we look at Orang-Utans and at the fact that there are two different types of males, one that express their additional sexual traits and those who do not express them, then there should be a difference in the age of sexual maturisation between those to types of males. So those males who do not develop all those addional sexual traits should become sexually mature at a much younger age then the "regular" males, if my first assumption is true. I hope this makes my point a little bit easier to understand. It's not really a question or anything like that, it's just something that went through my head after I read your Post.
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The color of the male mandrill is incredible! However, do you think that in this case, sexual selection has overpowered natural selection? The reason I ask this is because these bright colors, that seem to be sexually selected for, might result in higher predation rates and therefore lower survival. Due to a disadvantage in survival, these bright colors would be selected against in terms of natural selection, but it seems that these bright colors are still very prevalent in male mandrills today. What are your thoughts on this?