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