sexual dimorphism

Kevin Boilard

The Human Animal

Dr. Dolby

November 13th, 2009

Sexual Dimorphism

What’s the difference between a male and a female? A seemingly easy question, one may answer genitalia, testosterone, estrogen, and eggs or sperm. However, these are all differences that are not openly visible to us and still we can usually tell the difference between a male and a female. This is due to sexual dimorphism. Sexual Dimorphism is present in just about all species of animal. Although some examples are much more drastic than the dimorphism in humans, still the slightest difference can determine what one gender may find attractive in the other gender.

Sexual Dimorphism is defined as the systematic difference in form between individuals of different sex but of the same sex. Some dimorphic traits have a logical reason for survival but most traits are just made for reproduction purposes. In most cases these traits are used by males to attract females so they can mate with them. These traits are very important to the male because without them it is unlikely that the male would find a mate without them and therefore he would be unable to produce offspring. If he does not produce offspring then he fails his ultimate goal of passing on his genes to the next generation.

So as one can imagine the acquisition of these attractive sexually dimorphic traits is extremely important to the species and the individual is under a lot of environmental pressure to develop these traits in order to pass on his genes. However, a problem arises when every male individual of a species have the same dimorphic traits and they’re all using them to attract a mate. At this point, in most cases, a female individual would find the male with the most exaggerated traits also to be the most attractive and she will mate with him.

This is just the case in many species of birds. Take peacocks for example, the female peahen, with the exception of her green neck, is relatively plain in terms of coloration. They have mostly brown feathers with a few patches of white on their undersides and some black on the tips of their wings. These are all fairly neutral colors compared to the coloration of the male peacock. The male has a brightly colored blue neck and head and while courting a female they have a huge, beautiful display of colors on its fanned out tail. It is highly unlikely that a female would find a male without this elaborate display attractive, and in turn this peacock would not pass on his genes to the next generation.

Other interesting examples of sexual dimorphism in birds would be Mallard ducks and long-tailed widow birds. The females in both species are very plain and have neutrally colored wings. The male mallard duck looks exactly like the female mallard with one exception, a bright green head! Even more elaborate would be the extremely long tail that a jet-black male long-tailed widow bird has. It is believed that the reason only males have acquired these traits over time is because it is more of an advantage for females to have a natural camouflage since they have to protect their eggs and hide from predators.

Birds are probably the most spectacular and extravagant example of sexual dimorphism. In most mammals it is usually a subtle size difference between the sexes. Usually the male individual of a species is larger than the female individual when it comes to mammals. This begs the question; why are birds so much more sexually dimorphic than mammals?

The answer is simple; it all has to do with mating habits. On average mammals are less polygamous than birds. In other words mammals generally have fewer mating partners than birds in a lifetime. Without having to impress as many females, there is less competition and the male individual of a mammalian species has less environmental pressure to form sexually dimorphic traits than that of an individual of an avian species. Other factors that lead to being more or less dimorphic are the time spent looking for mates and the demands of taking care of young. The answer to the previously stated question is simply explained by the directly proportional ratio of polygamy to dimorphism. The more polygamous a certain species is the more sexually dimorphic that species is.

Humans, a particularly monogamous species, are only slightly dimorphic. At a young age there is almost no sexually dimorphic difference between male and female humans, but the same can be said for most species. It isn’t until about 13 years old, or when a youth reaches sexual maturation, when you begin to see the differences between the sexes. The only real visual cues as to whether a human is male or female, other than the genitalia of course, is the difference in the hips and chest. Males develop a much larger chest with much broader shoulders whereas females develop thinner waists and wider hips. Males are on average taller and larger than females as well. As far as facial structure goes, males have a squarer jaw while females have a thinner, more pointed jaw line.

Despite very few external differences between males and females the internal difference are actually quite numerous. For example men have a quicker metabolism than women, about 10% faster on average and the food men digest is converted into muscle more whereas women convert much of their food into fat. Other differences are tracheae and bronchi length (men have larger ones attributing to their deeper voices), and blood cell count. Men tend to have more red blood cells, which results in greater oxygen capacity. Women tend to have more white blood cells making them less vulnerable to disease.

As mentioned earlier there are other factors that lead to sexual dimorphism than polygamy and monogamy. One such factor is the amount of work the parent has to do raising the child. This is supposedly the reason why men and women have such a low degree of sexual dimorphism. When we look back at the peafowl, the mother does the majority of the parenting so the father can go and mate again without having to worry about having hundreds of offspring to tend to. In this case the relationship between sexual dimorphism and parental care is inversely proportional. As more parental care is required to raise offspring, the less sexually dimorphic the species is.

Other mammals, although less sexually dimorphic than birds, have some interesting differences between the sexes. Baboons for one have a relatively low degree of sexual dimorphism. The only really noticeable difference is that male baboons have canines that are twice as long as those of a female baboon. Like most sexually dimorphic traits this serves no survival purpose because baboons are herbivores. If the baboon’s diet consists of fruits and other plantation why would they have such large teeth? The only answer is because females find it attractive. Now, why the females find it attractive will forever remain a secret, whether they view it as a signal of being able to protect against harm or some other reason will never be known.

We are so used to the males being larger and more dominant than females because this is the trend depicted in both humans and most other mammals. It is hard to believe that the opposite is true in many bugs and insects. In the case of spiders females are usually many times larger than males. The mating ritual is more about survival to the male than anything else and in some cases survival is not an option if a spider wants to pass it’s genes on. An example of this would be the mating ritual between a male and female Australian Redback Spider. After the male courts the female he lets the female kill and eat him in order to provide food for his young. This is an extreme example of female dominance in the animal kingdom.

Sexual Dimorphism is just one of many extraordinary products of evolution. It is amazing to witness the results of millions and millions of years of adaptation and sexual selection. There are many, many species of animal on this earth and each one has it’s own examples of sexual dimorphism. Whether it be the low degree of dimorphism between male and female humans due to monogamy and child raising or the high degree of dimorphism between male and female peafowl due to monogamy and little male responsibility in terms of child raising; sexual dimorphism never ceases to intrigue scientists across the world.

Works Cited

“sexual dimorphism.” Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 12 Nov. 2009 <http://www.britannica.com/EBchecked/topic/537133/sexual-dimorphism>.

Evolution. Blackwell publishing. Web. 10 Nov. 2009. <http://www.blackwellpublishing.com/ridley/a-z/Sexual_dimorphism.asp>.

Encyclopedia.com. Allaby, 1999. Web. 10 Nov. 2009. <http://www.encyclopedia.com/doc/1O13-sexualdimorphism.html>.

Sexual dimorphism in birds: why are there so many different forms of dimorphism? (1998): 397-406. Print.

THE GENETIC BASIS OF SEXUAL DIMORPHISM IN BIRDS (2007): 214-19. Print.

HUMAN SEXUAL DIMORPHISM IN SIZE MAY BE TRIGGERED BY ENVIRONMENTAL CUES 5th ser. 37 (2005): 657-65. Print.

Equality for the sexes in human evolution? Early hominid sexual dimorphism and implications for mating systems and social behavior. Proceedings of the national academy of sciences in the united states, 2009. Web. 10 Nov. 2009. <http://www.pnas.org/content/100/16/9103.extract>.

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2 Responses to “sexual dimorphism”

  1. i might not have decided this was remarkable a couple years back then again it’s crazy how age alters the means by which you perceive abnormal concepts, thanks with regard to the article it is really good to go through anything smart occasionally instead of the usual garbage mascarading as blogs and forums on the web, i’m going to take up a few hands of facebook poker, adios for now

  2. Randolph King says:

    It’s interesting to me that you glossed over the male facial hair in humans; while many men today remove this, our ‘peacocks tail’, if you will, its presence suggests that humans may have not always been so monogamous as you imply, and that the current trends toward monogamy in ‘civilized’ people may have led to the barbaric practice of scraping ones face daily with a sharp steel razor.