fsem human animal

Kevin Boilard

The Human Animal

Dr. Dolby

November 13^th, 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>.

Kevin Boilard

The Human Animal

Dr. Dolby

October 9, 2009

Uniquely Human

It is no doubt that human’s have certainly set themselves apart from any other animal with the huge strides they have made over thousands of years. What they lack in size, speed, and other characteristics that help most animals survive generation after generation, they make up for in intelligence. Due to their extreme brainpower humans have become so accustomed to and adapted to their environment that they have come up with a solution to almost every problem that a normal animal would face. Because of this safety and comfort humans spend less time worrying about surviving and spend more time enjoying their life with thanks to the things that make them uniquely human.

One such example of an activity that is unique to humans would be their artistic work. People today have a tremendous appreciation for artwork and those who create art. Art is considered a way to express oneself and it is not limited to the most popular form of art, painting and drawing. Musicians, writers, poets, photographers, filmmakers, sculptors, and basically any work that someone does that is expressing themselves would be considered art. What makes art special is the fact that it has been unique to human activity since the Neanderthals drew the first cave paintings. However, just because humans may be the only animals that create art, that does not mean they are the only animals that appreciate beauty. For example peacocks have extravagant colorful tails in order to attract mates. The tail serves no other purpose other than reproductive advantages; it has no survival value and may even be detrimental to the peacock’s survival. Also, both elephants and chimpanzees have been taught to paint in captivity but it is doubtful that they are expressing themselves through the artwork.

The fact that humans are so much smarter than every other animal would be a moot point if not for man’s advances in technology and without tools that humans have created technology would not even exist. Early hominins used simple tools such as an ancient hammer like tool to break something open or a spear in order to make hunting easier. Thousands of years later humans have created the technology to harness and create artificial heat, light and innumerable other inventions that make our lives easier than they have to be. To be fair there are some species of primates that use large rocks to break open nuts to eat which is an extremely simple in comparison to what humans have created but still classifies as a tool.

Another activity that is specific to humans is drug and alcohol abuse. Humans are the only animal that uses drugs and unlike the past two activities drug and alcohol use is clearly harmful to one’s health and yet people still do it. There is no definite reason as to why humans do it but one such explanation is that humans would get natural highs from hunting and other survival activities that are no longer necessary for modern humans to take part in. In order to make up for these highs they replace it with drug use and alcohol abuse. Some animals do wait for fruit to be far passed its ripeness and as it rots and begins to ferment the animals begin to eat it. The reason this relates to human substance abuse is because the fermentation of this fruit produces alcohol and the animals become inebriated (National Geographic). Although it is doubtful that the animals do this on purpose it is the closest thing to human alcohol use.

It is no doubt that humans have set themselves apart from any other animal thanks to their extensive brainpower. They use art to express themselves and their abstract thoughts. Man has used tools to their advantage in order to further technology and make their lives easier. Humans are the only animal that use drugs and alcohol in a way that is clearly detrimental to their health. Although there are stretched examples of how other animals mirror human actions but it is clear that man is uniquely human.