Sexual Dimorphism

Sexual Dimorphism Definition

Sexual dimorphism is when the genders of a particular species have different characteristics, not related to their sexual organs. On the other hand, a sexually monomorphic species would look nearly identical, except for their sexual organs. Sexual dimorphism can be expressed in a number of different traits. Typically these include size, coloration, bone shape, and even bodies that hardly resemble one another. Sexual dimorphism is seen in birds, reptiles, mammals, fishes, humans, and many other sexually reproducing species. Sexual dimorphism is thought to be caused by sexual selection, a type of selection driven by the desire of every organism to find an evolutionarily fit mate.

Causes of Sexual Dimorphism

Sexual dimorphism is a product of evolution. There are many benefits and detriments to sexual dimorphism, and each species has evolved to further this trait or reduce it. In all sexually reproducing animals with distinct genders, there is a fundamental difference in the sexual organs. In hermaphroditic species, both sets of organs are present and these species are monomorphic. Sexual dimorphism then encompasses a huge spectrum. It goes from creatures which are only slightly different between the genders to animals which hardly look like they belong to the same species.

One force which usually drives sexual dimorphism in evolution is sexual selection. In this form or selection, the interaction between the genders seeking and choosing mates causes differences in the male and female populations. For instance, females seeking a fit partner might look for a large male, as confirmation of his success. Over time, males would tend to become larger, as a sign to females they were the most successful mate. This is not an active process on the part of the males, but simply the result of the largest males having the most offspring. Smaller males have less offspring, and thus have less of a contribution to the next generation. This type of size-based sexual selection can be seen in many animals.

Size is a commonly sexually selected trait for many reasons. Not only does it suggest to females that the male is capable of surviving, but also of fighting. Many males which compete for mates will develop high levels of sexual dimorphism. If not seen directly in their size, it can often be seen in other appendages, such as horns or enlarged appendages used for battles. Both Big Horn Sheep and Crabs share these enlargements caused by evolution, and both use them to battle for mates. However, size is far from the only trait selected for.

Other traits include color and other visual indications of success. A lion’s mane does not make the lion any bigger, or protect him in battle against other lions. However, a large mane simply looks more formidable. Manes may have developed as a type of sexually selected sexual dimorphism. Birds are often a clear example of sexual dimorphism. Males and females have starkly different feathers and markings, while their bodies are usually of similar size. Males are often brightly colored, with extravagant and cumbersome ornamental feathers. Think of the peacock. The extra weight of the tail feathers should be a detriment to his success. However, in attracting females, it is extremely useful and the benefits outweigh the consequences.

There are many other examples, from simple things like eye color all the way to very complex rearrangements of each organism. Remember too that each species is different. Not all birds are sexually dimorphic. There are many species which have evolved monomorphic patterns. The following examples will provide insight into several different species, and how evolution may have formed the sexual dimorphism seen.

Sexual Dimorphism Examples

Birds of Paradise

The Bird-of-Paradise is actually an entire family of bird species found from Indonesia to Australia. There are many different species, but almost all of them have evolved some of the most extreme forms of sexual dimorphism of any animals. Seen below is a male of one of the species.

Wilson's Bird of Paradise Best

The females are typically the same size, but contain no special coloration. Each species has a unique and complex pattern. In addition, many species have complex courtship rituals in which the male tries to seduce the female with his pattern. The extremely bright coloration is a dead give-away to predators, but it is also the only way to attract females.

This form of sexual selection displays one theory of sexual selection, the so-called sexy-sons hypothesis. In the theory, females try to select males which will produce offspring which will be most attract to the next generation of females. In other words, the mother wants her son to be able to attract a mate. Therefore, she selects the brightest and most flashy male. In doing so, the males who are the brightest and have the best patterns are selected and contribute more offspring to the next generation. The power of this form of sexual selection can transform similar species in a very short time. The family of the Birds-of-Paradise has around 42 different species, which have emerged over only 24 million years from a common ancestor. Each species has a unique and different pattern, which further drives their separation form the other groups.

Ornate Box Turtle

A box turtle is any turtle which has a hinge on its shell, allowing it to completely enclose itself within the shell. Ornate box turtles live on the plains of the United States, across several states. The turtle is a sexually dimorphic species, but only to the trained eye. Both genders have similar markings on their shell, and from afar would appear to be the same.

However, the males have several traits which are sexually dimorphic from the females. Males have red eyes, where females have brown. Like the Bird-of-Paradise, this may be a trait which has no real benefit, but females find attractive. It may also have benefits which scientists are not yet aware of. While the two genders are roughly the same size, they are not the same shape. This is for a very good reason: turtle sex.

Turtle sex is much more difficult than most forms of mating. The shell, which does so much to protect the turtle from predators, is a huge hindrance for male turtles. That is, until sexual dimorphism. Male turtle have a unique bend on the underside of their shell. Instead of being flat, like the female, the male has a large inward bend. This allows him to slide on top of the female. The curve of her shell fits into his curve, and they can balance. He also has a much longer tail, in which his penis is housed. This allows him to tuck it under the female once he is in position, and successfully mate. The sexual dimorphism in turtles, therefore, is caused less by sexual selection and more out of necessity for evolutionary success. This is a good example of the difference between sexual dimorphism and sexual selection.

Humans

Humans have clearly sexual dimorphic traits. Males are slightly larger than females, as in many primate species. However, in humans it is much reduced. A male orangutan is much larger than a female, where a human male is only slightly larger than a human female. Human breasts are different between males and females. Besides Bonobos, this is not necessarily common. A dog, for instance, will only develop visible breasts when it is breastfeeding or has in the past. Humans and bonobos develop larger breasts on females. This could be a symbol to males that the female will be a supportive mother for their offspring. This would give these females more opportunities to reproduce, and with the best males.

There are a few other areas which differ between men and women, particularly body hair and bone structure. Men tend to have more body hair, and narrower hips. While these traits aren’t related, both are sexually-distinct traits which were likely caused by sexual selection. While humans do have some sexual dimorphism, we are near the monomorphic side of the scale.

Quiz

1. Which of the following is an example of sexual dimorphism?
A. A female flamingo picks the pinkest male, a sign of success
B. The male Black Widow spider is about a tenth of the size of the female
C. A male and female Rattlesnake look identical, except their reproductive organs

Answer to Question #1
B is correct. Answers A and C are examples of sexual monomorphism. While it is a signal of success because the pinkness is derived from the food they have consumed, all flamingos are pink. In rattlesnakes, the reproductive organs are not considered when postulating sexual dimorphism. The spider, which is clearly different between the genders is the only solid example of sexual dimorphism.

2. Many species of spider, upon breeding with a male, will eat him. Scientist speculate that this process has driven the size difference in the spiders. What is this process called?
A. Sexual Dimorphism
B. Sexual Selection
C. Mating

Answer to Question #2
B is correct. While the outcome was sexual dimorphism, the process itself was sexual selection. Remember that dimorphism is the product of selection, not a process itself.

3. Why do some scientists argue that “sexual monomorphism” is an oxymoron (a phrase which contradicts itself)?
A. Sexual describes two forms already
B. That is a bad argument
C. Only hermaphrodites can be monomorphic

Answer to Question #3
A is correct. Organisms which reproduce sexually have two sets of reproductive organs. While hermaphrodites store these sets in the same body, the process of sex in most animals requires two different genders. Each gender is so named because of the differences in sexual organs. Ignoring those organs is considered arbitrary, and the argument is that the scale of sexual dimorphism should just be extended to include all sexually reproducing organisms with distinct genders.

References

  • Brusca, R. C., & Brusca, G. J. (2003). Invertebrates. Sunderland, MA: Sinauer Associates, Inc.
  • De luliis, G., & Pulera, D. (2007). The Dissection of Vertebrates. Amsterdam: Academic Press.
  • Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.
  • Pough, F. H., Andrews, R. M., Cadle, J. E., Crump, M. L., Savitzky, A. H., & Wells, K. D. (2004). Herpetology. Upper Saddle River, NJ: Pearson Prentice Hall.
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