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Final Draft of Wikipedia Assignment:

Brendan Giambruno-Fuge Thursdays @ 5:20 PM

The Evolutionary Enigma of Large Size in Dinosaurs The topic being discussed is the large size of the dinosaurs. The problem of large size has been an enigma for many scientists because large size in many cases offers many negative effects. Many large species of dinosaurs developed through the millions of years they were around. This is very confusing and interesting because without live specimens it is nearly impossible to know the exact reasons these creatures grew so big. The largest sauropods frequently weighed more than 30 tons. One of the largest known land mammals, an extinct rhino-like herbivore, grew to approximately one-third that size (Madhusoodanan, 2013)! This paper will be look into the problems of large size, the benefits of large size, and how it developed in dinosaurs. After looking into these three topics, a conclusion will be made as to why large size developed and why we haven’t seen any creature the size of the dinosaurs for a very long time. This ties into many evolutionary concepts such as sexual selection, fitness, social living, and extinction. The problem with dinosaurs developing such large size is the fact that there are so many problems associated with large size. Such problems would, in theory, be a negative evolutionary force in terms of large size. Negative problems that would affect the evolutionary force of large size include, but aren’t limited too, size and body support, breeding restrictions (non-egg laying), more food needs, more energy needs, and unacceptable high blood pressure and stress on the heart of the tallest dinosaurs. The problem of size and body support has to do with the fact that animals support their weight on their limbs. The ability to support weight is proportional to the cross-sectional area of the limbs, but weight is proportional to body volume. This implies that the larger that dinosaurs got, the larger limbs they needed to have, and according to bone structure this isn’t the case. Super large dinosaurs, such as the Sauropods, should have had huge limbs, but even todays elephants overcome the problem of weight with a simple modification: they change their behavior. This is why many scientists speculate that body support may not be an issue because it is likely that, like today’s large land critters, many terrestrial dinosaurs simply changed their behavior to match their body weight and volume. Breeding restrictions of large creatures is an interesting problem. Many large mammals today, such as whales, humans, and elephants, breed with only a single child (except in some cases as in twins). This means that we by nature breed more slowly because female bodies have a time period where they can no longer carry an additional child. It is possible that this is the reason why we can never become so large. As Matthew Carrano, the curator for the Museum of Natural History's Dinosauria, explains: “One thought is that dinosaurs laid eggs and so did not have any problems reproducing even at huge sizes. So population crises could be overcome simply by laying more eggs. Mammals have a terrible time because at large sizes they can only make one or two young every one or two years. If something goes wrong (diseases, etc.), it takes a long time for the population to recover. So mammals are constrained from being very big in a way that dinosaurs weren't.” (Misra, 2014) It is quite obvious that larger creatures need more food and water to sustain themselves, but energy is a bit more difficult to understand. A lot of a creature’s energy goes into growing, so naturally if there is more growing to do than you are going to need more energy. Another use of energy goes into movement. The larger you are the more energy needed to move. Large Sauropods would have needed a lot of energy to move their massive bodies to get from point A to point B. A final issue of being big is blood pressure problems. Dinosaurs, such as the Apatosaurus, who grew incredibly long necks should have had major problems with blood reaching their heads, and yet they survived. Some hypotheses that have been thrown around include a giant heart, many small hearts lined upon the neck, and even the idea that large Sauropods didn’t move their heads vertically but only horizontally (Esker). To solve this mystery we look at a creature today: the giraffe. Giraffes have special muscles in their necks that help with blood reaching their heads. It is very possible (and very likely) that dinosaurs had a similar muscle structure. So given that these issues are so against large size why would such gigantic creatures have existed? The answer is simple: there are enough benefits to large size that it outweighs the negative problems. Benefits to large size include resource storing, protection, and protection against cold. These benefits mean that large size would more easily develop because they outweigh the drawbacks. Resource storing is a lot easier in large creatures because there is more room to store food and therefore more room to store excess energy. An example of this is how humans can go about 3 days without eating because of stored energy (such as fat). This resource storage would then mean that creatures of large size would be able to do more movement with less access to food because they already have a supply stored. For example, large elephants are able to move over longer distances without stopping because they have so much stored energy. Protection is a simple concept to understand: larger creatures tend to be more difficult to take down. Many large creatures also have a form of defense built onto their bodies. Triceratops had horns, Sauropods had whip-like tails, and Stegosaurus had a spike tail. The larger bodies means that the creatures can more easily carry such weaponry without affecting their lifestyle much. One final benefit to large size is protection against the cold. The obvious explanation to this would be along the line of blubber (extra fat to protect against cold weather), but there is more to it than that. Bigger bodies are better heat retainers, because it takes longer for precious heat to travel from an animal's core to its extremities and dissipate (Moskowitz, 2008). This makes it more difficult for larger animals to freeze. This is why your fingers and toes get cold long before your chest, because heat stays near the important things (in this case, it is the heart). Larger creatures can, because of this ability to control their heat output, live in environments that fluctuate temperatures more often (UCSB Science Line). So assuming the pros of large size outweigh the cons of large size how did it come about? One explanation for the dawn of large size is sexual selection. It is possible that size gave enough benefits to offspring that females selected males to be larger and larger thus selecting for large size in the long run. This would eventually cause large size to become common among all members of the species. Another side of the same coin is that larger males fought off smaller males and therefore members of the population became bigger through intra-sexual selection. Another possible explanation for large size is side by side evolution with food. Many Sauropods had extremely long necks, and one explanation for this is that it was used to reach leaves in high trees, such as how giraffes eat today. It is possible that as selection for longer necks occurred, selection for bigger bodies followed, thus creating large herbivorous dinosaurs. In order to stay competitive, the carnivorous dinosaurs would have needed to become larger in order to still be able to hunt their meals (Strauss). This is an example of an evolutionary arms race or the Red Queen Hypothesis. In conclusion, large size has many drawbacks and pushes in terms of selection. It is obvious that the pros of large size outweigh the drawbacks for dinosaurs. How this came about has many different explanations such as inter or intra sexual selection or side by side evolution with and for food. However it occurred, the large sizes of dinosaurs has continued to baffle many scientists due to it strange nature because it should be too hard to control. Until we know more we can only go based off of previously held knowledge.

References

1.	Strauss, Bob. “Why Were Dinosaurs So Big? The Facts and Theories Behind Dinosaur Gigantism”. About Education. http://dinosaurs.about.com/od/dinosaurevolution/a/bigdinos.htm

2.	Sander, Martin and Marcus Clauss. “Sauropod Gigantism”. 10 October 2008. Vol 322. SCIENCE. AAAS

3.		Viegas, Jennifer. “Why Dinosaurs Were So Huge”. Discovery News. http://news.discovery.com/animals/dinosaurs/how-dinosaurs-got-so-big-120131.htm

4.	Esker, David. “The Paradox of Large Dinosaurs”. Dinosaurtheory.com. < http://www.dinosaurtheory.com/big_dinosaur.html>

5.	Madhusoodanan, Jyoti. “Study Reveals Why Dinosaurs Grew so Large”. 25 October 2013. Fox News. http://www.foxnews.com/science/2013/10/25/study-reveals-why-dinosaurs-grew-so-large/

6.	 “Why did large animal evolve in nature? Aren't they kind of useless like too big need lots of space and food?” UCSB Science Line. 

7.	 Moskowitz, Clara. “Bigger is Better, Until You Go Extinct”. July 17, 2008. Live Science. < http://www.livescience.com/2713-bigger-extinct.html>

8.	 Misra, Ria. “Why Were Dinosaurs So Much Bigger Than Animals Today?”. April 25, 2014. Kinja. < http://io9.com/why-were-dinosaurs-so-much-bigger-than-animals-today-1567725502>