SIZE and SHAPE LAB The adaptations that organisms develop to help them deal with abiotic and biotic environments often take the form of changes in the body size or shape of the organism itself. Changes in these characteristics may affect the success of organisms in such diverse ways as: controlling heat loss or gain in ectothermic and endothermic organisms increasing the ability of plants to disperse their seeds over longer distances decreasing the sinking rates of phytoplankton in water In this lab, we will use these three examples to examine how body size and shape may help determine the fitness of particular organisms. Part 1: Size and shape and heat exchange in ectothermic organisms As an ectothermic animal (e.g. a lizard), your body temperature is influenced by the temperature of the external environment. Unfortunately, in many habitats, the external temperature varies over a wider range than you are able to deal with, and you must find ways to regulate your temperature more closely. Many animals do this in part through behavior. For example, lizards may frequently move between sun and shade in an attempt to keep their internal body temperature within their preferred range. Ectothermic animals must, however, be concerned not just with simply reaching a certain temperature but also with the rate at which their body will reach that temperature. It does you no good to be able to heat up as much as you need in the sun if you do it so slowly that you freeze waiting for it to happen. As you will see, the rate at which organisms exchange heat with the environment is heavily influenced by their body size and shape. Exercise 1. As a class we will design “ectothermic organisms” that differ in two ways: size and shape. Thus we will end up with a total of 4 organisms: large with shape 1 (e.g. round) large with shape 2 (e.g. flattened) small with shape 1 small with shape 2 The two organisms with the same size will be constructed out of the same weight of modeling clay 2. We will embed I-button temperature dataloggers into the center of two sets of these organisms and put one set of four into an oven and one set into a refrigerator. 3. Be sure to note the time the organisms went into and came out of the new “environments” 4. I will give you a copy of the temperature graphs at the next class meeting. Questions – Part 1 1. Which body size do you predict should change temperature fastest? Which shape? Why would you expect to see these patterns? 2. For organisms of the same shape but different size, which changed temperature more quickly? Was this the same in both the warm and cold environment? Which reached a higher final temperature in the warm environment? Which reached a lower final temperature in the cold environment? 3. For organisms of the same size but different shape, which changed temperature more quickly? Was this the same in both the warm and cold environment? Which reached a higher final temperature in the warm environment? Which reached a lower final temperature in the cold environment? 4. Why do you think you saw the patterns you saw? Refer to the handout on surface area/volume ratio to help you formulate an answer. 5. What do your results suggest regarding the body sizes available to ectothermic animals? Would there be lower or upper limits on the sizes they can be? Why would this be the case? 6. What do your results suggest regarding the body shapes available to ectothermic animals? What body shapes might be better for them? Why would this be the case? Part 2: Size and shape and seed dispersal in plants All organisms need to disperse their offspring away from themselves to some extent. This acts to reduce competition between yourself and your offspring and spreads your genes into new habitats. Although it is likely that many of the habitats your dispersing offspring will encounter are not favorable to them, it is likely that at least some of them will be and your total genetic contribution to the next generation will be increased. Plants act to disperse their offspring, i.e. seeds, using a variety of mechanisms including: water transport (e.g. coconuts) wind transport (e.g. dandelions) transport by animals (either by defecation of seeds ingested while eating fruit or mechanically as in the case of seeds which stick to the fur of passing mammals) “explosive” dispersal as the fruit of some species dry out In this exercise, we will examine how plants which utilize wind dispersal may be able to control how far their seeds travel on average by altering their size and shape. Exercise 1. With a partner construct 20 seeds out of construction paper – 10 each of 2 different types. Use two different colors of paper for your two seed types. You can add weight to the seeds using Parafilm and tape. Your two seed types should differ in only one of the following ways: total size shape total weight the distribution of weight on the seed (e.g. is the weight concentrated at one end of the seed or centrally?) - e.g. if you choose to compare long thin seeds with oval seeds, the seeds should be the same length and have the weight distributed in the same way 2. I will throw your seeds in pairs off the roof of the Science Building and you should record: the time each seed spends aloft – i.e. the time between release and when it hits the ground the distance traveled by each seed from the “parent”. Using a meter tape you should measure the distance from the corner of the building to where the seed first landed. the “behavior” of the seed as it falls. Does your seed tend to “helicopter”, glide, tumble end over end, fall like a stone, etc? 3. Give me a copy of your data before you leave and I will compile the data for the class as a whole. Questions – Part 2 1. Which of your two seed types would you expect to spend more time aloft? Which should travel farther on average? Why would you expect to see these patterns? 2. Which seed actually did travel farthest on average? Which spent the most time aloft? What characteristics of these particular seeds (i.e. size, shape, behavior) might explain these patterns? 3. Is there a relationship between distance traveled and time spent aloft? As a plant, can you increase the distance your seeds travel simply by increasing the amount of time they spend aloft? 4. Is there a relationship between the average distance traveled by a seed and the variability in the distance it traveled? Which might be more important to a plant, increasing average distance or increasing variation in the distance? 5. When might a plant want to decrease the distance its seeds disperse? 6. What tradeoff might you have to make as a plant in order to increase the distance your seeds disperse? Think about the characteristics of the seeds which traveled the farthest. What costs might the plant have in making seeds of this type? 7. See if you can find real examples of seeds which look similar to the ones you made. How far do the seeds of this plant actually disperse on average? Part 3: Size and shape and sinking rates in phytoplankton There are a number of things organisms can do to change “drag”, or the resistance they experience from fluids flowing past their bodies. For example, the bodies of penguins are streamlined to allow them to move through the water more easily as them swim and to save energy. In the case of algae or phytoplankton, which are denser than seawater and would naturally sink to the ocean bottom, they have evolved many shapes and structures that may help slow their sinking rate and keep them in the sunlit surface waters. In this case, plankton may actually need to increase the drag on their bodies to stay afloat. This exercise explores some of the ways they may do this. Exercise 1. Create a minimum of 4 “plankton” out of modeling clay. Make sure that all of your “plankton” have the same weight, so that the only differences between your “plankton” will be due to the shapes you mold the clay into. 2. Fill a graduated cylinder with Karo syrup, and mark two points (or draw lines) with a wax pencil – one near the top and one near the bottom. (We will use syrup because it is more viscous than water and your “plankton” will sink more slowly overall, making it easier to measure the sinking times) 3. Time the sinking of each of your “plankton”. Use the two lines you drew on the cylinder as start and stop times for measuring the sinking time. 4. Calculate sinking rate for each of your “plankton”. (rate = distance / time) Questions – Part 3 1. Which “plankton” has the fastest sinking rate? Describe or draw its shape below. What do you think makes this “plankton” sink the fastest? 2. Which “plankton” has the slowest sinking rate? Describe or draw its shape below, and discuss why you think this “plankton” had the slowest sinking rate. 3. Now that you’ve experimented with 4 different shapes, and you have seen the outcome, do you have any thoughts about how to better design a “plankton”, so that it will have an even lower sinking rate? If so, what would you do differently to slow the sinking rate down? Go ahead and make at least one more “plankton”, making sure it has the same weight as the last four. See if you can build one that sinks slower than the others. 4. What shape(s) did you try? Did it work? 5. Can you relate any of the shapes you found successful to the shapes of actual phytoplankton or zooplankton? Look online or in marine biology textbooks to find pictures of real plankton. General Questions: 1. How else might size affect the success of organisms in different environments? Find a different example from what we discussed here. 2. How else might shape affect the success of organisms in different environments? Find a different example from what we discussed here.
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