Discussion and Pre-Lab Preparation: Arguably an animal's most ubiquitous behavioral capacity is the detection of environmental variables and the ability to orient or move relative to these. Taxis refers to the act of orienting towards some external stimulus or combination of stimuli. Spatial orientation, aided by different sensory modalities, is described by the corresponding term e.g. relative to light (phototaxis), smell (chemotaxis), sound (phonotaxis), or gravity (geotaxis). If orientation is towards the source, it is called a positive taxis, and away from the source a negative taxis. In such instances individuals move in a directed fashion along a particular stimulus gradient until they reach a perceived optimal range. In contrast, kinesisrefers to non-directional orienting reactions in the presence of a particular sensory stimulus. Animals which suddenly find themselves in an unfavorable environment (e.g. with regard to humidity, temperature, or salt concentration) may speed up or change direction by trial and error. In contrast, they may slow down or include a number of switchbacks when the going is good. Using such a method paramecia or woodlice tend to find themselves in an area with favorable environmental conditions without deliberately aiming towards it.
In this lab exercise you are asked to design and conduct experiments that explore the habitat preferences in one of a number of available model organisms. Choose a species for which you aim to test whether a particular stimulus effects the animal's spatial choices. None of these animals bite. Be careful not to squeeze them when you pick them up. You can carefully transport them in plastic tubes or petri dishes. Return them to their housing container after you have finished the experiment.
|Figure 1. T-maze used to study taxis or handedness. An individual is placed carefully at the end of the maze (S) and is allowed to walk toards the decision point (D) where it has to make the decision to turn right or left.|
Mazes are often used to study animal movement, learning and choice behavior. We have several types of mazes that can be used to study the behavior of T. molitor. T-mazes or Y-mazes can be used to study handedness or taxis (Fig. 1). For instance to explore the importance of phototaxis using a T-maze. Many insects can remember the direction they have been walking if an object in the environment forces them to turn off course. Such behavior can be studied with the L-T maze (Fig. 2).
|Figure 2. L-T maze used to study directionality of walking. A subject is placed at the starting point (S) and initially moves to the left. Mazes with different distances between the forced turn (F) and the decision point (D) can be used to determine if the distance of the detour influences whether or not an individual returns to the initial course.|
The first turn of the maze changes the course of the subject and the second turn allows the subject to either return to the original course or move in a direction opposite the initial course. Some animals appear to reset their course after a short detour, while others continue to travel in the initial direction even after a relatively long detour. Such behavior can be studied by use of L-T mazes of different sizes. These maze designs provide a subject with a dichotomous choice. The animal either turns left or right at a decision point on a maze or it selects one of two habitats in a petri dish. If we place a subject on a T-maze our goal is to determine whether subjects have a preference to turn in one of the two directions at the decision point. What is our expectation if subjects have no preference to turn right or left? The choice made by an impartial subject in this situation is analogous to a coin toss. The probability that an unbiased coin turns up heads P(H) is equal to the probability that it instead turns up tails P(T). Because the probability that the coin turns up heads or tails P(H or T) = P(H) + P(T) = 1, we know that P(H) = P(T) = 0.5. Furthermore, if we toss the coin repeatedly, say n times, we expect it to land heads on half of all tosses, or nP(H) = 0.5n. Thus, we expect half of our subjects to turn right and half to turn left if individuals have no preference to turn in either direction. This expectation forms the basis of a testable hypothesis.
Other structures, like petri dishes, can be set up to function like a simple T-maze or multiple arm maze. Animals generally exhibit preferences for certain habitat types. Habitats may vary with respect to useful resources, such as protection from predators, and animals are expected to discriminate between alternative places to forage, sleep or rear young. Preferences for colors or patterns by T. molitor can be studied using a petri dish (Fig. 3).
|Figure 3. Petri dish divided into two habitat types, A and B. A subject is placed in the center of the dish (S) and observed for a specified period of time. The time a subject spends on each side of the dish can be measured to score a preference or the side on which the animal is located after the specified period can be used to score habitat preference.|
Consider an experiment on the habitat preferences of meal worms in which subjects are observed in a petri dish with two environments, A and B, of equal area. We measure the time spent in each environment for some specified period t. Although time is a continuous variable we can our data to construct a dichotomous variable so that use of a Sign Test is appropriate to test the null hypothesis that subjects have no habitat preference. Our a priori expectation is that subjects will spend an equal amount of time in each habitat if there is no habitat preference because the area of each habitat is identical. We score trials in which subjects spend more time in habitat A as successes and trials in which subjects spend more time in habitat B as failures. Thus, there are two mutually exclusive outcomes and the probability of a success is equal to the probability of a failure under the null hypothesis, two of the conditions needed to justify our use of the Sign Test. A subject could potentially leave an odor trail on the surface of the petri dish, so we clean the dish after each trial to insure that all trials are independent. The third condition necessary for use of the Sign Test is consequently defensible. Suppose we observe n = 15 subjects for t minutes in the petri dish and find that 12 subjects spend more than half of t in habitat A and only three subjects spend more than half of t in habitat B. Can we reject our null hypothesis that there is no habitat preference? Inspection of Table I for n = 15 indicates that P(S < 3) = P(S > 12) = 0.0176 , so P(S < 3) + P(S > 12) = 0.0352, which is less than our rejection criterion of a < 0.05. We consequently reject the null hypothesis. The experiment suggests that subjects have a habitat preference. The fact that we observed 12 of 15 individuals in area A suggests that habitat A is preferred to habitat B.
The experiments described above will not necessarily reveal group preferences rather than individual preferences. Suppose half of all subjects placed in a petri dish divided into two habitats, A and B, prefer habitat A and half of the subjects prefer habitat B. Then we would expect half of the individuals tested to spend most of their time in habitat A and half to spend most of their time in habitat B, just as we would expect if individuals had no preferences for either habitat. The experiment described earlier consequently reveals an overall tendency for individuals to prefer one of the two habitats. You can test individuals repeatedly to discover individual preferences. Suppose you test a subject twice in identical petri dishes. If a subject has a preference for habitat A, then we would expect the individual to spend more time in habitat A than habitat B in each trial. Furthermore, if a subjects spends more time in habitat A in trial one and has no preference for either habitat, the probability that it spends more time in habitat A in the second trial is analogous to a coin toss. Thus, we can score a pair of trials with the same individual as a success if the subject consistently spends most of its time in one of the two habitats across the two trials. A pair of trials is scored as a failure if in the first trial a subject spends most of its time in habitat A and in the second trial most of its time in habitat B, or vice versa. Use a Sign Test to determine whether individuals have habitat preferences.
Choose an individual and present it with a particular spatial challenge that will tell you something about its ability to detect a particular visual stimulus. You may use a variety of tests, including mazes, structured habitats, or spatial puzzles.
Question: Does your animal of choice orient with regard to the darkness of the background?
Null hypothesis (Ho): No, the animal does not orient relative to the darkness of the background
Alternate hypothesis (Ha): Yes, the animal does orient relative to the darkness of the background
Experiment: Observe whether multiple individuals orient in a dish with respect to a difference (e.g. light vs. dark) in background
Preparation: Choose a particular effect and prepare an arena to test whether this influences spatial positions or movements of individuals. Before you initiate the experiment itself, formulate the expectations that would allow you to reject the null hypothesis. Formulate the conditions under which you will not reject the null hypothesis.
Procedures: Design and conduct an experiment that will allow you to test whether a visual stimulus effects spatial patterns.
Statistical Analysis: Consider first a single individual. What are the chances to find it on/near one particular environment - if the stimulus makes no difference. Now with two animals, what are the chances to find both on the black half, both on the white half or one on each side under the assumption that the background makes no difference. This mental exercise becomes more and more complicated with each aditional individual. Statistical tests help you with this decision. They allow you to estimate under a given null hypothesis whether a particular outcome of an experiment should be considered "rare" or "common". You must decide beforehand how rare the outcome would have to be so you would reject the null hypothesis in favor of the alternate hypothesis. You could use a Sign Test to test whether two outcomes are equally likely. Now, based on your results, formulate your conclusions on whether the stimulus affected the species' choices or not.
Answer questions: What were your conclusions? Would you be interested to repeat the experiment with 180 degree rotation of the arena? Why would you do that? Would you use new individuals for each experiment?
Preparation: Choose a particular odorant and test whether a model species of your choice responds to it. Before you initiate the experiment itself, formulate the expectations that would allow you to reject the null hypothesis. Formulate the conditions under which you will not reject the null hypothesis.
Procedures: Design and conduct an experiment that will allow you to test whether an olfactory stimulus effects spatial patterns.
Were movements of larvae directional or non-directional? Would you consider their movement more in the context of taxis or kinesis? How would you design an experiment that would allow you to scientifically distinguish between them?