Laboratory Exercises in Animal Behavior - Crickets and Territory Defense
Crickets and Territory Defense
Animals must compete for critical resources such as food or mates, but the form of this competition can differ from one kind of animal to another. In some animals the competition is in the form of a "test of efficiency" or "scramble competition" in which there are no direct confrontations. Instead, the "winner" of the competition is the animal that acquires the critical resource the fastest, or in the greatest amount. An analogy for such competition is the "cola war" in which competing soft drink companies spend money to advertise their products, and the most effective advertisers gain the greatest market shares.
In other cases, individuals or sometimes groups of individuals physically defend specific areas in an attempt to sequester sufficient resources. An analogy of such "resource defense competition" or "interference competition" is a street gang defending its "turf" against invasions by other gangs. The circumstances under which territory defense is advantageous to defending individuals have been studied by many animal behaviorists. In general, the resources being defended must be in short supply. There would be no advantage to spending time and energy defending something that was so common everyone could get enough. And it would be too difficult to sequester resources that moved from place to place, or whose distribution was unpredictable in time (here today, gone tomorrow). Territory defense, then, is a sort of economic practice-individuals and species attempt to defend specific areas and maintain exclusive access to them only when the benefits of doing so outweigh any costs.
Many different kinds of animals defend territories. Sometimes a single individual defends an area against all others of the same sex and species. In other cases one individual defends an area against individuals of other species. In still others, several individuals join together to defend the territory against others of the same, or of other species. Crickets are excellent animals to use in the study of territory defense because they exhibit a wide range of behaviors, they are readily available and easy to care for, and their territories are relatively small. Several different species are available for our use, but only winged adults defend territories. The most commonly used species are of the genus Acheta, such as A. pennsylvanius, the common large black cricket of late summer and fall in northeast North America, and A. veletis, a spring species. Another species, A. domesticus, can be obtained from biological supply houses, or from local bait dealers. This species is acceptable but less aggressive than other members of Acheta. In this exercise we will focus on the territorial behavior of crickets.
A territory may be defined as an area that is used more or less exclusively by an individual or a group as a result of overt defense (e.g., fighting) and advertisement (e.g., singing, posturing), which prevents others from entering. Territories have one common theme: they provide a valuable resource, such as nesting sites, food, or shelter, to the territorial resident that enables that resident (the territory owner) to survive and reproduce. We will examine territory defense by male crickets in a three-session exercise.
Methods
Session I.-We will use our knowledge of ethograms to construct one for the behavior of adult (winged) male crickets as they interact with each other. Take a few crickets from the main holding tank and examine each closely to identify the males. Both males and females have a pair of abdominal cerci (two short "spikes" sticking out the back end of the animal), but females also have a long "ovipositor" that extends from the tip of the abdomen between the two cerci. Females use the ovipositor to lay their eggs in the ground. Place the males in an observation arena (an aquarium), record their behavior, and construct an ethogram. Be sure that some of your observations are of lone males (one male in the arena) and some are of interacting males (two or more males in the arena). Note whether interacting males perform different behaviors, whether one seems to be "dominant" over another, and whether one seems to "win" fights with others. Be sure to get good descriptions of the behaviors associated with dominance versus subordinance, and winning versus losing.
Session II.-Each group will observe their crickets in an empty aquarium (the arena). Remove your subjects from the holding tank and place each separately in a plastic cup. Quickly cover each cup with screening and secure the top with a rubber band. Mark the cups so that you can tell which belongs to each male, and keep track of each male in all subsequent observations. Place a particular male in the arena and allow him to acclimate himself to the environment. This male will be your resident male or territory owner. Then introduce another, intruder male and observe the interactions as follow. Record the number of encounters instigated by the intruder and by the resident male. Record the number of encounters in which each cricket is dominant and subordinate. Record the number of fights and the winner and loser of each fight. Repeat the procedure with different pairs of males, and try reversing the order of introduction. That is, make male A the resident and B the intruder one time, then B the resident and A the intruder another. If you use a small number of males (say three or four) you may have time to run a complete round robin "tournament," in which all possible pairs are tested in all possible configurations (resident versus intruder). When you are done with this part, return all crickets to the holding tank.
Session III.-Place two match boxes at opposite ends of the arena, introduce three males, and observe who is able to establish a territory. Note how long it takes for territories to become established, and record the interactions of the two establishing males. Finally, place a female in the aquarium and watch the male-male and male-female interactions.
Data analysis and presentation of results.-The results of your two-cricket contests can be presented as percentages of encounters initiated and won or lost (or tied). Most of your results, however, will be descriptive. You must describe accurately and concisely what happened in each encounter you observed.
The most common way that scientists communicate their results with others is in written journal articles. The field of animal behavior has many such journals, which specialize in publishing studies of the four aspects of animal behavior. You will present your results in the form of a formal lab report, which will mimic a journal article. Your teacher will explain in detail how to prepare your lab report, but it should contain the following elements. (1) A short but informative title. (2) The names of the authors. (3) An "abstract" or one-paragraph summary of the report. (4) An introduction that describes briefly what you did and why you did it. (5) A methods section that describes in detail exactly how, where, and when you did your study. (6) A results section that presents your results (including graphs and tables) but does not interpret them at all. (7) A discussion section in which you talk about what your results mean and why they are important, and suggest what might be done next. (8) A list of any references used in the preparation of your report.
Things to Think about
What are the characteristics of territory defense, and do the crickets exhibit these characteristics (that is, are they really territorial)? How do crickets attempt to defend their territories? Do you see evidence ofboth overt defense and advertisement? Do you see evidence of exclusive occupation or that intruders avoid territory owners? How do crickets "advertise" their ownership of territories? Does the resident always successfully defend his territory? Is he able to do this against a male that was previously dominant over him? Where in the arena are the encounters taking place? Does the outcome of the encounter depend on where it occurs? Is fighting always the ultimate resolution to conflicts over territory, or can encounters be settled without recourse to fighting? Is there any way to predict who will win an encounter between two males before the encounter is staged?
Materials Needed
- crushed dry dog food (oatmeal and cracked corn are alternatives)
- water (a wet sponge works well) or slices of an apple or pear
- one aquarium (approximately 10 gallons) with screen top per three students
- match boxes and (cardboard) egg cartons
- plastic (not paper) cups, fiberglass screening, and rubber bands
- one large aquarium with a screen top, or a large plastic garbage can with a screen cover
Tips for Teachers
House crickets (Acheta domesticus) can be purchased from Carolina Biological Supply. Schools in the eastern United States (Minnesota, Nebraska, Kansas, Oklahoma, Texas, and all other states to the east) should order from Carolina at 800-334-5551. Schools in western states and Alaska and Hawaii should order from Powell Laboratories at 800-547-1733. Be sure to specify adult crickets (you may need to buy the House Cricket "Replacement Set," which includes adult crickets, sand, food, and peat pots). Nasco Science also sells crickets, but in mixed sets of adults and immatures. Eastern states (North and South Dakota, Nebraska, Kansas, Oklahoma, Texas, and points east) should order from Nasco (800-558-9595). Western states should order from Nasco West (the same 800 number will automatically connect you with Nasco West). All biological supply companies have restrictions on when and how they ship, so you should call well ahead to ensure timely delivery and the best price. In our experience, crickets shipped during very cold weather are likely to be DOA (Dead On Arrival), so you should consider the timing of this exercise carefully. An alternative source of crickets is a local bait or tackle shop. Bait crickets are usually cheaper, and are often available year-round, but many of them will be immature animals (and you need adults=winged crickets).
Housing can be provided simply using a large plastic garbage can. Cut a hole in the lid and use duct tape to attach fiberglass screening over the hole. Put a hand-full or two of crushed dry dog food in the bottom of the can along with two or three wet sponges on dishes, and then put cardboard egg cartons in the can so the crickets can hide from one another. Be sure to check the sponges every day-these are the water sources for all of the crickets. Alternatively, you can throw in slices of apple or pear as a water source as well as a food supplement.
A 10-gallon aquarium makes a reasonably good testing arena. It is not necessary to put any sand or soil in it, unless you want to mate the crickets to observe reproduction.
The observations are organized into three sessions, so you will need to use at least three class periods to complete them. It is possible that your students will become interested in running a lot of subjects in Session II (although a complete round robin with three crickets would be sufficient-it would require six different contests: 1 vs 2, 1 vs 3, 2 vs 3, each done twice with the residents and intruders switched, like a home-and-home conference basketball schedule), so be prepared to allot more time, or to tell them to go on to Session III. You might want to discuss with them the advantages of a larger sample versus faster completion of that part of the study.
Have your students think carefully about how long an acclimation period would be necessary to establish "residence." If the second cricket is placed in the arena just five seconds after the first, is the second one really an intruder? What constitutes "residence" in our study? Often territorial encounters end with a clear winner and loser. The loser is characterized by his determined attempts to escape. Remind your students that the arena test prevents losers from escaping, however, and may artificially prolong the encounter-the loser cannot escape, so the winner repeatedly attacks him. Once it is clear that one cricket is the loser, that animal should be removed from the arena to avoid undue harm and stress.
One of the most familiar aspects of cricket behavior is their singing. Your students will notice the singing right away (if the exercise goes as it should), and you should remind them that vocalization (singing) is a potential advertisement (that is, a way for a territory owner to declare his ownership). For this reason, you students should be reminded to take data on singing rates of both residents and intruders. They should also note whether singing behavior changes over the course of an interaction (for example, does one cricket stop singing, and does this change predict which will win and lose?).
You will need to spend some time explaining to your students how to write a formal lab report. Some might object, saying that this is a science class, not an English class, but you should point our forcefully that the best scientific study is worthless if it is not communicated to other scientists. Scientists are also professional writers, so science courses must stress scientific writing. As a first step, tell them that scientific writing is very different from creative writing in its primary goal. A science writer does not write to be understood, but to prevent misunderstanding. A clear, unambiguous, and economical style is the ideal, although there is room for individual difference too. Give them the following exercise-tell them to write directions on how to put on a coat, then have someone try to follow the directions. Tell the person to act as though s/he has never seen a coat before. Many of your students will find it very challenging to write unambiguous directions.
A scientific paper is a written and published report describing original research results. Scientific papers have been evolving for over three centuries under the influence of developing tradition, editorial practice, scientific ethics and printing/publishing procedures. Scientific papers are highly stylized, with distinctive and clearly evident component parts. At its present state of evolution, scientific papers are usually composed of the following sections: Title, Abstract, Introduction, Methods and Materials, Results, Discussion, Acknowledgments, and Literature Cited. This organization reflects the basic steps in the scientific process: stating a problem, forming a hypothesis, testing the hypothesis and interpreting the results.
Title.-The title is a one-sentence summary of the paper. It should give the reader a concise, informative description of the content and scope of the paper.
Abstract.-The abstract is an informative summary of the objectives and findings of the study. Usually the abstract is a single paragraph of about 100150 words that (1) states the purpose of the study and the hypotheses being tested; (2) contains a sentence summarizing the methods used; (3) emphasizes the major results of the study; and (4) ends with a closing sentence suggesting how the results may be interpreted. The abstract should be able to "stand on its own"; it should not refer to any graphs or tables, or cite any references.
Introduction.-The introduction prepares the reader for the rest of the paper. In this section the purpose of the study is stated and enough background information is given to clarify the significance of the study and the ideas being tested. Any hypotheses and their predictions should be described explicitly in the introduction.
Methods and materials.-The Methods and Materials section is a very concise summary of the subjects, equipment and procedures used in your study. This section should contain enough information for another researcher to repeat (replicate) your study exactly.
Results.-The Results section includes a narrative description of the results and any tables and graphs. In this section the results are described, but they are not discussed or interpreted. All further discussion of the results is saved for the next section. Tables are used to summarize numerical results in columns and rows. Each table should be numbered and given a concise heading. Graphs are visual representations (pictures) of numerical results, and are also numbered and titled. Data should be reported only once: in a paragraph of the Results section, or in a table, or in a figure (but not two of them and not all three).
Discussion.-The Discussion section should provide your interpretations of the results and place the results in context. The first paragraph should briefly summarize the major findings of the study. Subsequent paragraphs should deal with each major finding of the study. Each of these paragraphs should begin with a topic sentence that refers to or describes the main point, and should be followed by statements and information that support that point. The results of the study must be interpreted with respect to the hypotheses described in the introduction, and in light of the animal's biology. Avoid statements that cannot be supported by the data, and be sure none of your conclusions are contradicted by the data. The main points of the discussion should tie in with the main points of the introduction. The paper must come "full circle" and the discussion must give the reader a sense of completion. At least a paragraph should be devoted to a discussion of possible errors in the design and implementation of the study, and to alternative interpretations of the results. The final paragraph should provide a "take-home message" and/or suggest new areas for further research. This concluding paragraph must be both forceful and memorable, and should produce a sense of closure for the reader.
Acknowledgments.-Often the authors of a scientific paper receive help from many other individuals. The Acknowledgment section is a way for the authors to acknowledge the help and thank the individuals who provided it. Usually, authors will acknowledge those who have suggested improvements in their methods, provided useful information or data, gave permission for specific aspects of the work, or provided suggestions, criticisms and comments on earlier drafts of the paper. This section may not be necessary for your students' reports.
Literature cited.-Complete citations for all references cited in the text of the paper are given in the Literature Cited section. Citations are listed in alphabetical order by authors' last names (note that these citations are not endnotes).
Special features of scientific papers.-Papers written for scientific topics differ in several ways from other papers. For example, you should rarely, if ever, use quotations in scientific papers. Quoting another author's words is only appropriate when the exact wording (not the idea) is the point you are trying to make. Although quotations are rare, citations are common. You should cite any and all findings, interpretations and ideas that are not your own. Footnotes are NEVER used for citations; a Literature Cited section, which follows the discussion, is used to list references for all cited papers, as described above. The goal of scientific writing is more than understanding; the goal is to avoid misunderstanding. If ten people read a scientific paper, all ten should have exactly the same understanding of what was done, why it was done, what was found, and how the findings were interpreted.
