Territoriality and the factors that influence the outcome of animal contests are easy to study in a lab using convict cichlid fish (Archocentrus nigrofasciatus). I present a two-week laboratory experiment suitable for high school and college students. In the first week, students observe resident and intruder convict cichlids interacting in the presence of flowerpot territories, allowing them to develop hypotheses about factors that might determine the outcome of contests over resources, including relative size of competitors (“resource holding potential” hypothesis) and ownership (“resource value” hypothesis). They then work with their teacher to design and set up an experiment to test these two hypotheses, providing specific predictions that would support each hypothesis. In the second week, students observe contests between each owner and a larger or smaller intruder. The winner is defined as the individual that spends the most time in the pot and bites more. Students graph their data and decide what statistics are most appropriate for analyzing their results. They determine which hypothesis is supported by their findings and present their findings in scientific paper format.

Introduction

Territoriality is a familiar topic to humans; we patrol our borders, we erect fences around our properties, and teenagers put “Keep Out” signs on their bedroom doors. In nonhuman animals, territory holders often enjoy many fitness advantages over nonterritory holders, despite the energetic costs of defense, such as those experienced by territorial mountain spiny lizards (Sceloporus jarrovi; Marler & Moore 1989, 1991) and surgeonfish (Acanthurus lineatus; Craig, 1996). Territory owners may have exclusive access to food resources that promote rapid growth, as seen in American redstarts (Setophaga ruticilla; Marra & Holmes, 2001). They also can enjoy first access to females, which is the case for territorial male Arctic ground squirrels (Spermophilus parryii; Lacey & Wieczorek, 2001). Territories usually consist of an area or nesting site that can attract mates, and owners are better able to protect the resulting offspring from hungry predators (Grant, 1997).

Given that holding a territory is so beneficial, one might expect individuals without territories to try to expel territory owners and claim the resource as their own. However, it has been well documented that owners win most contests over intruders (Alcock, 2009). Two asymmetry hypotheses have been proposed to explain these observations: the “resource-holding potential” (RHP) hypothesis and the “resource value” (RV) hypothesis. These asymmetry hypotheses are so called because they focus on differences between the owner and the intruder in physical size and strength and in territory value, respectively (Huntingford & Turner, 1987).

The RHP hypothesis states that the ability both to obtain and retain a territory depends on an individual's physical and physiological traits, including size, strength, and fat reserves (Alcock, 2009). This hypothesis holds that resident owners have a competitive advantage over intruders, whether it is due to their larger size, better weaponry, or increased aggression (Maynard Smith & Parker, 1976). The RHP hypothesis has been supported in such diverse taxa as digger wasps (Philanthus basilaris; O'Neill, 1983), black-winged damselflies (Calopterix maculata; Marden & Waage, 1990), white rhinos (Ceratotherum simum; Rachlow et al., 1998), and fiddler crabs (Uca vocans hesperiae; Jaroensutasinee & Tantichodok, 2002).

The RHP hypothesis, however, does not explain why residents often are able to win contests against intruders of greater size and strength, as has been reported for European robins Erithacus rubecula; Tobias, 1997), green hairstreak butterflies (Chrysozephyrus smaragdinus; Takeuchi, 2006), and convict cichlid fish (Archocentrus nigrofasciatus; Figler & Einhorn, 1983). These observations instead can be explained by the RV hypothesis, which states that the territory is worth more to the owner than to the intruder; in other words, the owner has more to lose than the intruder has to gain (Alcock, 2009). This hypothesis proposes that owners are more likely to win a contest because the value of their resource increases over time (the “prior residence effect”; Figler & Einhorn, 1983), making the owner more motivated to defend it. The increase in territory value can be due to a variety of factors, including investment in mates and reproduction (Galvani & Coleman, 1998), and acquired knowledge of foraging and hiding areas (Grant, 1997).

Convict cichlids are an excellent model species to use in laboratory studies of territoriality because they are easy to maintain, readily set up breeding territories in clay flowerpots or other cavities, and exhibit resource defense against intruders (Gennaro & Winters, 1975; Mackereth & Keenleyside, 1993; Galvani & Coleman, 1998). Previous research has shown that prior residence plays an important role in determining the outcome of contests between two similarly sized convict cichlids over a desired resource such as a breeding territory. Indeed, even one day of prior residence can be sufficient time to establish ownership and ensure that the resource is defended successfully (Figler & Einhorn, 1983). However, when competing cichlids differ in size – even if by only 5% – larger male convict cichlids can win contests, especially when neither fish is a territory owner (Keeley & Grant, 1993a). In this two-week laboratory experiment, students work with their teacher to develop the two hypotheses described above on the basis of their own observations of cichlid behavior. They then design and carry out an experiment to test these hypotheses and determine which hypothesis is best supported by their results.

Observations, Hypotheses & Experimental Designs

In the first 3-hour lab, students observe single fish placed in 5-gallon (18.9 L) tanks with a small flowerpot 12–24 hours prior to lab (Figure 1). Each tank should have a bubbler, and a heater to keep the temperature at ~25°C. Each group of three or four students should spend some time observing the fish and discussing its appearance and behavior. After 15 minutes, the professor should place another fish in the tank. It does not matter if it is larger or smaller, but it must be the same sex (females have orange spots on the side). Students again make observations for 15 minutes, noting the relative sizes and locations of the two fish, and any distinctive behaviors and color changes as they interact. With guidance from the teacher, they should produce a list of behaviors, locations, and definitions (Table 1), some of which could be used during the following week's experiment. The behaviors can be discussed in the context of sequential assessment, in which two contestants can assess each other's size and strength, potentially avoiding unnecessary escalation to injury (e.g., Pratt et al., 2003).

Figure 1.

Male black convict cichlid in small tank for preliminary observation.

Figure 1.

Male black convict cichlid in small tank for preliminary observation.

Table 1.
Examples of behaviors, locations, and definitions for observation sessions.
Behavior or LocationDefinition
Facing Two fish face one another and raise their opercular flaps. (Size assessment) 
Side-by-side Two fish line up head to tail. (Size assessment) 
Circling Fish circle one another. (Size and strength assessment) 
Biting One fish “nudges” the other's body with its mouth. (Strength assessment) 
Mouth-wrestling Fish hold onto each other's mouths. (Strength assessment) 
Chasing One fish follows the other quickly around the pot zone. 
Body darkening Fish's body darkens under stress. 
In pot At least half of the fish's body is inside the pot. 
Outside pot Fish is completely outside the pot. 
Behavior or LocationDefinition
Facing Two fish face one another and raise their opercular flaps. (Size assessment) 
Side-by-side Two fish line up head to tail. (Size assessment) 
Circling Fish circle one another. (Size and strength assessment) 
Biting One fish “nudges” the other's body with its mouth. (Strength assessment) 
Mouth-wrestling Fish hold onto each other's mouths. (Strength assessment) 
Chasing One fish follows the other quickly around the pot zone. 
Body darkening Fish's body darkens under stress. 
In pot At least half of the fish's body is inside the pot. 
Outside pot Fish is completely outside the pot. 

Following these observation periods, the teacher should help the students pose a series of questions to discuss in their groups, starting with the idea that one fish is an owner and the other is an intruder (Table 2). At this point, the teacher can introduce the RV and RHP hypotheses for discussion. A reading assigned prior to lab (e.g., Takeuchi, 2006) would enhance this conversation, especially for college students.

Table 2.
Possible post-observation questions and answers.
QuestionAnswers
What characteristics of a fish might determine whether it keeps or takes over the flowerpot? Relative size and ownership (introduce RHP, RV hypotheses). 
How long does a fish have to be in the pot to establish ownership? One day is sufficient, but this is another hypothesis that could be tested experimentally. 
What behaviors might the fish use to decide who wins the pot? Behavioral tests of size and strength (facing, side-by-side, biting, circling, mouth wrestling), coloration. 
How might we decide who wins the pot? Whichever fish spends more time in pot and bites other fish more (loser turns darker). 
QuestionAnswers
What characteristics of a fish might determine whether it keeps or takes over the flowerpot? Relative size and ownership (introduce RHP, RV hypotheses). 
How long does a fish have to be in the pot to establish ownership? One day is sufficient, but this is another hypothesis that could be tested experimentally. 
What behaviors might the fish use to decide who wins the pot? Behavioral tests of size and strength (facing, side-by-side, biting, circling, mouth wrestling), coloration. 
How might we decide who wins the pot? Whichever fish spends more time in pot and bites other fish more (loser turns darker). 

The students then work together in groups to design an experiment to test these two hypotheses, and present their ideas on the blackboard. The class selects the best one with the teacher's guidance. In the experiment described below, larger and smaller intruders are allowed to challenge owners of flowerpots. Data collected during a trial include the difference in time that owner and intruder spend in the pot, the difference in size between owner and intruder, the number of bites by owner to intruder, and vice versa. The “winner” is the fish that stays in the pot longer and bites more. When designing any experiment, it is helpful to make predictions that are specific to the experiment and that support each hypothesis; the students should propose these predictions, which are not the same as the more general hypotheses (Table 3).

Table 3.
Predictions that support the “resource value” (RV) and “resource holding potential” (RHP) hypotheses.
HypothesisPrediction
RV The owner wins, whether it is larger or smaller than the intruder. 
RHP The larger fish wins, whether it is the owner or the intruder. 
HypothesisPrediction
RV The owner wins, whether it is larger or smaller than the intruder. 
RHP The larger fish wins, whether it is the owner or the intruder. 

Experimental Tanks & Fish

Test tanks (Figure 2) should be set up prior to the first lab so that students can help measure and establish owners for the following week's lab; make sure the water stands for a day or two to remove the chlorine. Intermediate-sized fish – male or female – should be selected as owners so that larger and smaller fish of the same sex as the owner can be used as intruders. Before placing each fish in its pot zone, it should be measured by placing it individually in a small, clear rectangular container with shallow water and holding it lightly in position while a student measures it with a ruler (Figure 3). Intruder fish are measured the same way on testing day. Write the size and sex of each owner on the side of each tank so that students can request a larger or smaller intruder of the same sex for test trials.

Figure 2.

Test tank setup, showing two flowerpot zones with owners at either end and the central zone for intruder acclimation. Removable opaque Plexiglas dividers are held in place with vertical guides attached with aquarium silicone. The bottom is covered with ~2 cm gravel, which also is piled behind the pot to prevent alternative hiding places. A heater in the central zone maintains the water at ~25°F. Bubblers in each pot zone provide aeration. The bubbler is removed during the 15-minute test trial.

Figure 2.

Test tank setup, showing two flowerpot zones with owners at either end and the central zone for intruder acclimation. Removable opaque Plexiglas dividers are held in place with vertical guides attached with aquarium silicone. The bottom is covered with ~2 cm gravel, which also is piled behind the pot to prevent alternative hiding places. A heater in the central zone maintains the water at ~25°F. Bubblers in each pot zone provide aeration. The bubbler is removed during the 15-minute test trial.

Figure 3.

Measuring an owner prior to setting up owner territories.

Figure 3.

Measuring an owner prior to setting up owner territories.

It is easy to obtain cichlids from a local pet store. Order all black convict cichlids; they show pattern changes more clearly than pink convicts. The number of fish to purchase will depend on the number of students and the number of 10-gallon (37.9 L) test tanks available. Ideally, students should work in pairs, each pair being responsible for testing the two owner fish held at either end of one tank. For example, if there are 20 students and ten 10-gallon tanks, purchase ~60 fish (Table 4). These fish can be held together in two large, 20-gallon (75.7 L) tanks prior to testing. Feed the fish with cichlid flake food once or twice a day. Each holding tank should have a gravel bottom, a flow-through aquarium filter, and one or two heaters to maintain the temperature at ~25°C. As is the case with any research involving vertebrate animals, approval must be obtained from the IACUC (Institutional Animal Care and Use Committee) prior to obtaining the fish.

Table 4.
List of supplies needed for the two lab periods.
ItemNumber
Black convict cichlids (4–8 cm) 60 (20 owners, 40 intruders) 
Omega One cichlid flake food (62 g) 
5-gallon (18.9 L) aquaria (pre-test tanks) 
10-gallon (37.9 L) aquaria (test tanks) 10 
20-gallon (75.7 L) aquaria (holding tanks) 
Flow through filters (large tanks) 
Flowerpots (5.7–6.4 cm diam.) 20 
Nets 10–12 
Fish bowls for transport between tanks 10 
Aquarium thermometers 22 
Aquarium heaters 14 
Air stones with airline tubing 20 
Double air pumps 10 
Opaque Plexiglas dividers, with twenty ~8 mm holes drilled in each for water and heat circulation 20 (2 per test tank) 
Heavy plastic guides for dividers 40 (4 per test tank) 
Aquarium silicone (to attach guides) 
Aquarium gravel Several bags 
ItemNumber
Black convict cichlids (4–8 cm) 60 (20 owners, 40 intruders) 
Omega One cichlid flake food (62 g) 
5-gallon (18.9 L) aquaria (pre-test tanks) 
10-gallon (37.9 L) aquaria (test tanks) 10 
20-gallon (75.7 L) aquaria (holding tanks) 
Flow through filters (large tanks) 
Flowerpots (5.7–6.4 cm diam.) 20 
Nets 10–12 
Fish bowls for transport between tanks 10 
Aquarium thermometers 22 
Aquarium heaters 14 
Air stones with airline tubing 20 
Double air pumps 10 
Opaque Plexiglas dividers, with twenty ~8 mm holes drilled in each for water and heat circulation 20 (2 per test tank) 
Heavy plastic guides for dividers 40 (4 per test tank) 
Aquarium silicone (to attach guides) 
Aquarium gravel Several bags 

Test Trials

The experiment is carried out during the second 3-hour lab period. Unless you have access to many tanks and many cichlids, students will have to test each of their owners twice: one with a larger intruder first, the other with a smaller intruder first. Testing each owner twice means that each owner will provide two data points for statistical analyses, which is known as pseudoreplication (see below). Prior to the start of a test trial, an intruder fish of an appropriate size and same sex as the owner is selected and measured by the teacher, then placed into the center zone of the aquarium to acclimate for 10 minutes (Figure 4). Ask the students how to control for handling the intruder fish (the owner fish should also be caught in a net and then replaced).

Figure 4.

Students placing an intruder fish in the central zone for acclimation.

Figure 4.

Students placing an intruder fish in the central zone for acclimation.

At the start of a test trial, the divider is raised, the intruder is encouraged gently to enter the pot zone by moving a net behind it, and the divider is replaced. The locations of the owner and intruder are recorded for 15 minutes. After the test trial, the intruder is removed and placed in a separate large holding tank for “used” fish. The second owner at the other end of the tank is then tested with its first intruder, which gives the first owner at least 30–45 minutes for recovery before its next test trial. Data collection during trials can be carried out using stopwatches and a chart (Table 5), but most cell phones now have simple behavioral event-recording apps that are either free (“Observe to Learn”) or quite inexpensive (“Animal Behaviour Pro” for iPhone).

Table 5.
Example data-collection chart for a 2-minute trial (actual trials are 15 minutes). One student calls out the locations of each fish; the other student enters the data. O = owner; I = intruder.
TimeOwner in potIntruder in potBoth in potNeither in potBites
O → II → O
0:00 X    //  
0:11    X /// / 
0:26   X    
0:33  X    / 
0:35   X    
0:41 X    //  
1:02    X //// / 
1:10 X      
1:55   X    
Total(s) 77 18 23   
Summary for data sheet (Figure 5):
Owner in Pot: 77 + 18 = 95 s
Intruder in Pot: 2 + 18 = 20 s
Owner Bites Intruder: 11
Intruder Bites Owner: 3
Winner: Owner 
TimeOwner in potIntruder in potBoth in potNeither in potBites
O → II → O
0:00 X    //  
0:11    X /// / 
0:26   X    
0:33  X    / 
0:35   X    
0:41 X    //  
1:02    X //// / 
1:10 X      
1:55   X    
Total(s) 77 18 23   
Summary for data sheet (Figure 5):
Owner in Pot: 77 + 18 = 95 s
Intruder in Pot: 2 + 18 = 20 s
Owner Bites Intruder: 11
Intruder Bites Owner: 3
Winner: Owner 

The data can be summarized to provide the total time in seconds that each fish spends in the pot during the trial. Students should also tally the number of times each fish bites the other. (It should be noted that these “bites” are light pecks or nudges; the fish never injure one another.) Students may also report whether other “sequential assessment” behaviors occur, such as facing, side-by-side (lateral display), circling, and mouth wrestling. The winner of the pot contest is defined as the individual that spends the most time in the pot and bites more (these values are virtually always in agreement). All data should be summarized on a data sheet (e.g., Figure 5), which the teacher can use to compile the results of all test trials.

Figure 5.

Example data sheet to be filled out by students following each trial (two sheets per owner). Additional behaviors that are observed can be added as desired.

Figure 5.

Example data sheet to be filled out by students following each trial (two sheets per owner). Additional behaviors that are observed can be added as desired.

Tables, Graphs & Analyses

Once all trials are completed, students take the remaining hour of lab time to discuss how they might present the compiled data in table and graphical formats, using the blackboard. Unfortunately, “Observe to Learn” only does interval recording (e.g., every 15 seconds, enter a behavior), and it summarizes results as pie charts of the percentage time for each situation (e.g., percent time owner in pot). This is fine for high school students if no statistics are planned; they can simply compare the categories. College students should decide what would be the most appropriate statistical analyses, guided by their professor. As mentioned above, an owner usually must be tested twice, which means that each owner will provide two data points for most statistical analyses. This statistical “sin” is known as pseudoreplication, due to the artificially inflated sample size. It is important to discuss this issue with students because it means that a Type I error (finding a significant result by chance) is more likely.

Table 6 presents an example of the number of owner fish that won or lost their pot, depending on relative size. These discrete data normally would be analyzed with a chi-square test of independence. In this example, most owners retained their pots, regardless of size, so there was no significant effect of relative fish size on the outcome of the contest (χ2 = 0.067, df = 1, P = 0.795). However, note that if the owners always win, regardless of size, there will be zeros in both rows of the Losses column. In this case, the expected values will be <5, so a Fisher's exact test would be needed to analyze these data.

Table 6.
Number of owner fish that won or lost their pot as a function of their size in relation to the intruder's size.
OwnerWonLost
Larger 22 
Smaller 14 
OwnerWonLost
Larger 22 
Smaller 14 

Students may also choose to present the average number of bites that the owner makes toward the intruder – and vice versa – within a specific contest (e.g., Figure 6). A Wilcoxon matched-pairs test would be the most appropriate statistical test to compare these averages because the data are discrete. However, students at this level may not be familiar with this test; therefore, a paired t-test or a sign test may also be chosen. In these example data, owners bit intruders significantly more than intruders bit owners in each contest (t = 2.209, df = 22, P = 0.038).

Figure 6.

Mean number of bites inflicted by owner (O) and intruder (I) cichlids on each other during each contest (N = 23). Error bars represent standard error.

Figure 6.

Mean number of bites inflicted by owner (O) and intruder (I) cichlids on each other during each contest (N = 23). Error bars represent standard error.

Finally, the relationship between relative pot times of owners and intruders can be plotted as a function of relative sizes of owners and intruders within each contest, as shown in Figure 7. A regression analysis of these example data shows that there is no significant relationship between relative size and relative pot time. In other words, owners spent more time in the pot than intruders, no matter how much larger or smaller they were in comparison to the intruders (R2 = 0.017, P = 0.410, N = 42).

Figure 7.

Relationship between the difference in size between owners and intruders and the difference in time spent in the flowerpot territory during each trial (N = 42).

Figure 7.

Relationship between the difference in size between owners and intruders and the difference in time spent in the flowerpot territory during each trial (N = 42).

Scientific Report

Student pairs are required to write a four- to five-page report in scientific-paper format with five sections:

  1. Introduction – Provide background information about animal contests and territoriality, focusing on the two main hypotheses. State the purpose of the experiment, how the hypotheses were tested, and the predictions that would support each hypothesis. Refer to at least one or two primary research articles, as well as lecture material.

  2. Materials & Methods – Explain how the experiment was carried out, and how the data were analyzed statistically.

  3. Results – Describe the findings presented in each table and figure, state whether the analysis was significant or not, and provide the statistical result in brackets as shown above. Mention that the analyses involve pseudoreplication and consider the implications of this issue.

  4. Discussion/Conclusions – Discuss findings with respect to the hypotheses and previously published research.

    • If owners almost always win regardless of size, this finding supports the RV hypothesis. If the owner wins if it is bigger but loses if it is smaller, this finding supports the RHP hypothesis.

    • If owners bite intruders significantly more than vice versa, then the RV hypothesis is supported. If there is no significant difference between owner-to-intruder bites and vice versa, then the RV hypothesis is not supported by the data. (In the latter case, it would be necessary to check whether larger individuals bite more; if so, then the RHP hypothesis would be supported.)

    • If there is no significant relationship between relative owner–intruder size and relative owner–intruder pot time, then this result supports the RV hypothesis. Alternatively, a significant positive relationship between size differences and the differences in pot time supports the RHP hypothesis.

    • Conclude by proposing future research questions that could be addressed using this model (see Extensions below).

  5. Literature Cited – Cite a few research articles in the introduction and discussion using the format required by the journal Animal Behaviour. The References section should also use a consistent format for all references.

Extensions

Variations that students can examine on their own, either in subsequent labs or as independent research students, include manipulating (1) the presence or absence of holes in dividers and the transparency of dividers (Keeley & Grant, 1993b), both of which can affect whether owners sense the presence of intruders ahead of time, either chemically or visually; (2) the relative size of owner and intruder; (3) sex (females tend to be more aggressive than males); and (4) territory value. Territory value may depend on a variety of factors, such as the duration of ownership (Figler & Einhorn, 1983), the presence of a mate (Keeley & Grant, 1993b), or eggs in the pot (Galvani & Coleman, 1998).

Learning Outcomes

This two-week laboratory activity promotes performance expectations required in a secondary-school science education (Table 7) as defined by the Next Generation Science Standards (http://ngss.nsta.org/), as well as the National Common Core Standards for mathematics (http://www.corestandards.org/Math).

Table 7.
Summary of how activities in this laboratory support NSTA, NGSS, and Common Core science education frameworks (Pratt, 2012; Weber et al., 2013).
K–12 Essential Science PracticesLab Activities
Asking Questions, Defining Problems Based on observations of two cichlids interacting, students pose a research question: What factors influence which fish wins the territory? 
Constructing Explanations During discussions, students develop two hypotheses to answer the research question above: relative size (RHP) or ownership (RV). They learn that these hypotheses differ from the predictions posed below. 
Planning & Carrying Out Investigations Students design and carry out an experiment to test their hypotheses by quantifying behavior. They pose specific predictions that would support their hypotheses: RHP, larger fish wins; RV, owner wins. 
Analyzing & Interpreting Data Using Math & Computational Thinking Students use descriptive statistics to summarize their data in graphical and tabular format and then decide which inferential statistical tests to run. 
Using Evidence to Support Arguments Students determine which hypothesis is supported based on the results of their statistical tests. 
Communicating Information Students collaborate in pairs to present their findings in the context of previous research in scientific-paper format. 
K–12 Essential Science PracticesLab Activities
Asking Questions, Defining Problems Based on observations of two cichlids interacting, students pose a research question: What factors influence which fish wins the territory? 
Constructing Explanations During discussions, students develop two hypotheses to answer the research question above: relative size (RHP) or ownership (RV). They learn that these hypotheses differ from the predictions posed below. 
Planning & Carrying Out Investigations Students design and carry out an experiment to test their hypotheses by quantifying behavior. They pose specific predictions that would support their hypotheses: RHP, larger fish wins; RV, owner wins. 
Analyzing & Interpreting Data Using Math & Computational Thinking Students use descriptive statistics to summarize their data in graphical and tabular format and then decide which inferential statistical tests to run. 
Using Evidence to Support Arguments Students determine which hypothesis is supported based on the results of their statistical tests. 
Communicating Information Students collaborate in pairs to present their findings in the context of previous research in scientific-paper format. 

Conclusion

Territoriality and the factors that influence the outcome of animal contests can be studied in a lab using convict cichlid fish. Students make observations and develop two alternative hypotheses to explain their observations. They collaborate to design an experiment that tests these hypotheses simultaneously by exposing owners to larger and smaller intruders. They gain experience quantifying behavioral observations, graphing and analyzing their data, and writing up their findings in scientific paper format.

I thank Knox College photographer Peter Bailley, all the BIOL/PSYC 312 students who have tested this lab, and the reviewers for their comments. The Knox College IACUC approved the use of animals in this lab.

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