Eye Concealment - NSTA

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Concealment of Give-Away Parts

The outline of an animal is not the only feature that might give it away. Often some part of it, perhaps its eyes or its legs or its tail, might also be a clue. In this activity, these parts are called give-away parts. Their concealment often involves special features.

Eye Concealment

In some environments, round objects are unusual and likely to attract atten‑tion. In nature this characteristic of a round object is quite likely an advantage to many flowers whose round blooms must be visited by nectar feeders if the flower is to reproduce. It may also be an advantage to certain plants whose round brightly‑colored fruits and berries attract animals that disperse the seeds in their droppings. It may be a disadvantage, however, when the object is the eye of an animal, as many hunters can attest.

In many animals the conspicuousness of the eye is diminished, if there is a match of the color of the iris to the color of the head. However, this still leaves the pupil, which is usually round and black, as a potential give‑away feature.

A few animals, such as many geckos (small harmless lizards) and some snakes, not only have an iris that blends with the color of the head but also a pupil that closes down to a narrow inconspicuous slit during the daytime (see Figure 6a, sidewinder). Since such animals are primarily nocturnal, their need for vision during the day is small. Therefore, the size of the pupil would not affect their vision and may prove advantageous for concealment.

Most animals have round rather than slit pupils, and therefore if they are to see they must expose a black, round spot. Many variations in color appear to make the black spot of the pupil and the rest of the eye structure less conspicu‑ous, or at least less recognizable as part of an animal.

One of the most common features that effectively conceals the eye is the eye stripe. Figures 6a and 6b show a number of eye‑striped animals. Note that the stripe runs through virtually unrelated structures. Besides the pupil, it can in‑volve a portion of the iris and the skin, scales, feathers, or hair on the body. In view of the involvement of such different structures, the incidental occurrence of such stripes seems unlikely. Their functional character is further suggested by their occurrence in many diverse animal groups.

National Science Teachers Association

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Activities on the Evolution of Concealment108 Activities on the Evolution of Concealment

Figure 6a. Eye Concealment

Activities on the Evolution of Concealment

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Figure 6b. Examples of False Eyes


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The presumed advantage of the stripe is that it replaces a small but circular spot with a larger but ambiguous stripe. Often the stripe acts as a disruptive mark, obscuring the profile of the animal. Dark stripes harmonize with many features of the environment and their shadows—rock crevices, fissures in bark, twigs, and slender leaves.

Also shown in Figures 6a and 6b are several other kinds of eye concealment. Often the eye is hidden in a black patch as with the American Kestrel of Figure 6b. The mask of a racoon may be advantageous in concealing its eyes.

Protection of the Eye

Besides being a possibly conspicuous spot, the eye is also a vulnerable spot. Therefore features of coloration that hide the eye may also serve primarily to protect the eye. In some instances concealing the eye may be at the expense of revealing the animal. The butterfly fish in Figure 6b, for example, is made conspicuous by a larger‑than‑life “eye” toward its tail. Possibly such a mark‑ing misleads an inexperienced predator that, responding to the spot as an eye, miscalculates the direction in which the prey will flee. The true eye is hidden by a stripe. There is also the possibility that the marking may function in species or sex recognition.

Coloration that protects the eyes is often advantageous for predators that prey on animals that can retaliate—particularly those that might peck the predator’s eyes. The eye stripe of the Shrike in Figure 6a may have evolved as a result of its predatory habits. The Pygmy Owl and the American Kestrel (Figure 6b) also prey on smaller birds and may owe the eye‑like marks on the backs of their heads to this feature of their existence. Should an owl or hawk be attacked by other birds, their chances of losing an eye would probably be less be cause of the deceptive markings.

Leg Concealment

A frog’s leg, very useful in jumping, may be an obstacle to concealment. The un folded leg might be recognized as a part of a frog and thus be a give‑away part. For concealment, therefore, hiding of the legs may be advantageous.

As might be expected, frogs in general are observed to fold their legs when resting. Although such behavior greatly reduces the conspicuousness of the legs, some frogs also have disruptive marks that slash across the folded legs. These marks may provide an advantage to the frog as they help to obscure the


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still discernible outlines of the legs. Like the eye stripes previously discussed, these markings run through several distinct structures—in this instance three parts of the leg and foot (see Figure 6c). Disruptive patterns that join up differ‑ent parts of the body in this way are often called coincident disruptive coloration or joining patterns.

Figure 6c. Coincident Disruptive Coloration


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Wing Concealment

The wings of moths also often exhibit coincident disruptive coloration. Again pat tern elements run through more than one structure. In the moth, markings often cut across both pairs of wings, sometimes involving the back of the ab‑domen as well. Figure 6c shows a moth that has coincident markings on the wings only. Note that the markings on the hind wings extend to complete the pattern when the moth rests with wings folded.

Coincident disruptive markings are observed on a great variety of animals. Since such marks appear impossible to explain as incidental (or physiological) coloration, they offer some of the best arguments that the coloration evolved as a result of their visual effect.

Activity 6-1 Eye Concealment

In the last activity the students may have discovered that a badly concealed eye can spell disaster. They may also have found that a stripe can make all the difference. The chances are better, however, that many have failed to conceal the eye effectively and that few, if any, have used a stripe for that purpose. The function of the present activity, therefore, is to consider the problem of eye con‑cealment more carefully and to investigate the usefulness of an eye stripe.

Students may first develop and try their own ideas for testing their models. But again a proven experiment is presented for use if the students’ ideas don’t work well.

ACTIVITY 6-1 NOTES

Procedure Props and Supplies

1. Discussion of eye concealment

2. Design of experiment Sample “fish” with and without eye stripe

3. Preparation of props

60 double sheets of newspaper, black watercolor or tem-pera, black construction paper, several 9" × 12" sheets, 3 or 4 yardsticks, 3 or 4 scissors, glue, standard size ¼" paper punch

4. Notebook entry


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Preparation for Activity

Paper fish: Using Figure 6d, cut out two fish from newspaper. Use a section of the paper in which photographs or headlines will not create prominent pat‑terning on the fish. The Want Ad section is recommended.

Figure 6d. Pattern for Fish

Put on eye stripes with black watercolor or tempera mixed with a trace of white so that it dries a dark grey rather than jet black. Stripes should be placed on both sides of the “fish.” Intensely black stripes sometimes do not work well. Do not allow the stripes to intersect the outline of the fish (see Figure 6e).

Figure 6e. Preparation of Models for Eye-stripe Experiment


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The next step is to equip the models with eyes. These may be prepared from black construction paper. Circular “pupils” can be punched out with a ¼" pa‑per punch.

Glue the “pupils” to both sides of both “fish.” The final models should look like parts A and B of Figure 6e.

Suggested Teaching Procedure

1. Discussion of eye concealment: Point out several conspicuous eyes on the fish that the students have patterned. Ask the students for sugges‑tions of ways in which the conspicuousness might be reduced. Have those students with ideas illustrate them on the chalkboard. Remind the class that the pupil is a black opening and cannot be eliminated.

At this time there is no point in trying to elicit descriptions of all the vari‑eties of eye‑concealing devices that are observed in animals. A variety of actual colorations that appear to provide some measure of eye conceal‑ment are shown to the students in the next activity. In view of the activity to follow, however, the eye stripe should be among those considered.

2. Design of experiment: Raise the question of how the effectiveness of an eye stripe might be tested. Encourage the students to suggest specific experimental techniques that might be tried. If interest runs high, and time permits, work out the details of the most popular idea and carry it through.

If the students’ experiment doesn’t work well (or time is not available for it), display the striped and unstriped “newspaper fish” and suggest that they might be used as models in an experiment. Explain that with shred‑ded newspaper as a habitat, the general color of the fish will match the background almost perfectly. Therefore the most conspicuous feature of the fish will probably be its eye. We then may have a good chance of finding out whether an eye stripe can delay capture. The shredded newspaper may be thought of as seaweed.

Explain that the black dot is an attempt to simulate a real animal’s pupil, which usually appears black and round. The eye stripe is made a shape, size, and color that will serve to disrupt the circular appearance of the pupil without being too prominent itself. The stripe does not intersect the outline, for then the stripe might be successful partly because of its


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disruption of the fish’s shape rather than its masking of the pupil, and the purpose of the experiment is to test the effectiveness of the eye strip only.

Indicate that the plan is to distribute equal numbers of each kind of “fish” throughout the “seaweed” habitat and have the students search for them for a specified time.

3. Preparation of the props: The class might be divided into groups as‑signed to specific tasks. One group could make the “seaweed.” This is accomplished by folding a double sheet of newspaper once and tear‑ing off strips ½" wide (see Figure 6e). Use all parts of the newspaper but avoid colored sections. Shredded newspaper used in packing can be used if some wider strips are added. Another group could cut out the fish. Be sure no pencil outlines are present. Four or five can be cut out at one time. A third group could put on the eye stripes. A fourth could make and glue on the pupils.

Approximately 20 to 25 models of each type should be made. After they are all made, they should be checked to make sure that they are pat‑terned the same on each side and that exactly equal numbers of each type have been made.

To make the habitat, spread newspapers on the floor to cover an area about 3 feet wide and 15 feet long. Tape the outer edges to the floor so that the papers won’t move about. The newspaper shreds should then be spread evenly over the area to a depth of 2 or 3 inches. Include short pieces 2 or 3 inches long as well as long strips.

4. Notebook entry: Have the students complete the following notebook entry.


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ACTIVITY 6-1 EYE CONCEALMENT

Name:

Date:

Title of Experiment:

My Hypothesis:

Description of Experiment:

Results:

Conclusions:


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Activity 6-2 An Experiment With Eye Stripes

In the present activity, the students try to discover whether the eye‑striped “fish” prepared in the last activity are less likely to be “eaten” than the “fish” without eye stripes. The “predators” are permitted to forage for a limited time after which the contents of their “stomachs” are inspected.

At the end of the activity, the students are shown examples of eye stripes that occur in real animals, as well as other colorations that may serve to conceal or protect the eye.

ACTIVITY 6-2 NOTES


1. Discussion of basic assumptions made in experimenting with eye stripes

2. Hunting for “fish” Fish models and seaweed habitat

3. Presentation of concealment of fish

4. Searching for living examples of eye stripes

5. Demonstration of eye concealment

6. Demonstration of false eyes


Models: The students’ efforts from the last activity should be gathered together into equal sets of the two varieties. Cull out any poorly made or improperly marked fish. Count the number in each set and make sure that it is the same for each.


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Hiding the “fish”: If the class members are to do the hunting, the teacher should hide the “fish” ahead of time. Thoroughly mix the two kinds of fish. Pat down the surface of the habitat to even it. Then distribute the models as evenly as possible throughout the “seaweed” habitat. Place the models on their sides on the surface of the habitat. If instead of serving as their own guinea pigs, the students elect to try the experiment on students from another class, the hiding can be done by the students during class time.

Notebook forms: A sample form for the report on the experiment follows on the next page.


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ACTIVITY 6-2 AN EXPERIMENT WITH EYE STRIPES

Name:

Date:


My Hypothesis:


Results:

Conclusions:


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1. Discussions of basic assumptions made in experimenting with eye stripes:

a. The fish match the color of their background.

b. Side lighting on the flat fish produces the same visual effect as normal light from above on fish that are perfectly countershaded.

c. It is the pupil that is conspicuous in those fish that do not possess the eye stripe.

d. The natural predator is one with visual perception similar to our own.

2. Hunting for “fish”: Have the students hunt in groups of two to four. They should be spaced out along one side of the foraging area facing the class. On signal they should begin catching the fish as rapidly as pos‑sible, placing them in a row in the order caught. Side lighting is best in conducting the experiment.

One fish should be caught at a time by each predator. There should be no pawing through the habitat. The predator should turn away and place the prey on a table or on the floor. The prey are lined up so that capture rates of the two types of models can be com pared. The teacher should see that the proper order is maintained.

When roughly half the total number of fish have been caught, stop the foraging and analyze the results (see Figure 6f). List the results on the chalkboard. Replace the captured fish and allow another group to hunt.


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Conclusion: As captures progressed the striped fish became “more avail‑able” to the predators since they constituted the majority of the popula‑tion. However, even in the 6 to 10 capture category they still were found less frequently than the unstriped fish.

3. Notebook entry: Experiment report. At the conclusion of the experiment the notebook form on page 119 may be filled out. The students may also include samples of each model in their notebooks.

4. Searching for living examples of eye stripes: Have the students look for examples of eye stripes in wild animals. This can take place as an in‑ternet search or the teacher can bring in a variety of pictures for students to examine. Make a list of eye‑striped species based on their personal observations.

5. Demonstration of eye concealment: Display Figures 6b and 6c and point out the different ways in which coloration appears to reduce the conspicuousness of the eye. Eye stripes such as the students have experi‑mented with are seen in three of the animals—the frog, the strike, and the pike. The butterfly fish has a vertical eye stripe. In the sparrow hawk the eye is situated in a dark blotch. The eye color of the sidewinder (a

Figure 6f. Sample Results of Eye-Stripe Experiments

Experiment Number Number of Participants

Totals Captured

Unstriped Striped

*1 26 168 (64.6%) 92 (35.4%)

(6 groups of 4 each)

2 6 43 (64.2%) 24 (35.8%)

3 4 30 (75.0%) 10 (25.0%)

4 6 36 (64.3%) 20 (35.7%)

Grand Totals 277 (65.5%) 146 (34.5%)

* Changes in capture frequency as predation progressed in Experiment 1:

Captures 1—5 Captures 6—10 Unstriped Striped Unstriped Striped 90 (69%) 40 (31%) 78 (60%) 52 (40%)


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rattlesnake) blends with its head scales, and in daylight the pupil closes down to an inconspicuous slit. These colorations may be advantageous in concealing the animal by obscuring the give‑away eyes. They may also serve in part to protect the eyes. Whether either or both of these functions is present, however, in any of these animals could only be de‑termined by extensive observation.

6. Demonstration of false eyes: Point out the eye‑like markings of the ani‑mals in Figure 6b. Ask the students what function they might serve (di‑version of attack, species recognition, sex recognition). Note that in most of these animals the true eye appears to be concealed.


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Activity 6-3 Coincident Distruptive Coloration

The central character of the activity is a frog. Many frogs exhibit coincident markings on their legs. When the legs are folded, dark marks on upper and lower portions join to gether to form bold markings that slash across the legs (See Figures 6c and 6g). The chief activity is to observe the markings on a frog and then to make a jointed paper model of such a frog.

ACTIVITY 6-3 NOTES


1. Discussion of problem of frog’s give-away parts

Paper frog model

2. Demonstration of coincident markings on model

Black felt pen

3. Frog pictures Pictures of frogs with coincident leg markings

4. Inspection of other animals exhibiting coincident disruptive coloration

Pictures of insects alive or from collections, etc.

5. Notebook entry

6. Making a model frog

Photocopied frog patterns (one/student), 9" × 12" sheet of green tag board (one/student) plus 1¼" paper fasteners six/student, box of colored crayons including black (one/student), scissors (one/student)


Frog patterns and models: Prepare copies of the pattern on Figure 6g for the students. Also use the pattern to prepare one model out of tagboard for class‑room demonstration. Leave the model unpatterned.


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Figure 6g. Pattern for Frog with Movable Legs


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1. Discussion of frog’s give-away parts: Hold up the tagboard form with its legs partially unfolded and ask what it is. When the class identifies the model as representing a frog, determine what characteristics identify it as a frog. Conclude that like the eyes, the legs may be give‑away parts. Discuss means for concealing the legs: (1) folding them, (2) adding de‑ceptive pattern, (3) doing both.

2. Demonstration of coincident markings on model: With black crayon or felt pen place black bars across one or both legs of the model frog with the legs in the folded position. Then unfold the legs to show how the pattern breaks up when the legs are not in the resting position.

3. Frog picture display: Using pictures of frogs obtained from books or the internet, ask the students to notice whether the markings on the frogs’ legs join up to form a single pattern when the legs are folded. There is considerable variation in frog leg patterns. Sometimes the markings do not join up or they may be absent.

4. Inspection of other animals exhibiting coincident disruptive color-ation: Display Figure 6c or other pictures or books showing various examples of coincident disruptive marking. Have students search the internet for examples of animals that exhibit various examples of coinci‑dent marking. Students should sketch the animal and describe the habi‑tat in which it is normally found.

If an insect collection is available (in particular, one containing moths), have the students inspect it to see if they can discover examples of coin‑cident disruptive coloration. As insects are usually mounted, the wings are not put in the normal resting position but are spread out to reveal them more fully. As a result, any coincident pattern is likely to be sepa‑rated into its parts. The students should be encouraged to look for living examples of animals with coincident markings. Moths attracted to lights about students’ homes at night may have coincident markings. As with the eye stripe, a list of examples should be prepared as the students re‑port their observations.

5. Notebook entry: Have students complete the notebook entry on page 127.


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6. Making a model frog: Distribute the materials for making the tagboard frogs and have each student construct and color one frog (see Figure 6h). The students may make up their own color scheme or (probably better) copy the pattern of an actual frog. Either the frog on Figure 6d or other pictures of frogs may be copied. The placing of the pattern on the legs should be done with the legs in the resting position so that the pattern elements will line up properly. Use black crayon or felt pen.

The completed frogs may be mounted in the students’ notebooks. To show how the pattern works, the frog may be mounted with one leg folded in the rest position and one leg partially extended.


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ACTIVITY 6-3 COINCIDENT DISRUPTIVE COLORATION

Name:

Date:


My Hypothesis:


Results:

Conclusions:

Eye Concealment - NSTA

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