One of the early contributions of ethologists was to emphasize that many actions in the natural behavior of animals were evoked by species-specific stimulation provided by social partners.   A stimulus that evokes a particular response in the natural behavior of an animal is called a releaser.   Konrad Lorenz realized that such releasers were responsible for many features in animals' social behavior.   He further emphasized that many social releasers were species-specific, because they only affect the behavior of other individuals of the same species.  

Experiments by Niko Tinbergen and his students around the middle of the century demonstrated that releasers often included only parts of the complete natural stimulus.   They could often show that a simplified version of the natural stimulus, a version with just a few critical features of the natural stimulus, evoked a full response.   A simple but sufficient stimulus for a response was called a sign stimulus.

These concepts proved to be very fruitful as investigation of the neural basis of the natural behavior of animals (a subject known as neuroethology) began to develop in recent decades.   In a number of cases it has been possible to discover the neural mechanisms (often called releasing mechanisms) that recognize releasers.

In this exercise, we use the schooling responses of several species of small fishes in aquaria to investigate releasers for this behavior.

Schooling behavior in fish has probably evolved for a number of different reasons.

1. For small fish, schooling can reduce the risk of predation faced by each individual.   Schools, for instance, might confuse a predator unable to single out any one individual for pursuit.   Also, those individuals inside a school would run less risk of having a predator snatch them; the individuals in a school might thus vie with each other for positions in the middle, behavior that would result in large, dense groups.   This possibility is called the selfish herd hypothesis.
   

2. Schooling might aid in feeding.   In some species, schools herd prey into tight bunches in which they are caught more easily.   Obviously, whether it pays to school or not depends on what you eat and what eats you!

The subjects of our experiments will include two or three species of small fish, for instance, zebra danios (Brachydanio rerio), tetras (Hyphessobrycon callistus), and tiger barbs (Barbus tetrazona).

Procedures

First, observe the fish in the large aquaria.   Learn to recognize the different species; some are easier to tell apart than others.   Try to answer some basic questions:

Do the fish form schools?  

How closely spaced are the individual fish?

How cohesive are the schools when they move?

Do the species differ in any of these features?

If more than one species is present in the same tank, do they school together or separately?

Social releasers for schooling.   Each group will have an empty aquarium in which to experiment.   Make sure your acquarium is at the same temperature as the large aquarium.  

During each experiment, you will place one (or perhaps two) subjects into the aquarium.   Always transfer fish in a container filled with aquarium water -- not in tap water or in a net -- in order to minimize disturbance to the fish.  

You will also place test fish or various species and numbers in containers at the ends of the aquarium.   The object is to see with which of the test fish the subjects prefer to school.

To allow you to record the location of the subject, make sure that marks on the front of the aquarium separate four equal sections of the aquarium.  

Gently place your subject fish in the aquarium.   Add your test fish in two containers at each end.   Every five seconds for five minutes, record which of the four equal sections of the aquarium your subject occupies.  

This information (called scan samples because at fixed intervals you scan your subjects and record their current behavior) will allow you to estimate the proportion of time spent by your subject in each section.

Try these experiments:

1. Use 4-5 test fish of the same species together at one end of the aquarium and an empty container at the other end.   Why is it important to put the empty container in the aquarium?   Flip a coin to decide which end of the aquarium has the test fish.   Why do this?
   

2. Try one fish of the same species as the subject at one end and 4 at the other end.   To start a new experiment, remove both containers for test fish from the aquarium and allow the subject a few minutes without a chance to see any other fish.   How should you decide at which ends of the aquarium to put the solitary test fish and the group?
   

3. If you found a clear response by the subject, try the same two test groups but switch each to the opposite end of the aquarium.   Why?

Now, try a subject of a different species and repeat these experiments.

Do the fish respond to conspecific (same species) individuals?   Do they prefer a group over a single fish?   Do both species respond equally strongly?

Species-specificity of the response.   So far you have used test fish of the same species as the subject.   Now try two test groups with 4 fish each, one group of the same species as the subject and one of a different species.

Try switching the test groups to the opposite ends of the aquarium.

Now try the same two test groups but use a subject of the second species.

Do these fish preferentially school with their own species?   Do the two species have equally strong preferences?

Additional possibilities.   Place all the fish back in the large aquaria and once again observe their schooling.   Sprinkle some food on the surface of each aquarium.   Does the schooling behavior change?   Do the species differ in their behavior?   What effect would the presence of a predator have?

Taking care of your fish.   Maintaining animals in good condition is an important part of studying animal behavior.   Your Teaching Assistant will arrange for students to help in feeding our fish and cleaning their tanks.

References

Chivers, D.P., G.E. Brown, and R.J.F. Smith.   1995.   Acquired recognition of chemical stimuli from pike Esox lucius by Brook Sticklebacks Culaea inconstans (Osteichthyes, Gasterosteidae).   Ethology 99(3):   234-242.

Hager, M. C. and Helfman, G. S.   1992.   Safety in numbers:   shoal size choice by minnows under predation threat.   Behavioral Ecology and Sociobiology   29:   271-276.

Helfman, G. S.   1984.   School fidelity in fishes:   the yellow perch pattern.   Animal Behaviour   32:   663-672.

Keenleyside, M. H. A.   1955.   Some aspects of the schooling behaviour of fish.   Behaviour 8:   183-248.

Partridge, B. L., and T. J. Pitcher.   1979.   Evidence against a hydrodynamic function for fish schools.   Nature 279:   418-419

Partridge, B. L.   1982.   The structure and function of fish schools.   Scientific American 246 (Number 6):   114-123.

Pitcher, T. J.   1976.   Blind fish can school.   Science 194:   963-965.

Pitcher, T.J., A.E. Magurran, and J.I. Edwards.   1985.   Schooling mackerel Scomber scombrus and herring Clupea harengus choose neighbors of similar size.   Marine Biology (Berlin) 86(3):   19-322.

Pitcher, T. J., and J. K. Parrish.   1993.   Functions of shoaling behavior in teleosts.   In:   Behaviour of Teleost Fishes (T. J. Pitcher, editor).   New York:   Chapman and Hall Publishers.

Shaw, E.   1962.   The schooling of fishes.   Scientific American 105 (Number 6):   128-138.

Shaw, E.   1978.   Schooling fishes.   American Scientist 66:   166-175.

Weihs, D.   1973.   Hydromechanics of fish schooling.   Nature 241:   290-291.