EXERCISE 2
AGONISTIC BEHAVIORS
INTRODUCTION
Aggression is any behavior intended to threaten or cause harm to another organism. Agonistic
behaviors are aggressive acts among individuals of the same species. Such behavior usually involves
displays of threat, submission, or even chases between individuals, but may also include physical combat.
Agonistic behaviors often result in dominance hierarchies within populations or groups. For example, if
animal ‘A’ threatens both ‘B’ and ‘C’, and B threatens only C, and C is submissive to both A and B, the
dominance hierarchy would be A < B < C. Typically, aggressive behaviors are used when animals are
competing for some limited resource and are actively trying to exclude rivals (what ecologists call
interference competition). Often such behaviors arise from the need to hold territory, dominate food
resources, or sequester mates, but may include protection of young from intruders.
Behavioral studies often begin with the construction of a list of the observed behaviors called an
ethogram. Using the ethogram as a guide, the observer can then quantify the behavioral interactions of
individual animals by counting the number of times each act on the ethogram occurs per unit time.
Behavioral ecologists are also interested in sequences of behaviors, that is sequential patterns of interactions
that seem to follow each other. For example, courtship behavior in most animals represents a series of
behavioral interactions between males and females where each successive behavior depends on the one
before it (i.e., if behavior ‘A’ does not occur, neither will ‘B’ or ‘C’, etc.).
In this lab, we will examine agonistic interactions found between individuals of the blue crab,
Callinectes sapidus. We will attempt to construct dominance hierarchies based upon sex and/or size of
individuals. Blue crabs have already been studied ethologically and the following ethogram has been
constructed:
1.
Avoidance -- subservient ‘A’ retreats as another individual, ‘B’, approaches; ‘A’ shows no
observable aggression.
2.
Dominant Meral Spread -- ‘A’ shows threat posture with chelipeds (claws) outstretched
towards approaching individual ‘B’; when ‘A’ is dominant over ‘B’, ‘B’ often retreats.
3.
Subservient Meral Spread; ‘A’ shows threat posture with chelipeds (claws) outstretched
towards approaching individual ‘B’; when ‘A’ is subservient to ‘B’, ‘B’ will either ignore
‘A’ or interaction may escalate to a fight in which ‘A’ ultimately retreats.
4.
Dominant or Subservient Strike -- dominant or subservient ‘A’ strikes at ‘B’ with cheliped;
when ‘A’ is dominant, ‘B’ often retreats; when ‘A’ is subservient, ‘B’ will either ignore ‘A’
or interaction may escalate to a fight in which ‘A’ ultimately retreats.
5.
Fight -- both animals strike, grasp, or one animal may totally envelope the other with its
chelae; after mutual striking and grasping, one individual (the subservient) usually retreats.
6.
Advance -- one crab (usually the dominant) moves intentionally towards the other. The key
here is “intentionally”.
7.
Tapping and flicking -- the dominant animal taps the dorsal surface of the other, the
subservient animal often flicks its chelae or appendages (a rapid, jerky motion) in response.
You may observe behaviors not represented in this ethogram. These should be recorded,
added to the ethogram, and tallied as they occur.
METHODS
Each group will be assigned two “observation arenas” (a wash tub or aquarium for observing the
crabs) and shown how to make observations. All crabs will be numbered and the carapace width measured.
Each group will then systematically pair the crabs in the arena and observe their behavior for 15 minutes (or
possibly longer if significant interactions are still occurring). You should record data from all possible
pairwise combinations of crabs. You will need to record the sex and size (longest width of carapace) of
each individual in each bout and note the number of behaviors that each individual performs during the
In your lab notebooks, you should construct a table for each pair wise interaction such as:
Fight Indiv.#
1
1
2
Sex
M
F
Size(cw)
10.5cm
11cm
Advances Avoidances Spreads
1111
11
11
1
1111
1
Strikes
111
Fights
Data Analysis.
Analyze the data using a Goodness-of-Fit G-test in the following manner:
(1)
Prepare tables contrasting the number of interactions against size or sex of individuals; for
example
(2)
NOTE:
Animal (size)
Threats Observed
Threats Expected
1 (10cm)
26
22
2 (8cm)
18
22
Total
44
44
This particular example setup will allow a test of the null hypothesis H0: there is no
difference in aggressiveness between differing sized crabs.
Set up tables and null hypotheses for the affects of sex, size, and dominance hierarchy on
behavior. In order to do this, you will need to come up with an “index” of aggression. This
could be number of threats, number of strikes, number of threats - number of avoidances, etc.
Be sure to record and keep all notes, data, calculations, hypotheses, conclusions, etc. in your
lab notebooks.
QUESTIONS (Please answer in a word document and email it to me by 9am Friday)
1.
Is their a dominance hierarchy among the blue crabs in the laboratory?
2.
What might be the evolutionary advantages of such agonistic behaviors? How does the dominant
individual benefit? How does the subservient benefit?
3.
What “costs” might be associated with the observed agonistic behaviors?
4.
What differences would you have expected if some limited resource like food had been placed in the
arena?
5.
Why should an animal in the middle of the dominance hierarchy display agonistic behaviors to both
dominant and subservient individuals?
SELECTED READINGS
Allee, W.C. and M.B. Douglis. 1945. A dominance order in the hermit crab, Pagurus longicarpus Say.
Ecology 26: 411-412.
Imafuku, M. 1991. Shell fights in the hermit crab Pagurus geminus: Aggressive act with mutual gain.
Journal of Ethology 9: 67-76.
Jachowski, R.L. 1974. Agonsitic behavior of the blue crab, Callinectes sapidus Rathbun. Behaviour 50:
232-253.
Parker, G.A. 1974. Assessment strategy and the evolution of animal conflicts. Journal of Theoretical
Biology 47: 223-243.