BETTA BEHAVIOR
Epigamic Display of the Male Betta: A Study of Sexual Selection
PURPOSE

To examine aggressive behavior in male Siamese Fighting Fish, Betta splendens

To correlate male dominance hierarchies with female preference
MATERIALS
Betta splendens individuals (3 male, 2 female)
Glass bowls
Mirrors
Transparency centimeter rule
Nets
Rectangular “arena” tank
Copyright © Dr. Robert R. Glesener
INTRODUCTION
On a worldwide basis, females and males are distinguished by the gametes they make, egg and
sperm, respectively. Although both carry roughly equivalent amounts of genes, eggs are enormous,
nutrient-rich cells that will provide for the energetic needs of early development. Yolk is heavy and
takes up much space, making eggs almost completely immobile. This would create a fertilization
problem if all gametes were of this type. Sperm, on the other hand, are nothing more than vehicles
for delivering the father's genes. Their small size and lack of excess baggage allows them to be
mobile, swimming "in pursuit" of eggs (as in animals), or being carried by wind and other outside
agents (as with pollen in plants). Because eggs are so much larger and more expensive to produce,
females make many fewer of them, i.e., eggs are in short supply. Every egg produced has a good
chance of becoming an embryo.
On the other hand, there are more than enough sperm to "go around". The average male
produces enough sperm to fertilize, theoretically, all adult females of his species. Of course, it is not
usually physically possible, even if he were the only male of that species available. What's more,
many breeding populations usually consist of roughly 50% males, each with the same capacity. The
vast majority of male gametes will be wasted, and males must compete among themselves for access
to the limited supply of eggs. Thus, male and female adults adopt different reproductive strategies.
Both of these strategies seem adaptive, i.e., appropriate for the perpetuation of their respective
gametes. At the same time, they are mutually interdependent in the sense that in a sexual species,
neither is "good" without the other.
This has lead to the evolution of distinctive features, particularly in males, whose primary
function is for male/male competition and/or male/female attraction. Without these accessory
features, the sperm produced by a male’s primary sexual organs (testes) would be worthless; thus,
they are called secondary sexual characteristics. The female may also evolve accessory features
that, rather than being used in "combat", are signals of her receptiveness and for the male
competition to begin. It is these secondary sexual characteristics that we usually use to identify male
and female phenotypes of a species. Intermediate phenotypes of either sex would send confusing
messages to the opposite sex, eliciting inappropriate behavior. In males, intermediate phenotypes
probably would not be successful in male/male competition, either, since many of these "masculine"
features (discussed below) are determinants of success in dominance hierarchies or in defending
territories. Thus, selection will maintain the two extreme phenotypes (male and female) at the
expense of intermediates; this is one of the few examples of disruptive selection operating in nature
(Glesener, unpublished #1). To the extent that the male and female bodies differ, we say they are
sexually dimorphic (di = two, morph = form).
Sometimes advantage over other males is obtained through the establishment of "pecking orders"
or dominance hierarchies, with the male on top of the totem pole being the alpha () male, the next
beta (), and so on. This arrangement favors males that are large and intimidating (aggressive) in
order to get primary access to valuable resources, e.g., females about to ovulate, yet still capable of
cooperative interactions in a social environment, e.g. chimpanzees. The selection for males to be
larger and more aggressive than females is particularly noticeable in openly polygynous species
where competition for females is greatest (in some species 4 of 5 males born die without siring
Copyright © Dr. Robert R. Glesener
offspring). The advantage of such a system to the female is still a topic of debate. Can you think
how mating with the alpha male might increase her fitness?
In other species, females are attracted to males holding territories established by driving out
other males using acts of aggression. Here the advantage to the female is more immediate; the
resources of the territory can be used to raise her young. Some evidence in birds suggests that the
female selects a male based solely on the quality of his territory (Orians 1980). If the female is not
"sizing up" the male, why are such systems often associated with sexual dimorphism in color and
vocalization? Vocal and color displays in these systems may be intended for other males, not
females (although females can use them as cues that there is a potential suitor in the vicinity with a
territory to be examined). Most male intruders are not looking for a fight; they are looking for a
territory. If they had forewarning that this real estate was occupied, they would probably steer clear.
After all, the incumbent, with his knowledge of escape routes, perches, etc. is more likely to be
victorious. By having contrasting plumage or "singing" a song, the resident makes his presence
known. Males pay a price for this conspicuousness. Besides females and other males, they make
themselves known to predators as well. This is one reason why males of most species usually have
higher mortality rates. Females, who don't need to compete, are usually cryptically camouflaged, as
are juveniles (and, often, males out of breeding season).
Still in other cases, the female simply seems to choose a male on the basis of appearance, often in
the form of elaborate costumes, e.g., tail feathers of peacocks or of the African long-tailed widow
birds or the bower of bowerbirds (Borgia 1998). This presentation of exaggerated features is known
as an epigamic (epi = toward, gam = spouse) display (or sometimes as lek behavior) and is the result
of sexual selection. Sexual selection is the selective pressure exerted on one sex by the other, often
operating on males by females, in order to be successful in reproduction. The resulting exaggerated
features may hinder the operation of that organism under ordinary circumstances. Nevertheless,
preference by females may favor more and more exaggerated phenotypes until the selective
advantage of being attractive to the female is outweighed by its physical encumbrance (e.g.,
interference with flight); that is, until sexual benefits are balanced by utilitarian ones arriving at a
compromise of sorts (Glesener, unpublished #2).
Betta Breeding Behavior
The Siamese Fighting Fish, Betta splendens (Phylum Chordata, Class Osteichthythes, Order
Labyrinths, Family Anabantidae) is an ideal organism to study the above topics in the laboratory.
First, they are clearly sexually dimorphic, as will be demonstrated in this exercise. Second, their
reproductive behavior incorporates many of the above elements. Third, they are easy to keep and
manipulate in small maneuverable tanks. The latter is the result of their unusual ability to breathe
air. Although store-bought Bettas (which has replaced fighting fish as their common name) are bred
in captivity, their native habitats in SE Asia are stagnant pools, swamps and marshes whose low
oxygen content have selected for the ability to gulp air. The fish come to the surface and swallow
air, which rises above their gills into a highly vascularized, branched chamber, the labyrinth organ,
where gas exchange takes place. If prevented from surfacing, the fish allegedly may suffocate even
in more oxygenated waters.
Copyright © Dr. Robert R. Glesener
Of course, the swallowed air must be "exhaled". When doing so, the male coats it with mucous
from his pharynx, forming bubbles of spit which float on the surface. This habit has become
incorporated into the fishes’ breeding behavior. Aggregations of bubbles may actually rise out of the
water into the atmosphere forming a bubble nest. During the breeding season (monsoon season,
usually July or August in nature but much extended in domestic varieties), the male's interest is
focused on this bubble nest. The nest itself requires continuous maintenance. He will exclude all
competitors and potential egg predators from the column of water beneath the nest to the bottom
several feet below. Thus, he can be sure that at least in the vicinity of his "territory", he is the only
male available to fertilize any eggs that become available.
When a gravid female (one ripe with eggs) approaches, he attempts to position her below the nest
where gentle pressure from their entangled bodies squeezes her eggs out. Being heavier than water,
the eggs fall to the bottom. He temporarily leaves her to retrieve the eggs. First he fertilizes them,
and then, using his mouth, he transfers them to the bubble nest where the surface tension of the
bubbles will hold them away from aquatic predators. Meanwhile, she remains “mesmerized” until he
returns and repeats the process. With diminishing returns, the male eventually chases the female
away. If she has eggs left or produces more, she may mate with another male in another territory.
The male remains with the bubble nest and its eggs (anywhere from 20-200), maintaining it and
driving away potential egg predators until the young hatch and swim away as fry. Such parental care
by the male is unusual in nature. The Betta male is apparently "willing" to provide paternal care, due
to the increased certainty of paternity associated with this courtship procedure. Since courtship and
fertilization took place in the immediate vicinity of the nest, an area he actively defends, the male
Betta can be relatively certain that his will be the only sperm having access to those eggs.
Aggressive Behavior
While aggressive behavior (and color intensity) of this fish has been amplified by a hundred
years of selective breeding, the tendency toward sexual dimorphism in size, aggressive behavior,
exaggerated fin length, and color are natural. Also, the general pattern of movement and postures
used to communicate intentions remains that of a typical male fish. One such aspect of its behavior
common to most animals is the presence of two levels of aggression: (1) the exchange of displays, as
opposed to (2) physical violence. In most animals, the majority of "fights" is settled by exchanges of
signals called displays. Displays are evolved communication signals between individuals of a
species; a behavior is said to be a display if it alters the probability of behavior by another individual.
Usually a territorial encounter is settled with the retreat of one individual before any physical
violence is necessary. Neither does the victorious individual pursue, to do so would be to risk injury
while also abandoning one’s territory and nest. The tendency of fighting fish to fight in captivity is
largely due to their inability to retreat when confined in small enclosures. Also, their aggression is
primarily directed to males of their own species (and other closely related species, e.g., gouramis,
paradise fish, etc.). In a community tank where there is no more than one male Anabantid (family
Anabantidae) present, Bettas rarely fight. In fact, their long draping fins are apparently tempting and
are nibbled on by other fish species.
Copyright © Dr. Robert R. Glesener
Study Purpose
In this exercise we will be examining aggressive behavior in the male Siamese Fighting Fish,
Betta splendens. You will observe interactions between males and their own image, males and
males, and males and females. You will attempt to determine if there is a correlation between male
dominance hierarchies and female preference. From these observations, you may also be able to
make conclusions regarding the type, intensity, or context of stimuli capable of invoking responses in
these fish.
Since the male will be defending her eggs along with his territory, it is in the female’s interest to
choose a male with a demonstrated ability to hold a territory (and build a suitable bubble nest).
Accordingly, our working hypothesis for his lab will be that the female will choose the most
dominant male. To test this hypothesis, it will be necessary for you to devise ways to independently
measure (1) a male dominance hierarchy and (2) female preference.
If time permits, we might also ask the question: if a male Betta with an established territory was
simultaneously confronted with a challenging male intruder and a potential female mate, which
should he confront first? Should he drive the contender away before turning his attention to
acquiring her eggs or should he court her before she turns her attention elsewhere? According the
above explanation for the evolution of paternal care in this species, the certainty of paternity is a
critical factor in shaping male behavior. If he courts the female in another male's presence, he risks
the possibility that the contender may fertilize some of the eggs. By driving him off, he ensures that
any eggs she does provide (assuming she stays in the vicinity), will be fertilized by his sperm.
STUDY METHODOLOGY
You will be designing experiments to examine aggressive and/or courtship behavior in male
Siamese Fighting Fish. The actual course of events is up to those in your group, but think it out first;
a well-designed experiment is easier to write up. For female preference, you should use procedures
#3 and #5 (and #7, if available) below. For male dominance hierarchies, I recommend using
procedures #1, #2, and #4 in that order (and #6 if available). What follows are some of the
procedures you may choose to follow. You are not expected to do all of them; only those that are
appropriate for your experimental design.
Display Characteristics:

Displays involve primarily body orientation and extension of the gill covers (opercula) and fins
(Figure 1).

Watch each fin individually to see what it is doing.

Pay close attention to angles of approach:
 "broadside" displays involve primarily the unpaired dorsal, caudal and anal fins, making the
fish look much bigger along the midline
Copyright © Dr. Robert R. Glesener
 "frontal" displays utilize the gills covers, the extended red gills (beard-like in appearance),
and paired pectoral fins, making the fish look much stouter from the front.

See if you can detect any difference in frequency of these two approaches when displaying to
another male versus a female.

Note when each fish first notices the other and how long the various displays continue (pay
particular attention to the length of time each male extends his operculum and gills).
Figure 1.
Diagram of lateral view of a Betta, with positions of
gill and fins indicated.
Procedures:
1. Reflection in Mirror and Measurement of Size.
|| | ||
To give you some idea of what to expect, place a mirror along the broad side of the fish bowls.
Begin with the males, but also observe females for comparison. Try to judge from the degree of
activity, color enhancement, length of operculum extension, etc., the expected rank order of the
males in a "dominance" hierarchy. Actually, this is probably the least reliable method of estimating
dominance, but it is interesting since the mirrored image is the perfect match for each fish, movement
for movement; it usually produces the most sustained response. This will also give you a good
chance to measure your fish. Place the calibrated transparency between the mirror and the bowl.
Wait until the fish gives a broad side view and attempt to obtain measurements of body and fin
length as indicated in Figure 2. Also attempt to measure the fish's girth or width when it faces the
mirror. Record these measurements for all fish, males and females, and enter in Table 1.
Copyright © Dr. Robert R. Glesener
Figure 2. Lateral and anterior views of male Betta, with needed
measurements as indicated.
2. Male Bettas: Separate Tanks. ||||
The first interaction is between two male Bettas. Place the broad sides of two fishbowls, each
containing a single male, adjacent to one another. Do this for all pairwise combinations. Record the
results of both males. Does either one back down. Which displays more rapidly? more
impressively? Pay particular attention to the length of time the operculum is extended.
3. Male(s)/Female: Separate Tanks.
||||||
Line up three bowls in a row, with a male on either side of a single female. Observe from the
side. Using a stopwatch, record in which half of the bowl she spends more time or preferably toward
which male she orients. This might indicate the male in which she shows more interest. How does
this indication of "preference" correlate with earlier predictions of dominance? Do this with all
pairwise combinations of males for both females. Leaving the three tanks arranged as above, try
placing mirrors on the "open" ends of both of the males' tanks. The male must now choose whether to
display to a competing male (his own image) or to the female. Record what percent of the time his
Copyright © Dr. Robert R. Glesener
attention is directed to each. You need not do this for all combinations: simply use your most
"receptive" female and two most responsive males.
4. Male Bettas: Same Neutral Tank.
||
Using the small nets, gently transfer the males (two at a time) from their small bowls into the
larger tank. Use the large net to partition the "arena." Care should be taken so as not to damage or
traumatize the fish. After both males have been placed in a separate partition, remove large net. Be
prepared to separate them immediately if they lunge at one another or appear to be on the verge of
physical contact. If one fish consistently pursues while the other retreats, the former is dominant
(physical attack is not a reliable measure of dominance; often it is a frustrated subordinate that strikes
"the first blow” - don't let it go that far.) Is the "dominant" one the same as the one that displayed
most aggressively when alone?
5. Male(s)/Female: Same Tank.
|| and ||
Place a male and female into the large tank and carefully observe the male's action. Specifically,
note his speed of approach and method of display in comparison to a male-male confrontation as
observed in 4. You should note that he primarily uses side displays instead of the more threatening
frontal display. Once again, pay particular attention to the length of time the operculum is extended.
What reasons can you think of to account for the differences in a male-male and a male-female
display? Also note the female's response. How does this relate to the male's actions? Remove this
male and observe male-female interactions with the other male. Is their any difference? Now put
both males in together with the female. Do they seem more concerned with the presence of the other
male or the female?
6. (Optional) Male/Male: Same Tank(s), "Homefield."
||
At first thought, placing competing males in a neutral arena seems the appropriate way to
guarantee fairness. However, in nature we would not expect a male to defend a neutral territory, one
with which that he is unfamiliar. His territory is to some extent defined by the dimensions of the
bubble nest he builds and whose appearance and oder might be expected to motivate him. In
captivity, the water itself is conditioned by his secretions and moving him abrubtly to a tank with
clean water might only serve to make him indifferent. Unfortunately, the small fish bowls which are
convenient for fish care and some of the earlier treatments do not lend themselves for testing home
field advantage. If enough arena sized tanks are available in your lab, each male should be allowed
to condition one. In determing male dominance, each pair of males should be tested twice, once in
each's "home field." Does the outcome depend on the location of the encounter? Is there a "home
field advantage"? In addition to determining rank order, compare individual performances as
defender versus intruder. Defenders would seem to have more at risk. Do they behave accordingly?
Copyright © Dr. Robert R. Glesener
7. (Optional) Male/Female, Same Tank, "Homefield."
||
In the same way that "homefield" might alter a male dominance hierarchy, it might also be
expected to effect female preference. The above methods of determining female preference (#3 and
#5) assumed that females respond primarily to male behavior and/or secondary sexual characteristics
as indicators of his ability to defend her eggs. Although it has never been demonstrated, it might be
expected that she would also take notice of the size and quality of his bubble nest. The most
aggressive defender of a territory would still be a poor mate selection if he failed to build or maintain
a bubble nest. Yet, from a separate bowl, females can not inspect a male's nest, and the nests do not
transfer to neutral arenas. If enough arena size tanks are available, females can be carefully
introduced into each males territory in such a way as to not disturb the existing bubble nest. The
presence of a female in his vicinity of his territory may also effect the way he displays to her.
Caution:
Appropriate behavior is performed on cue from environmental stimuli. A stimulus is a perceived
piece of information. The sum of all stimuli represents an organism's picture of the world. In
ethology (the comparative study of animal behavior), the German word "umwelt" is used to refer to
this world as perceived by an organism. The umwelt of each species is different to some degree,
since the sensory apparatus of each species is different.
During this experiment, be aware that the "umwelt" of these fish will include you, the noises and
movements you make, and the vibrations you impart to the table. They will not behave socially if
frightened. Fish behave differently in their individual bowls than when placed in the same tank with
another fish; in their separate bowls they are not only restricted in their maneuvers, but can not smell
or "feel" (by detecting vibrations associated with movement in water) the other fish. Remember that
these fish are complicated living beings. To some extent their behavior is predictable in a broad
statistical sense, but do not expect them to "perform" repeatedly as robots.
ACKNOWLEDGEMENTS
This exercise was adapted and extensively modified from the Biology 112 & 114 Laboratory
Manual, 2nd Edition, produced by Members of the Division of Biological Sciences, University of
Michigan. Figure 1 was transcribed from the same reference with minor re-labeling.
LITERATURE CITED AND OTHER REFERENCES
Alcock, John. 1989. Animal behavior : an evolutionary approach. Sinauer Associates, Sunderland,
MA. Brevard College Call Number: 591.51 A354a.
Benn, John E. 1990. How to succeed at breeding Bettas. Aquarium Fish Magazine 2(3): 40-45.
Copyright © Dr. Robert R. Glesener
Borgia, Gerald. 1998. Why do bowerbirds build bowers? Pp. 167-172 In Paul W. Sherman and
John Alcock (eds.). Exploring Animal Behavior: Readings from American Scientist (2 nd
Edition). Sinauer Associates, Inc., Sunderland MA. Brevard College (Moore Science) Call
Number: 591.5 E96.
Glesener, Robert R. First Came Sex, Then the Sexs: Speculations on the Origin of Heterogamy.
Unpublished memo.
Glesener, Robert R. Sexual Selection and Sexual Dimorphism. Unpublished memo.
Innes, William T. 1966. Exotic Aquarium Fishes. Metaframe Corporation, Maywood, New Jersey.
pp. 404-407.
Kelly, Darcy B. and Dennis L. Gorlick. 1990. Sexual selection and the nervous system. Bioscience
40(4): 275-283.
Orians, Gordon H. 1980. Some Adaptations of Marsh-nesting Blackbirds. Monographs in
Population Biology 14, Princeton University Press, Princeton NJ. Brevard College (Moore
Science) Call Number: 598.881 O69s.
Swift, E.M. 1988. Sport in the land of Sanuk. Sports Illustrated 68(15 February): 130-132.
Wolfsheimer, Gene. Enjoy the Fighting Fish from Siam. The Pet Library, LTD., N.Y.
Copyright © Dr. Robert R. Glesener
Exercise — Betta Behavior
Length in millimeters of male and female Betta bodies and fins. List the appropriate measurement,
and identify each fish studied. Be sure to indicate its sex. Codes correspond to measurements
indicated on Figure 2.
Code
Measurements
A
Body length from tip of snout to base of tail
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
B
Body height at highest point
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
C
Body height at base of tail
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
D
Longest axis of dorsal fin
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
E
Longest axis of caudal fin
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
F
Longest axis of anal fin
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
G
Longest axis of pectoral fin
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
H
Longest axis of pelvic fin
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
I
Girth (widest point side to side not including expanded gill cover)
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
J
Frontal Width (same as I but including expanded gill cover)
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
K.
Sex.
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
L.
Length of Time Operculum Extended (with Gills Flaring).
(1)__________ (2)__________ (3)__________ (4)__________ (5)__________
Copyright © Dr. Robert R. Glesener