Teacher Copy, Level 2
Name_________________
Beetle Battles
Featured scientist: Erin McCullough from the University of Western Australia
Research Background:
Male animals spend a lot of time and
energy trying to attract females. In some
species, males directly fight with other
males to become socially dominant. They
also fight to take over and control
important territories. This process is known
as male-male competition. The large
antlers of male elk are an example of a
trait that has been favored by male-male
competition. In other species, males try to
court females directly. This process is
known as female choice. The flashy tails
of male peacocks are a good example of a
trait that has been favored by female
choice. Lastly, in some species, both malemale competition AND female choice
Erin has always loved beetles! Here she is with a
determine which males get to mate. In
dung beetle in Tanzania, during a graduate school
order to be successful, males have to be
class trip.
good at both fighting other males and
making themselves attractive to females.
Erin is a biologist interested in these different types of mating systems. She wondered if she
could discover a single trait that was favored by both male-male competition and female choice.
In horned dung beetles, male-male competition and female choice are both important in
determining which males get to mate. Females dig tunnels underneath fresh piles of dung where
they mate and lay their eggs. Beetles only mate inside these underground tunnels, so males
fight with other males to become the owner of a tunnel. Males that control the tunnels have a
better chance to mate with the female that dug it. However, there is often more than one male
inside a breeding tunnel. Small males will sneak inside a main tunnel by digging a connecting
side tunnel. Additionally, the constant fights between large males means that the ownership of
tunnels is constantly changing. As a result, females meet many different males inside their
tunnels. It is up to them to choose the male they find the most attractive, and with whom they’ll
mate. In this species of dung beetle, males try to persuade females to mate by quickly tapping
on the females’ back with their forelegs and antennae. Previous research has found that
females are more likely to mate with males that perform this courtship tapping at a fast rate.
Because both fighting and courtship tapping take a lot of strength, Erin wondered if the trait of
strength was what she was looking for. Would stronger male dung beetles be favored by both
male-male competition and female choice?
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Teacher Copy, Level 2
Name_________________
To test her hypothesis, Erin
conducted a series of
experiments to measure the
mating success, fighting
success, and strength of male
dung beetles. First, Erin
measured the mating success
of male beetles by placing one
male and one female in an
artificial tunnel (a piece of clear
plastic tubing). She watched
the pair for one hour, and
measured how quickly the
males courted, and whether or
not the pair mated. Second,
Erin measured the fighting
Two dung beetle males fighting for ownership of the artificial tunnel.
success of males by staging
Why is the photo pink? Because beetles mate and fight in dark,
fights between two males over
underground tunnels, Erin carried out all of her experiments in a
dark room under dim red-filtered light. Beetles can’t see the color
ownership of an artificial
red, so working under red-filtered light didn’t affect the beetles’
tunnel. Beetle battles consist of
behavior, and allowed Erin to see what the beetles were doing.
a head-to-head pushing match
that results in one male getting
pushed out of the tunnel, and the other male remaining inside. To analyze the outcome of these
fights, Erin randomly selected one male in each pair as the focal male, and scored the
interaction as a “win” if the focal male remained inside the tunnel, and as a “loss” if the focal
male got pushed out of the tunnel. In some cases, there was not a clear winner and loser
because either both males left the tunnel, or both males remained inside. These interactions
were scored as a “tie”. Finally, Erin determined each beetles’ strength. She measured strength
as the amount of force it took to pull a male out of an artificial tunnel. To do this, she superglued a piece of string to the back of the beetle, had it crawl into an artificial tunnel, attached the
string to a spring scale, and then pulled on the scale until the beetle was pulled out of the tunnel.
To keep beetles alive in the lab, Erin set up a bucket with sand, and placed one pile of dung in the
center (left). Female beetles dug tunnels below the dung (right).
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Check for Understanding: After reading the introduction, students should be able to:
• Describe the two methods (male-male competition and female choice) that male animals
use to try and get mates. How are the two methods different, and how are they alike?
• Explain how traits such as a peacock’s tail, or a dung beetle’s strength could change
over time due to male-male competition or female choice.
• Describe why Erin thought that strength might be a trait favored by male-male
competition and female choice. How might strength affect the outcome of fighting for
tunnel territories and attracting females?
• In their own words, describe the three experiments that Erin ran to test her hypothesis.
What were the three variables measured in these experiments? Mating success, fighting
success, and strength.
Scientific Question: How does strength affect a male horned dung beetles’ chances of
winning fights or being chosen as a mate?
What is the hypothesis? Find the hypothesis in the Research Background and underline
it. A hypothesis is a proposed explanation for an observation, which can then be tested
with experimentation or other types of studies.
Scientific Data:
Use the data from the two tables below to answer the scientific question:
Table 1. Strength measurements in millinewtons (mN) for
males that did and did not mate with a female.
Mated
Not mated
Strength (mN)
94.9 ± 7.3 113.1 ± 8.7
(mean ± standard error)
Table 2. Difference in strength (measured in millinewtons [mN]) between a focal male
and his opponent for trials where the focal male won, tied, and lost male-male fights. (A
positive value means that the focal male was stronger than his opponent, while a
negative value means that the focal male was weaker than his opponent.)
Win
Tie
Loss
Difference in strength (mN)
27.9 ± 11.6 -8.0 ± 17.4 -28.7 ± 12.4
(mean ± standard error)
* Standard error (SE) tells us how confident we are in our estimate of the mean, and
depends on the number of replicates in an experiment and how much variation is in the
data. A large SE means we are not very confident, while a small SE means we are
more confident.
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Check for Understanding: After reading about how the data was calculated and presented,
students should be able to:
• Identify whether high or low values indicate greater strength in Table 1.
• Explain what the positive and negative values in Table 2 tell us. What would a value of
“0” mean? How can we use this information to understand how strength affects the
outcome of fights?
• Summarize how the interpretation of the values in Table 1 is different than in Table 2.
Understand that the measure of strength in Table 1 is for a single male, while the
measure of strength in Table 2 is the difference between the 2 male opponents.
• Describe what you would predict the values to look like if Erin’s hypothesis is true?
Teacher Note: You can have students add error bars to their graphs to deepen this
discussion or remove SE from the table for younger students. Error bars based on standard
deviation (SD) give us information on the amount of variation in the data. They can be used
to draw attention to large or small amounts of variation around the mean. Standard error (SE)
is the SD divided by the square root of the study’s sample size (SE=SD/√n). Unlike SD, SE
reflects the uncertainty in our estimate of the mean. The larger our sample size and the less
variation in the data, the more confident we can be in our estimate of the mean.
Upper error bars are calculated by adding one SE to the mean, and lower bars are
calculated by subtracting one SE from the mean. In the data there is a lot of variation,
signified by large error bars. Relative to the signal, the noise is strong. Graph 1: The SE bars
do not overlap, but Erin found no statistical difference in strength between males that mated
and those that did not. Students will not be able to conclude this from the data directly, but
you can have a class discussion about statistical versus biological significance. Do students
think a difference of only 18.2 mN is something that female beetles could tell about a male?
Would that difference affect the quick back taps males use to attract females? There is not
just one answer to this question, but a class discussion might help students decide if this is
an important difference in light of the biology of the system. Graph 2: The difference in
strength was statistically different between males that won battles and those that lost. The
males that tied were not statistically different from either the winning or losing males. This
means that the difference in strength between a focal male and his opponent did partially
determine which male would win in a beetle battle.
What data will you graph from Table 1 to answer the question?
Independent variable: Average strength (mN)
Dependent variable: Mating success (mated or not mated)
What data will you graph from Table 2 to answer the question?
Independent variable: Difference in strength (mN)
Dependent variable: Fighting success (win, tie, or loss)
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Teacher Copy, Level 2
Draw your graphs below:
Graph 1:
Strength (mN)
140
120
100
80
60
40
20
0
Mated
Unmated
Mating trial outcome
Difference in Strength
Graph 2:
50
40
30
20
10
0
-10
-20
-30
-40
-50
Win
Tie
Loss
Fighting trial outcome
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Interpret the data:
Make a claim that answers the scientific question.
Strength had no effect on a male’s chances of mating. In
contrast, strength had a big effect on a male’s chance of
winning fights. If a male was stronger than his opponent, he was
more likely to win the fight, and if the male was weaker than
his opponent, he was more likely to lose the fight.
Teacher Note: It is ok if students conclude that the strength of males who mated with
females is different than the strength of males who did not mate. Because students do not
have access to the statistics, it is up to them to determine if they feel there is a large enough
difference between the bars in Graph 1 to be important biologically.
What evidence was used to write your claim? Reference specific parts of the tables or
graphs.
Strength was very similar for males that did and did not mate.
Males that mated had an average strength of 94.9 mN, while males
that did not mate had an average strength of 113.1 mN. There is
a lot of variation in these data (the standard error bars are
almost overlapping), and this difference is not statistically
significant.
The difference in strength between a male and his opponent was
not the same among males that won, tied, or lost fights. Males
that won fights had strengths that were on average 27.9 mN
higher than their opponents. Males that lost fights, on the
other hand, had strengths that were on average 28.7 mN lower
than their opponents. Finally, males that tied fights had very
similar strengths to their opponents. The average difference in
strength among males that tied was only 8 mN.
Explain your reasoning and why the evidence supports your claim. Connect the data
back to what you learned about the different strategies used by male dung beetles to
attract females and fight off males.
Females do not prefer to mate with strong males. There was no
difference in strength between males that mated versus males
that didn’t mate. We know that female dung beetles are attracted
to males when they tap their backs, and maybe strength does not
come into play in this trait. Maybe it is another aspect of
physical performance, such as endurance, that females would find
more attractive. The results from this experiment just looked at
strength, and suggest that strength does not affect a male’s
likelihood of being chosen as a mate.
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On the other hand, strength did affect a male’s likelihood of
winning fights with other males. In other words, males that won
fights were able to resist more force than their opponents
before being pulled out their tunnel. Males that were stronger
than their opponents were more likely to win fights, and males
that were weaker than their opponents were more likely to lose
fights. When a male fights another male that is equally strong,
neither male won the fight, and the interaction ended in a tie.
What does the data from this study tell us about the scientist’s hypothesis?
The data do not fully support the hypothesis that strength is
favored by both male-male competition and female choice.
Strength appears to be favored by male-male competition, because
strong males are more likely to win fights. But strength does
not appear to be favored by female choice, because strong males
are not chosen more often as mates.
Your next steps as a scientist: Science is an ongoing process. What new question do
you think should be investigated? What future data should be collected to answer this
question?
See Teacher Note below.
Teacher Note: In this study, Erin used strength as a measure of the beetles’ physical
performance. This measure of performance is probably very biologically important, because
males that are stronger and can resist more force should be better at defending their
breeding tunnels. However, strength is only one measure of physical performance. In
addition to being strong, it may also be beneficial for beetles to be fast, or to have high
endurance. Erin is now interested in looking at how other measures of physical performance
(like speed, agility, or endurance) may also affect a male’s likelihood of winning fights and
being chosen as a mate. Students can discuss how they would design an experiment to
measure speed or endurance in the dung beetles.
Students will probably generate a wide diversity of questions for in this system. You can
have a class discussion where you jot down all the questions up on the board. Be prepared
to ask your students to clarify or justify another student's response in a class discussion. Do
students see any ways to improve each other's questions? Are some questions untestable?
Remember, if your class wants to send their questions about the study system to Erin, the
scientist studying dung beetles, they can email them to [email protected]!
Additional teacher resources related to this Data Nugget:
A scientific paper published on this research:
McCullough, E.L. and L.W. Simmons (2016) Selection on male physical performance
during male–male competition and female choice. Behavioral Ecology
• http://datanuggets.org/wp-content/uploads/2016/08/McCullough-and-Simmons2016.pdf
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