Sexual Selection in Soldier Beetles
Sexual Selection
• Characterized by…
– A difference in the reproductive success of
individuals of the same sex
• Hypothesized to influence…
– Development of ornamental/showy traits
– These traits may reduce an individual’s survival,
but increase their chance of reproducing
Hypotheses for Sexual Selection
• Male-Male Competition:
Hypotheses for Sexual Selection
• Female Choice:
– Good Genes Hypothesis
– Runaway Selection
RUNAWAY SEXUAL
SELECTION
RUNAWAY SEXUAL
SELECTION
RUNAWAY SEXUAL
SELECTION
RUNAWAY SEXUAL
SELECTION
RUNAWAY SEXUAL
SELECTION
Today’s Agenda
• Look for evidence of sexual selection on body
size in soldier beetles
– Measure body length of mated and unmated
males and females
Soldier Beetles
• Males vs. Females
– Size
– Abdomen striations
Hypotheses
• Sexual selection on males:
– HO: There is no difference between the sizes of
mated and unmated males
– HA: Mated males are larger than unmated males
Hypotheses
• Sexual selection on females:
– HO: There is no difference between the sizes of
mated and unmated females
– HA: Mated females are larger than unmated females
Hypotheses
• Relationship between male and female size:
– HO: There is no relationship between the sizes
of mated male and female beetles
– HA: Larger females mate with larger males and
smaller females mate with smaller males
Methods
• Collect mating and nonmating pairs
– NOTE: keep each mated pair in an individual vial
– NOTE: Put all nonmating males in one vial and all
nonmating females in a second vial
• Measure body length using calipers
• Write down observations of beetle behavior
• Release the beetles at the end of lab
– NOTE: keep the beetles out of the sun while you are
collecting/measuring other individuals
Data Analysis
• Pick one hypothesis about sexual selection
• Compare the average size of mating and
nonmating individuals (either male or female)
• Evaluate the relationship between male and
female size
Data Analysis
• Comparing body size:
– Calculate the mean body size for paired and
unpaired individuals
– Calculate confidence intervals for each mean
– If the confidence intervals do not overlap, then the
means are significantly different from each other
at p=0.05
– Create a bar graph showing the mean body size of
paired and unpaired individuals
Data Analysis
• Comparing body size:
Data Analysis
• Comparing male and female sizes:
– Create a scatter plot showing the relationship
between paired male and female body sizes
– Use ALL of the data points
– Female size should be on the x-axis
– Male size should be on the y-axis
Data Analysis
• Comparing male and female sizes:
Lab Report
• Clearly state your hypothesis
• Write a 1-page mini-discussion about whether
your results support your hypothesis
• Cite at least 1 scientific article
• Include the appropriate figures