Sexual Selection and Mate Choice

Concepts Instructor Prep Student Protocol

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Herbert Spencer first coined the phrase survival of the fittest in 1864, to describe Darwin's observations of natural selection. One way to quantify the fitness of an individual is in the number of offspring that it can contribute to the next generation - so an organism can improve its fitness by producing more offspring that are then able to successfully reproduce themselves. Reproduction is a critical element of fitness, and sexually-reproducing organisms must mate in order to pass their genes on to the next generation. This means that for sexually-reproducing organisms, competing for mates becomes a key part of fitness.

This pressure to find a mate leads to a phenomenon known as sexual selection - a type of selection that acts upon traits associated with mating and courtship. Typically, sexual selection is more intense for the sex that invests the least amount of energy in reproduction. Since sperm is energetically inexpensive to produce, males generally bear the brunt of sexual selection pressures. Conversely, eggs are energetically expensive to produce and then gestate. This means that females are expected to be more selective when choosing mates, and males must invest substantial resources in attracting females. This observation is related to the operational sex ratio, or the OSR - which is the ratio of sexually-mature males to fertile females. Often the OSR is male-biased, due to their longer reproductive lifespans.

Due to the often-intense sexual selection pressures males face, many species of animals display extravagant ornamentation that distinguishes males from females. When males and females of the same species appear phenotypically different, we call this sexual dimorphism. Sexual selection, however, isn't limited to only male ornamentation. There are two types of sexual selection, intersexual and intrasexual. In intersexual selection, one sex, typically the males, will display a certain trait or behavior with the goal of attracting and mating with the opposite sex. Intrasexual selection, on the other hand, occurs between members of the same sex. For example, male sea lions compete for dominance over rookeries of females. Here, intrasexual selection acts on the physiology of the sea lion, to make it large enough to compete with other males for a group of females. But with attributes like vibrant colors and larger size come costs. Like increased visibility to predators, or decreased immunity due to the overproduction of androgens. This means that natural selection and sexual selection are often at odds with each other, pulling traits in opposite directions. Because of this, individuals with exceptionally high- or low-quality scores are rare. Instead, individuals of average quality are expected to make up the majority of a population.

In this lab, you will perform simulations of mating scenarios in which females and males can select their mates based on varying amounts of information on their potential mate's quality.

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Sexual Selection Class Simulation
- Note: In this activity, you and your classmates will portray a sexually reproducing population consisting of an equal number of male and female animals. You will simulate several different mating scenarios to determine how relative quality, attractiveness, sex, and parental commitment affect mate choice and reproductive success. The goal of this activity is to maximize your progeny.
- Hypotheses: There are several overall hypotheses for female and male behavior for this activity. The main experimental hypothesis for female behavior is that females will mate with males of high quality more frequently than those of low quality, yielding a higher reproductive success. The null hypothesis is that females will not demonstrate preference towards high- or low-quality males in any of the four rounds of mating. For males, one experimental hypothesis is that males with a higher visible score will have greater reproductive success. Another could be that males will be more selective for females in scenarios in which they join females to care for offspring as opposed to scenarios in which they exhibit no parental commitment. The null hypothesis for male behavior might be that males will not demonstrate preference towards high- or low-quality females in any of the four rounds of mating.
- Activity Overview: To begin, each student will be assigned a sex as well as a relative quality and attractiveness score. Remember - the goal is to maximize your reproductive success in each round. You do this by mating with partners of the highest possible quality which is rated on a scale of one to three with three being the highest possible score. Attractiveness scores, which follow the previously described scoring paradigm, correlate with quality but do not exactly predict the score. Though useful, they are not a perfect way to choose the best mate. If you are assigned a male ID holder, you will have 16 gametes. However, if you are assigned a female ID holder, you will only have four. A successful mating occurs when one sperm and one egg are paper-clipped together and deposited at the fertilization area.
- Note - Rules of the activity:
  - Assigned male and female identities for these experiments are not representative of students' personal gender identities.
  - Running is not permitted.
  - Talking is not permitted.
  - Hidden values cannot be shared but individuals can attempt to hide their visible scores.
  - Respect the space around each individual and remain an arm's length away.
  - Offspring will only be counted towards an individual score if both gametes are paper-clipped together and dropped off in the designated fertilization area before the end of the two-minute round.
Round 1 – Hidden Quality Score
- To begin the first simulation, take the clip-on ID holder you were randomly assigned and make sure it contains four gametes if you are assigned female or sixteen gametes if male. Males should also have a strip of white paper and two paperclips.
- Next, males should paperclip the white strip of paper over their quality score to hide it, but keep the attractiveness score visible.
- Record your ID number for round one in table two. Click Here to download Table 2
- Next, clip on the ID holder and move to the designated mating area.
- When the instructor says begin, you may commence the mating simulation. You will have exactly two minutes to produce as many offspring as possible. Males may freely give their sperm to females, and females may choose to use these sperm immediately after which they would walk to the fertilization area and paperclip it to one of their eggs, or they may choose to wait until they receive other sperm to do so.
- Once the gametes have been deposited in the fertilization area, females may resume mating.
- When the two minutes have elapsed, all matings must stop and any gametes not in the fertilization area at this time will not be counted.
- The instructor should now tally all of the matings and record the numbers of the individuals who produced each offspring.
- Now, find all the gametes that came with your ID holder and repackage them for the next round.
Round 2 – Hidden Attractiveness Score
- For the second round, obtain a new randomly assigned ID holder and again ensure it contains the proper number of gametes.
- Next, males should use the paperclip and strip of white paper to hide their attractiveness score (instead of the quality score, which was covered in round one).
- Record the ID number of your clip in table two under round two. Note: The mating simulation process for round two will be the same as round one.
- Move to the designated mating area and with for the instructor to signal the start of the round. As before, males may freely give away their sperm while females will choose to use it now or wait for different sperm. When a female decides to fertilize an egg, she will walk to the fertilization area and paperclip the gametes together.
- Proceed with the simulation for two minutes, and at the signal from the instructor all mating activity should stop.
- The instructor should now tally all of the matings from this second round.
- Now, find and return the gametes to your ID holder, and then return the holder to the instructor.
Round 3 – Visible Quality and Attractiveness Scores
- For the third simulation, you should again obtain a new clip-on ID holder and check that it contains the appropriate number of gametes. Record the ID number of your clip in table two.
- For this round, leave both attractiveness and quality scores visible on male IDs. Note: The mating simulation process for round three will be the same as the previous two rounds.
- Move to the mating area and wait for the signal to begin the simulation.
- Once the instructor starts the timer and gives the signal, commence mating. As before, males are allowed to freely give away their sperm to females and females may use the sperm immediately or wait for different sperm. Again, fertilized eggs only count if placed in the fertilization area.
- After the two-minute time limit, the instructor should tally all matings.
- Now, find and return the gametes to your ID holder, and then return the holder to the instructor.
Round 4 – Increased Male Parental Care
- In the final round, males will again leave their attractiveness and quality scores visible.
- Obtain an ID holder and ensure it contains the appropriate number of gametes.
- Clip on the ID holder and move to the mating area. Note: As before, males may freely give away their sperm to females. However, in this round, once a female chooses to mate with a male, both the male and female will walk to the fertilization area to paperclip their gametes together. After placing the paper-clipped gametes on the table, both male and female actors may return to the mating area.
- The instructor should signal the start of the round and begin the timer – at this point participants can begin the simulation.
- Once the two-minute time limit is up, all matings must stop.
- The instructor should tally all mating events.
- Find and return the gametes to your ID holder. Give your ID holder to the instructor and notify them if any gametes or supplies are missing.
Results
- To analyze the data generated during the mating activities, first split up into groups of four.
- Collect the class data from the instructor for ONE of the four rounds of mating. Note: In this activity, there were several types of females and males distinguished by different letters. These letter types correspond to specific combinations of quality and attractiveness scores. In the compiled data, the average number of matings and reproductive success has already been sorted by letter type for you by the instructor.
- Use these data to generate four graphs for the round you were assigned. First plot two graphs for the average number of matings against female and male type.
- Then plot the reproductive success for males and females against male or female type, giving a total of four graphs.
- Once these graphs are complete, each group should present their data to the class, making sure to state the original hypothesis for your assigned round and whether or not the hypothesis was upheld.
- After each group presents, discuss the overall results of the four rounds together.
- Note the affect that hiding the quality score had on the overall quality of mates chosen.
- Note how hiding or revealing the attractiveness score affected the results.
- Discuss the effect of male parental investment on the number and overall quality of mating pairs.