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Featured scientists: Raisa Hernández-Pacheco from University of Richmond and Damián A. Concepción Pérez from Wilder Middle School

Background:

It is easy to identify a deer as male when you see his huge antlers, or a peacock as male by his stunning set of colorful tail feathers. But you may wonder, how do these traits come about, and why don’t both males and females have them? These extravagant traits are thought to be the result of sexual selection. This process happens when females mate with males that they think have the sexiest traits. These traits get passed on to future male offspring, leading to a change in the selected traits over time. Because females are only choosing these traits in males, sexual selection often leads to sexual dimorphism between males and females. This means that the sexes do not look the same. Often males will be larger and have more elaborate traits than females.

One species that shows strong sexual dimorphism is rhesus macaques. In this species of monkey, males are much larger than females. Cayo Santiago is a small island off the shore of Puerto Rico.

On this island lives one of the oldest free-ranging rhesus macaque colonies in the world. This population has no predators and food is plentiful. Scientists at Cayo Santiago have gathered data on these monkeys and their habitat for over 70 years. Every year when new monkeys are born they are captured, marked with a unique tattoo ID, and released. This program allows scientists to monitor individual monkeys over their entire lives and record the sex, date of birth, and date of death. Once a monkey dies and its body is recovered in the field, skeletal specimens are stored in a museum for further research.

These skeletal specimens can be used by scientists today to ask new and exciting questions. Raisa and Damián are both interested in studying sexual dimorphism in rhesus macaques. They want to find out what causaes the differences between the sexes. They chose to focus on the length of the very large canine teeth in male and female macaques. They expected that canine teeth may be under sexual selection in males for two reasons. First, rhesus macaques are mostly vegetarians, so they don’t need long canines for the same purpose as other meat eating species that use them to catch prey. Second, male rhesus macaques often bare their teeth at other males when they are competing for mates. Females could see the long canines as a sign of good genes and may prefer to mate with that trait. Excited by these ideas, Raisa and Damián set out to investigate the museum’s skeletal specimens to check whether there is sexual dimorphism in canine length. This is the first step in collecting evidence to see whether male canines are under sexual selection by females. 

They measured canine length of four male and four female rhesus skeletal specimens dating back to the 1970s. Measurements were only taken from individuals that died as adults to make sure canines were fully developed and that differences in length could not be attributed to age. Raisa and Damián predicted that males would have significantly longer canines compared to those of females. If so, this would be the first step to determine whether sexual selection was operating in the population.

Scientific Question:  Is there evidence for sexual dimorphism in canine length in Cayo Santiago rhesus macaques?

Scientific Data:

1. What is the hypothesis?  Find the hypothesis in the Research Background and copy/paste it below. A hypothesis is a proposed explanation for an observation, which can then be tested with experimentation or other types of studies.

2. Click on the graph tab, and explore the available variables. 

   What data will you graph to explore the first hypothesis?

   Independent variable:
   Dependent variable:

3. Create the graph of your data, and paste your graph below:

4. To find the mean of the data, add descriptive stats, select SEM, and paste it below:

5. Identify any changes, trends, or differences you see in your graph. Write one sentence describing what you see.

Interpret the Data:

6. Make a claim that answers the scientific question.

7. What evidence was used to write your claim? Reference specific parts of the tables or graph.

8. Explain your reasoning and why the evidence supports your claim. Connect the data back to what you learned about the role canine teeth play for rhesus macaques.

9. Did the data support Raisa and Damián’s hypothesis? Use evidence to explain why or why not. If you feel the data were inconclusive, explain why.

Your next steps as a scientist:

10. Science is an ongoing process. What new question(s) should be investigated to build on Raisa and Damián’s research? What future data should be collected to answer your question(s)?

These questions are a digital extension of the original Data Nuggets activity. The data manipulation and graphing tasks within are best completed here on DataClassroom.

Although we observed that the mean canine length of males and females is different, the value of the mean may change depending on the number of individuals sampled, the degree of error in measurement or the range of variation in canine length found among the two sexes. Thus, scientists rely on conducting statistical tests of significance to assess whether the measured difference of canine teeth between the sexes, is an accurate representation of sex differences in tooth length in this species. Below we will go through some questions exploring statistical analysis of this dataset. 

11. Consider a hypothetical scenario in which Raisa studied a different population of macaques and measured the average canine length of 6 males and 6 females. They found that the average canine length of males was 2.20 cm and SD was 0.30  whereas females had an average canine length of 1.50 cm with an SD of 0.21. Explain at least one possible reason why their estimates were different from those on Cayo Santiago.

12. Would you say that this population also demonstrates sexual dimorphism in canine length? Justify why or why not?

13. Typically, scientists make use of statistical analysis to determine that any difference between two values is reflective of natural phenomena regardless of the population being surveyed. Go to the Graph Driven Tests located at the upper right corner of your screen and list 2-3 reasons why a t-test is most appropriate to test if canine tooth length is sexually dimorphic.

14) What is your conclusion? How would you interpret the P - value obtained with this dataset? Paste results from the analysis here.

15) If we compare canine length in males and females across ages, we find the following pattern. Based on this graph, can you say that mean canine length changes as individuals grow older? Explain why or why not.

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This dataset and content is provided our by our friends at Data Nuggets.

Visit DataNuggets.org to see the original activity and additional materials