As an example of field observation in evidence of theories of sexual selection, S.J. Arnold and M.J. Wade presented the following data set on size and number of mates observed in 38 bullfrogs.
A data frame with 38 observations on the following 2 variables.
Body size (in mm)
Number of mates
Ramsey, F.L. and Schafer, D.W. (2013). The Statistical Sleuth: A Course in Methods of Data Analysis (3rd ed), Cengage Learning.
Arnold, S.J. and Wade, M.J. (1984). On the Measurement of Natural and Sexual Selection: Aplications, Evolution 38: 720–734.