We are accustomed to thinking that the proportion of males at birth is fixed by genetics in birds and mammals to be close to 50%. Some have suggested, however, that the sex of offspring can be adjusted by females, such as in response to the quality of her mate or the number of helpers she has. West and Sheldon (2002) found a total of 15 studies, all done on birds, that have measured changes in the sex ratio in response to such social factors, and each has expressed its results in terms of a coefficient ranging from −1 to 1. The coefficient is positive if the change in the sex ratio of offspring is in agreement with evolutionary theory, and negative if the data disagree with the theory. A coefficient of zero indicates no association with social factors in the data. The measures are as follows:

The frequency distribution of the coefficients is shown in the following histogram.

a. What method could be used to test whether these data are consistent with a mean or median coefficient of zero? Discuss why you would use this method in contrast to specific other methods.

b. Apply the best method to these data to test whether the mean coefficient differs from zero.

Transcribed Image Text:

-0.160,-0.160,-0.037,-0.037, 0.034,0.034, 0.144,0.144, 0.137,0.137, 0.118,0.118, 0.395,0.395, 0.363.0.363, 0.350,0.350, 0.376,0.376, 0.253, 0.253, 0.440,0.440, 0.453,0.453, 0.460,0.460, 0.563.0.563.