Prairie voles are monogamous and social, whereas their close relative the meadow voles are polygamous and solitary. The species differ in their expression of a vasopressin receptor gene (V1aR) in their forebrains (expression measures the rate of protein production by a gene). The scientific hypothesis is that this receptor of vasopressin, an important neurotransmitter, might influence the voles’ social behavior. Geneticists were able to experimentally increase the expression of the V1aR gene in a sample of meadow voles, and they compared the behavior of the resulting individuals with that of control individuals without excess V1aR (Lim et al.

2004). They measured the time each vole spent huddling with a partner, under the assumption that greater huddling time is indicative of a more social animal, as described in Assignment Problem 14 in Chapter 3. The following graph shows the bootstrap sampling distribution of the difference in median huddling time between the two groups, based on 1000 bootstrap replicates. Estimate by eye the bootstrap standard error of the mean difference in median huddling time.

Data from Assignment Problem 14

How do the insects that pollinate flowers distinguish individual flowers with nectar from empty flowers? One possibility is that they can detect the slightly higher humidity of the air—produced by evaporation—in flowers that contain nectar. von Arx et al. (2012) tested this idea by manipulating the humidity of air emitted from artificial flowers that were otherwise identical. The following graph summarizes the number of visits to the two types of flowers by hawk moths (Hyles lineata).