In garter snakes, some males emerging from overwintering dens mimic females by producing female pheromones. Males might mimic females to warm up: males tend to emerge sooner than females and warm up in the sun while they wait for the females; the females are surrounded by males soon after emergence and soak up their warmth.

A prediction based on this idea is that males that mimic females should be covered by more males than males that don’t mimic females. Observations on newly emerging garter snakes in Manitoba found that, on average, 58% of a male’s body was covered by other males if he emitted female pheromones (with standard deviation 28%, measured on 49 males). In comparison, 32 males that did not emit female pheromones had, on average, 25% of their bodies covered by other males, with standard deviation 24% (Shine et al. 2001).

a. On average, how much more covered by other males are female mimics compared with nonmimics? Give a 95% confidence interval for this parameter.

b. Test the hypothesis that female mimicry has no effect on the proportion of body coverage in these garter snakes. What assumptions are you making?