The study from Example 9.6.6 of the textbook was conducted by quantifying how a garter snake’s ability to escape predation is affected by both the color pattern on its back and by its behavior.
Garter snakes have a wide range of color patterns, from checkered patterns (left image below) to patterns with bold longitudinal stripes (right image below).
Likewise, when garter snakes flee a predator they also exhibit a wide range of behaviors. Some flee directly away from the predator while others make a number of reversals in direction while fleeing.
Brodie (1992) found that certain behaviors and color patterns work well together as a means of avoiding predation while others do not. Specifically, he found that striped snakes that flee directly away from predators survive well but striped snakes that exhibit reversals during fleeing do not. Likewise, checkered snakes that exhibit reversals during fleeing survive well but checkered snakes that flee directly do not. Thus, if you are a striped snake you should flee directly but if you are a checkered snake then reversing your direction while fleeing is a good idea.
Example 9.6.6 of the textbook illustrates these conclusions by showing that the critical point of the survivorship function F is a saddle point. Below we reproduce a plot of this function and identify where snakes with different color patterns and behaviors lie on the plot.
A straight snake indicates one with few reversals while a curved snake is one that exhibits many reversals. The two color patterns shown are striped and checkered. You will notice that striped snakes with no reversals and checkered snakes with many reversals survive best.
References
Brodie, E. III. 1992. Correlational selection for color pattern and anti predator behavior in the garter snake Thamnophis ordigoides. Evolution 46:1284-1298