(This exercise is based on Losos, J. B., T. W. Schoener, and D. A. Spiller. 2004. Predator-induced behaviour shifts and natural selection in field-experimental lizard populations. Nature 432: 505–508.)
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In addition to influencing population sizes and dynamics of prey, predation also can affect behavior of the prey. The presence of predators can cause individuals to change how, where, and when they feed in ways that reduce the risk of predation. As described in the textbook, predators can also exert selection pressures on populations of their prey, and thus change the morphology or behavior of the prey over evolutionary time.
Jonathan Losos from Washington University in St. Louis, Tom Schoener from the University of California at Davis, and their associates have been studying the ecology and behavior of Anolis lizards in the Caribbean islands for many years. In this particular study, they addressed the effects of predators on the behavior of one species of these anoles. This species, Anolis sagrei (henceforth called the anole) normally lives on or near the ground.
In June 2003, Losos and his colleagues released a predatory lizard, Leiocephalus carinatus, to six islands, leaving six comparable islands that lacked this predator as controls. L. carinatus (henceforth called the predatory lizard) is substantially larger than the anoles and can cause considerable mortality. On four of the experimental islands, individual anoles were observed to determine their initial reaction to a predatory lizard or an inanimate control object of about the same size. For these observations, a predatory lizard or control object was introduced 50–100 centimeters away from an adult anole. Based on previous studies, the researchers thought that no individual anole on the four islands had ever seen a predatory lizard. Thus, any changes in anole behavior were unlikely to be learned responses to the dangers posed by that predatory lizard.
Figure 1 The mean and standard error for the change in perch height of anoles after anoles were presented with a predatory lizard (experimental) or an inanimate object of the same size (control). The values are pooled from all individuals across the four islands.
Question 1
Describe the results.
Question 2
Is the inanimate object the proper control for this experiment? Discuss.
Figure 2 The percentage (and standard error) of adult anoles seen on the ground in May, July, and November of 2003 in islands with (experimental) and without (control) addition of predatory lizards.
Question 3
The immediate response of the anoles to the predatory lizard was not permanent, and soon many of the anoles returned to the ground. The researchers were interested in whether there were longer-lasting behavioral responses to the predatory lizard. To do this, they monitored anoles on the experimental and control islands a month before release of the predatory lizard (May), a month after release (July), and five months after release (November). Describe the results. What is the effect of the predatory lizard on anole habitat use?
Figure 3 The frequencies of adult anoles (those that measured greater than 36.5 mm stout-to-vent length) and hatchings (those that were less than 30 mm in stout-to-vent length) at different perch heights in both control and experimental islands during November 2003.
Question 4
The researchers were interested in whether hatchling anoles and adult anoles showed similar or different patterns of behavior (perch height). Describe the results. Do adults and hatchlings show different responses to predation?
