Island tameness: living on islands reduces flight initiation distance

Abstract

One of Darwin's most widely known conjectures is that prey are tame on remote islands, where mammalian predators are absent. Many species appear to permit close approach on such islands, but no comparative studies have demonstrated reduced wariness quantified as flight initiation distance (FID; i.e. predator-prey distance when the prey begins to flee) in comparison with mainland relatives. We used the phylogenetic comparative method to assess influence of distance from the mainland and island area on FID of 66 lizard species. Because body size and predator approach speed affect predation risk, we included these as independent variables. Multiple regression showed that FID decreases as distance from mainland increases and is shorter in island than mainland populations. Although FID increased as area increased in some models, collinearity made it difficult to separate effects of area from distance and island occupancy. FID increases as SVL increases and approach speed increases; these effects are statistically independent of effects of distance to mainland and island occupancy. Ordinary least-squares models fit the data better than phylogenetic regressions, indicating little or no phylogenetic signal in residual FID after accounting for the independent variables. Our results demonstrate that island tameness is a real phenomenon in lizards.

Keywords: antipredatory behaviour; body size; escape behaviour; flight initiation distance; island tameness; lizard.

Figure 1.

Three-dimensional scatter plot of FID on SVL and distance to mainland for all data. FID (m) decreases as the distance of the population from the mainland (km) increases, and increases as SVL (mm) increases. Note that mainland species are represented by a large cluster at log(distance + 0.5) having variable body length and FID.

Figure 2.

Two-dimensional scatter plots of FID on SVL and distance to mainland for all data. These graphs complement figure 1 in two-dimensional views. (a) FID (m) decreases as the distance of the population from the mainland (km) increases. (b) FID increases as SVL (mm) increases.

Figure 3.

Three-dimensional scatter plot of FID on SVL and distance to mainland for island taxa. This figure complements figure 1, which includes data for mainland species in addition to the island species shown in figure 3. FID (m) decreases as the distance of the population from the mainland (km) increases, and increases as SVL (mm) increases.