Linking locomotor performance to morphological shifts in urban lizards

Abstract

Urban habitats are drastically modified from their natural state, creating unique challenges and selection pressures for organisms that reside in them. We compared locomotor performance of Anolis lizards from urban and forest habitats on tracks differing in angle and substrate, and found that using artificial substrates came at a cost: lizards ran substantially slower and frequently lost traction on man-made surfaces compared to bark. We found that various morphological traits were positively correlated with sprint speed and that these same traits were significantly larger in urban compared to forest lizards. We found that urban lizards ran faster on both man-made and natural surfaces, suggesting similar mechanisms improve locomotor performance on both classes of substrate. Thus, lizards in urban areas may be under selection to run faster on all flat surfaces, while forest lizards face competing demands of running, jumping and clinging to narrow perches. Novel locomotor challenges posed by urban habitats likely have fitness consequences for lizards that cannot effectively use man-made surfaces, providing a mechanistic basis for observed phenotypic shifts in urban populations of this species.

Keywords: Anolis cristatellus; Puerto Rico; adaptation; performance; urban evolution; urbanization.

Figure 1.

Sites sampled in four Puerto Rican municipalities with approximate sampling areas outlined in blue. Canopy cover, impervious surface cover and roads are shown within 2 km² from the centre of the sampled area. Grey solid lines represent coastline.

Figure 2.

(a) Decrease in sprint speed by track. Percentages above each represent the per cent of maximum sprint speed compared to performance on the 37° wood track, on which lizards sprinted the fastest. (b) Mean number of stops, slips, and slides on each track, with whiskers showing ± s.e. of each (there were no slips or slides on wood tracks). C, concrete; M, metal; W, wood.

Figure 3.

Trade-offs of morphology at different inclinations and the interaction of traits in producing different velocities (across all substrates). Darker blue shades are slower velocities, brighter red shades are faster velocities and contour lines are labelled with the average velocity (in m s⁻¹) across the three substrates. On gradually inclined tracks (a(i)–c(i)), velocities are greater with relatively longer forelimbs (x-axis) only when combined with long hindlimbs (a(i)), larger rear toepads (b(i)) or more front lamellae (c(i)). On steeply inclined tracks (a(ii)–c(ii)), relatively longer forelimbs have a consistently negative effect counteracted to varying extents by other traits: lizards with long limbs and larger toepads or more lamellae still attain fast velocities.

Figure 4.

Urban lizards had relatively longer limbs, larger toepads, more lamellae and wider bodies compared to forest lizards. Significance levels were determined from MANOVA of log-transformed trait values with log-transformed body size and municipality as covariates. Effect sizes (Cohen's d) are provided for each comparison. Significance levels: **p < 0.01, ***p < 0.001.

Figure 5.

The angle of inclination (a) and perch roughness (b) of random potential perches (white) and perches used (grey) in urban populations, contrasting perch use by fast and slow lizards as a test of the habitat breadth and constraint hypotheses. Poorly performing lizards (slowest on the most challenging track, 60° concrete) used less vertical and rougher perches than were common while lizards that performed best (fastest on the most challenging track) did not discriminate based on either factor. Significance levels of contrasts to randomly available perches: p > 0.05 ‘n.s.’, p < 0.05 ‘*’.