Anolis cristatellus on a smooth, vertical substrate in Puerto Rico (photo by K. Winchell)
In urban areas, the number of natural substrates (e.g. trees) is reduced. In their place are novel manmade substrates (e.g. walls, metal gates). These surfaces undoubtedly have different properties relevant to anole locomotion: they are smoother, harder, and (in the case of walls) much broader and flatter compared to natural surfaces in a forest. In urban areas lizards still use these substrates at high frequency, but do they do so effectively? Kolbe and co-authors began to dive into this complex topic in their recent publication, “City slickers: poor performance does not deter Anolis lizards from using artificial substrates in human-modified habitats” (Kolbe et al. 2015).
The relationship between habitat use, morphology, and performance for anoles has been extensively studied in natural environments (reviewed in Losos 2009). Urban environments add new dimensions to this area of research. Resource distribution and abundances differ drastically compared to natural areas. For example, the distribution of available perches and what they are made of in urban habitats is very different from a forest. Moreover, the properties of these resources differ drastically as well: urban substrates are smoother, broader, and have different thermal properties, to start. Understanding these differences in habitat use and how they influence performance and, ultimately, adaptive responses in anoles is the topic of ongoing collaborative research that I (K. Winchell) and the Kolbe lab have been conducting.
Anolis cristatellus and Anolis stratulus on rough natural substrates in the forest.
We know from the literature that perch diameter influences both sprint speed (e.g. Losos and Sinervo 1989) and cling force in anoles (e.g. Kolbe 2015) and that morphology influences this relationship. In general, long-limbed lizards sprint more slowly, but cling more strongly to thinner perches. In natural habitats, the long-limbed anoles use broader perches to avoid the sub-optimal narrow-perch habitat (“habitat constraint hypothesis”; Irschick and Losos 1999). Yet we do not know how morphology and performance interact with habitat use in urban areas, where there is an abundance of broad, flat, and smooth perches. Is performance impacted by these different substrates? Do lizards avoid habitat in which they perform poorly? These are the main questions Kolbe et. al sought to answer, building on previous research demonstrating relationships between long hindlimb length and broad urban perch use in both Anolis sagrei and Anolis cristatellus (Marnocha et al. 2011, Winchell et al. accepted).
To test this, Kolbe et. al conducted sprint speed trials of two lizards commonly found in anthropogenically disturbed areas in the British Virgin Islands: Anolis cristatellus and Anolis stratulus. They conducted their sprint trials on rough substrates (meant to mimic surfaces encountered in natural forest habitats) and smooth substrates (meant to mimic common, smooth urban surfaces) at two different inclinations using lizards from an anthropogenically disturbed site (although not strictly “urban”). They measured maximum velocity, pauses, slips, and falls on these substrates. The authors also quantified habitat use to determine if lizards occupied the portions of the habitat in which they perform best (“habitat constraint” hypothesis). With this study design, they make three predictions about performance, morphology, and habitat use: (1) lizards will perform more poorly on smooth and steeper tracks, (2) lizards will avoid smooth vertical surfaces in the wild because of this poor performance, and (3) larger lizards will be more sensitive to the roughness and inclination of the track.
Figure 1 from Kolbe et. al (2015): Mean (+/-SE) maximum velocity on tracks differing in surface roughness and inclination. Both species performed better on the rougher and more horizontal tracks. A. cristatellus performed worse than A. stratulus on the smooth vertical tracks.
Kolbe et al. found that both species ran slower on smoother surfaces and on vertical surfaces and that A. cristatellus frequently slipped and fell on smooth vertical surfaces. For both species, lizards were fastest on the 37o rough track and slowest on the 90o smooth track and A. cristatellus performed worse on the smooth vertical track than A. stratulus. They also found that A. cristatellus paused after a shorter distance on the vertical smooth track compared to the other tracks, and that both species, but particularly A. cristatellus, paused and slipped more frequently on the 90o smooth track. Overall, surface roughness was more important in reducing maximum sprint speed than inclination – on vertical tracks, lizards ran up to 70% slower on smooth surfaces (29-31% slower for A. stratulus, 59-70% slower for A. cristatellus).
Figure 3 from Kolbe et al. (2015): Substrates in disturbed areas (within 1m or 4m of built structures or artificial substrates) were much smoother than those in natural areas.
Although both species performed poorly on the smooth vertical substrate, they both utilize such substrates in the urban habitats at high frequency. Kolbe et al. found that A. cristatellus used manmade substrates 73% of the time in human-modified areas! These manmade perches were often flat (mainly walls) and were smoother than those found in undisturbed forest habitats. In contrast, A. stratulus mainly used natural substrates such as tree trunks and branches. As a result, they reject the habitat constraint hypothesis since both species utilize habitat in which they perform poorly.
Figure 4 from Kolbe et. al (2015): A. cristatellus with relatively longer limbs, wide heads, and wide pectoral regions (PC1) run faster on the smooth vertical surface. Open circles females, solid circles males.
Lastly, their morphological data suggest that morphology plays an important role in this story as smaller-bodied A. cristatellus with proportionally longer limbs performed better. This same relationship did not exist for A. stratulus suggesting that selection is acting differently on the two species, perhaps because of the smaller body size of A. stratulus. Among all lizards tested, larger lizards (SVL and weight) performed more poorly on the 90o smooth track. Their principal component analysis indicated that lizards with longer limbs, wider heads, and wider pectoral regions (PC1) ran faster on smooth vertical surfaces.
In summary, Kolbe and coauthors have provided us with persuasive evidence that morphology-performance-habitat use relationships may shift in urban habitats. Lizards perform poorly on common urban substrates (e.g. walls), yet they do not avoid these habitats. This result, coupled with the fact that longer limbed lizards with wider heads and pectoral regions perform better on such surfaces, suggests that natural selection may be shaping these phenotypes in human modified habitats. In this case, poor performance on manmade substrates appears to be mediated by adaptive phenotypic responses, although behavioral responses such as habitat avoidance or altered activity on these substrates may also be relevant.
Works Referenced:
Irschick, D.J. & Losos, J.B. (1999) Do lizards avoid habitats in which performance is submaximal? The relationship between sprinting capabilities and structural habitat use in Caribbean anoles. American Naturalist, 154, 293–305.
Kolbe, J.J. (2015) Effects of hind-limb length and perch diameter on clinging performance in Anolis lizards from the British Virgin Islands. Journal of Herpetology, 49, 284–290.
Losos, J.B. (2009) Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles. University of California Press, Berkeley, CA, USA.
Losos, J.B. & Sinervo, B. (1989) The effects of morphology and perch diameter on sprint performance of Anolis lizards. Journal of Experimen- tal Biology, 145, 23–30.
Marnocha, E., Pollinger, J. & Smith, T.B. (2011) Human-induced morphological shifts in an island lizard. Evolutionary Applications, 4, 388–396.
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