Terrestrial Locomotor Evolution in Urban Environments

The structural habitat of terrestrial urban environments can differ drastically from environments less impacted by human activities. Whether or not urban species use anthropogenic structures, they are subject to novel selection pressures to effectively locomote. Urban environments are distinctly more open than non-urban habitats, they offer few refuges, and habitat space is patchy with clustered perches. Animals must either change their behaviour to use only natural substrates or contend with manufactured substrates. Arboreal species are particularly impacted because the anthropogenic structures with which they interact, even if infrequently, differ from trees in structural, material, and surface properties. The chapter explores potential adaptive responses to the spatial structure and properties of climbing substrates in urban environments relevant to terrestrial and climbing locomotion. For each, the authors first discuss differences between urban and non-urban terrestrial habitats relevant to locomotion. They then discuss how these differences influence behaviour and locomotor demands, providing a mechanism through which natural selection shapes morphology. Lastly, they discuss the morphological traits most likely to be impacted by these altered demands and predict how natural selection may affect these traits in urban environments based on biomechanical principles. As there have been very few studies investigating urban morphological adaptation related to locomotion, the chapter draws on trait–environment relationships in natural environments. The discussion provides a starting point for developing rigorous hypotheses about functionally relevant trait shifts in urban environments and future directions for investigating locomotor adaptations in urban species.

Convergent Evolution of Claw Shape in a Transcontinental Lizard Radiation

Species occupying similar selective environments often share similar phenotypes as the result of natural selection. Recent discoveries, however, have led to the understanding that phenotypes may also converge for other reasons than recurring selection. We argue that the vertebrate claw system constitutes a promising but understudied model system for testing the adaptive nature of phenotypic, functional, and genetic convergence. In this study, we combine basic morphometrics and advanced techniques in form analysis to examine claw shape divergence in a transcontinental lizard radiation (Lacertidae). We find substantial interspecific variation in claw morphology and phylogenetic comparative statistics reveal a strong correlation with structural habitat use: ground-dwelling species living in open areas are equipped with long, thick, weakly curved, slender-bodied claws, whereas climbing species carry high, short, strongly curved, full-bodied claws. Species occupying densely vegetated habitats tend to carry intermediately shaped claws. Evolutionary models suggest that claw shape evolves toward multiple adaptive peaks, with structural habitat use pulling species toward a specific selective optimum. Contrary to findings in several other vertebrate taxa, our analyses indicate that environmental pressures, not phylogenetic relatedness, drive convergent evolution of similarly shaped claws in lacertids. Overall, our study suggests that lacertids independently evolved similarly shaped claws as an adaptation to similar structural environments in order to cope with the specific locomotory challenges posed by the habitat. Future biomechanical studies that link form and function in combination with genomic and development research will prove valuable in better understanding the adaptive significance of claw shape divergence.

Do structural habitat modifications associated with urbanization influence locomotor performance and limb kinematics in Anolis lizards?

Urbanization significantly alters habitats for arboreal species, increasing the frequency of very smooth substrates by substituting artificial objects, such as metal poles and painted walls, for some trees. Because they experience these novel substrates more often, urban animals may use strategies to overcome challenges from substrate smoothness that animals from natural habitats do not. We assessed locomotor performance and two-dimensional hindlimb kinematics of two species of Anolis lizards (Anolis cristatellus and Anolis sagrei) from both urban and natural habitats in Miami, Florida. We ran lizards on six racetracks, crossing three substrates of increasing smoothness (rough bark, concrete blocks, and smooth, unpainted wood) with two inclinations (37° and vertical). We found that on vertical tracks with smooth substrates, lizards ran slower, took shorter strides and exhibited more contracted limb postures at the end of their stance than when running on the inclined track. Urban lizards, which are likely to be exposed more often to smooth substrates, did not adjust their movement to increase performance relative to lizards from natural habitats. This result, and the similarity of kinematic strategies between the two species, suggests the locomotor responses of lizards to substrate properties are highly conserved, which may be a mitigating factor that dampens or obviates the effects of natural selection on locomotor behaviour.