James Baxter-Gilbert and colleagues from down under — Julia Riley and Martin Whiting — have been digging into the ecological and evolutionary responses to urbanization in a charismatic and common (at least if you’re in Australia) group of lizards: Australian water dragons (Intellagama lesueurii). Previously, we told you about their findings that urban dragons are innately bold. In their latest study, “Shrinking into the big city: influence of genetic and environmental factors on urban dragon lizard morphology and performance capacity“, along with coauthor Celine Frere they tackle another aspect of urban adaptation in dragons: morphology and performance.
Urban morphological changes?
Baxter-Gilbert et al. predicted that adaptive morphological differences might arise in urban dragons as a consequence of natural selection on locomotor performance (sprint speed, endurance, and clinging ability). Given findings in other lizards, like anoles (see: Do Anoles Adapt to Urbanization in Similar Ways? and Are Anoles Evolutionarily Adapting to Urban Habitats?), they predicted that urban dragons would be larger bodied and have relatively longer limbs, differences they suspected would lead to improved sprinting and clinging performance.
The researchers found that in the wild hatchling dragons differed morphologically between urban and non-urban populations: urban hatchling dragons were smaller (shorter body length) and had longer limbs. In adult dragons, the difference in limb lengths disappeared but urban dragons were still smaller than non-urban dragons (but only in males and not in females). These data suggested that the traits observed in hatchlings were innate but that phenotypic plasticity might be important in shaping adult phenotypes. Perhaps the hatchling phenotype is only beneficial at that young life stage, or perhaps it is strongly influenced by parental effects such as maternal diet.
“Urban-style enclosures” — A novel rearing experiment
Baxter-Gilbert and team explored this morphological variation further with a rearing experiment in which they raised hatchlings from urban and non-urban populations in “urban- and natural-style enclosures”. What exactly does an urban-style and natural-style enclosure look like? The researchers constructed six 5×2.5 m pens. In the urban enclosures they placed common urban substrates: concrete blocks, paving stones, steel sheets, and roofing tiles. In the natural enclosures, in contrast, there was abundant natural vegetation and wooden perches of different diameters. With this setup, they were able to tease apart whether differences in morphology and associated performance were attributable to differences in natural selection between habitats or developmental conditions.
Baxter-Gilbert et al. found that the dragons from both urban and natural origin reared in urban-style enclosures were smaller than those reared in natural-style enclosures. This result suggests that the morphological differences observed in body size in the wild are attributable to developmental conditions. Interestingly, even though the lizards differed in body size, they did not differ in performance between rearing treatments. Moreover, these effects were seen only in the male dragons and not in the females. So why were the urban-treatment lizards smaller if this didn’t translate into performance differences? The authors speculate that smaller body sizes may have consequences for thermal tolerance and desiccation in urban environments, an intriguing possibility that I hope they are following up on!
Innate differences with environmental reinforcement?
This latest study on Australian dragons suggests that morphological differences arising in urban populations may be attributable to both innate and phenotypically plastic mechanisms. Urban hatchlings differed in their limb morphology and body size, differences that disappeared as the lizards matured and were strongly influenced by developmental environment. The authors conclude that some aspects of urban dragon morphology are likely innate at hatching, but that environmental differences between urban and natural habitats influence adult dragon morphology. In other words, the persistence of urban phenotypes to adulthood requires ongoing environmental cues.
Read the study:
Featured image:
Australian Water Dragon (Intellagama lesueurii) CC-BY-NC iNaturalist user joannab011
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