We’ve talked before here on Life in the City about behavioral shifts related to fear and boldness in urban animals (check out: Skirting Skinks: Are Lizards Learning to Live Among Humans?, No city for shy dog, Concrete Escape: Increased Wariness of Anoles). Although it is clear that urban species experience behavioral modifications in response to urban pressures, it is still unclear in many species if these behavioral differences are genetically based or shaped by life experiences. A recent paper by James Baxter-Gilbert and colleagues on urban Australian water dragons (Intellagama lesueurii): “Bold New World: urbanization promotes an innate behavioral trait in a lizard“, tackles the outstanding question of whether behavioral traits in urban dragons are innate. This new research fills an important gap in our understanding of these evolutionarily important behavioral shifts.
Here be dragons… in the city
Australian water dragons are successful urban exploiters – found in human-altered landscapes where they also experience morphological shifts. Baxter-Gilbert and colleagues sampled dragons from 12 different sites across the Sydney, Australia metropolitan region from three different habitat types: natural (native bushland with minimal human disturbance), semi-natural (protected green-spaces adjacent to urban areas), and urban (locally dense human populations with landscape dominated by buildings and impervious surface). From each of these sites they collected female dragons, which they brought into the lab and collected eggs as they were laid. They then incubated the eggs and reared the hatchlings in a standardized common garden experiment.
Quantifying fear
During the first year of development, they conducted repeated individual behavioral assays for traits they expected to differ between habitat types: exploration, boldness, and neophilia, in a series of what sound like very fun experiments.
In the exploration experiment, they wanted to see how willing a dragon was to explore a novel environment. Their test arena was set up similarly to an open-field test (an assay to see how willing an individual is to be in exposed environments and away from refuge). They provided two refuges within the test arena and covered the ground with different types of novel substrates for every trial, conducted every 2 months: plain paper, eucalyptus mulch, sugar cane mulch, topsoil, and pine-bark mulch. The researchers then quantified the amount of time the dragon spent exploring in this novel environment.
In the boldness assay, they wanted to see how long it took a dragon to leave an unfavorable refuge after being chased by a predator. They provided two refuges in the arena, a warm (favorable) and a cold (unfavorable) and then chased the dragon around the arena with a gloved hand until it entered the cold refuge. They then timed how long until the dragon emerged from the unfavorable refuge as a proxy for boldness, again repeating trials every two months.
Finally, in the neophilia assay, they tested to see how willing the dragons were to investigate novel objects. In the test arena they placed an unknown object in the center representing commonly encountered novel objects from urban environments (i.e., refuse): paper coffee cup, aluminum pie tin, empty water bottle, unopened bag of potato chips, unopened soft drink can, using a different item for each of the periodic trials. The dragons were allowed 30 minutes to investigate the novel objects and the closest distance they got to the object was recorded as the neophilia score.
Cities are no place for shy dragons
It turns out that some of the behavioral traits studied were innate and differed by habitat type while others were not. Dragons from urban, semi-natural, and natural parents did not differ in neophilia or exploratory behavior. Boldness, however, was strongly different between these groups. Dragons from the semi-natural and urban habitats exited their unfavorable refuge in significantly shorter time compared to those from natural habitats indicating that these lizards were significantly bolder than those from the natural habitats. While urban and semi-natural populations had similar mean boldness, urban dragons had a much larger amount of variation in boldness responses across individuals. In addition to this key result, the authors also found that of their three behavioral traits, only boldness was consistently repeatable throughout development.
The authors conclude that boldness in Australian water dragons is an innate trait that is modified in urbanized environments and likely contributes to urban success. Determining the heritability of this trait and whether it is genetically determined or the result of maternal effects will further shed light on this interesting finding. Regardless of the mechanism, evolutionarily speaking, if bolder dragons are more successful in cities (i.e., have higher fitness) then we may expect bolder dragons to be the most likely to adaptively respond to urban selective pressures as they exploit habitat space that differs more drastically from their natural environment. Natural selection may further mold other important behavioral traits, or even related morphological and physiological traits of dragons in human-modified environments.
Want to read more? Check out the full manuscript:
Cover photo: Andrew Mercer, Wikimedia Commons CC-BY-SA
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