Are behavior and stress-related phenotypes in urban birds adaptive? This question is the title of a study published in The Journal of animal ecology. The researchers explain: "Urbanisation is a world-wide phenomenon converting natural habitats into new artificial ones.
Environmental conditions associated with urbanization represent great challenges for wildlife. Behavior and stress tolerance are considered of major importance in the adaptation to novel urban habitats and numerous studies already reported behavioral and stress response phenotypes associated with urbanisation, often suggesting they represented adaptations, while rarely demonstrating it.
The main goal of this study was to test the adaptive nature of urban shifts in behavioral and stress-related traits, and by adaptive we mean phenotypic change favoring traits in the same direction as selection. Using 7 years of monitoring of urban and forest great tits, we first tested for differences in exploratory behaviour, aggressiveness and breath rate, between both habitats.Second, we performed habitat-specific analyzes of selection on the three former traits using rep productive success and survival estimated via capture-mark-recapture models, as fitness estimates, to determine whether shifts in these behavioral and stress-related traits were aligned with patterns of ongoing selection.
We found that urban birds displayed higher exploratory behavior and aggressiveness, and higher breath rate, compared to forest birds. Selection analyzes overall revealed that these shifts were not adaptive and could even be maladaptive.
In particular, higher handling aggression and higher breath rate in urban birds was associated with lower fitness. Higher exploration scores were correlated with lower survival in both habitats, but higher reproductive success only in forest males.
Overall, differences in patterns of selection between habitats were not consistent with the phenotypic divergence observed. Taken together, these results highlight that phenotypic shifts observed in cities do not necessarily result from new selection pressures and could be maladaptive.
We hypothesise that divergences in behavioral traits for urban birds could result from the filtering of individuals settling in cities. We thus encourage urban evolutionary scientists to further explore the adaptive potential of behavioral traits measured in urban habitats by replicating this type of study in multiple cities and species, by implementing studies focusing on immigrant phenotypes and by measuring selection at multiple life stages."