Behavioral DiverCity: Individual differences in behavior change along an urbanization gradient

Urbanization is occurring globally at an unprecedented rate and, despite
the eco-evolutionary importance of individual variation in adaptive
traits, we still have very limited insight on how phenotypic variation is
modified by anthropogenic environmental change. Urbanization can increase
individual differences in some contexts, but whether this is generalizable
to behavioral traits, which directly affect how organisms interact with
and respond to environmental variation, is not known. Here we examine
variation across three behavioral traits (breath rate, handling aggression
and exploration behaviour) in great tits Parus major along an urbanization
gradient (n > 1000 phenotyped individuals accross nine years) to
determine whether among-individual variance in behavior increases with the
degree of urbanization and spatial heterogeneity. Urban birds were more
aggressive and faster explorers than forest birds. They also displayed
higher among-individual variation for breath rate and aggression (1.5 and
1.8 times increase, respectively), but lower among-individual variation
for exploration (3.3 times decrease). Only individual variation in
exploration clearly changed along the continuous urbanization gradient;
individual differences in exploration declined with increasing impervious
surface area. Collectively our results suggest that individuals in the
city may have more diverse behavioral stress responses, yet display
stronger similarity in their behavioral responses to novelty. Our results
suggest that generalizations about urbanization’s impacts on behavioral
variation are not appropriate. Instead our results suggest that
urbanization can shape individual variation differently across behavioral
functions and we may expect decreased individual diversity in urban birds
for traits related to behavioral response to novelty.