With their often unexpected and novel stimuli, urban environments can be chaotic, variable, and harsh. This is why only some species are able to thrive in urban areas, while others vacate disturbed habitats and retreat to more rural locations. But why don't all species respond to cities in the same way? A new study suggests that it may be a matter of brain power.
Researchers from Sweden's Evolutionary Biology Centre and Spain's Estación Biológica de Doñana analyzed the relationship between brain size and success in urban habitats for 82 songbird species found in and around 12 representative cities in France and Switzerland. "Success" was defined as the ability to breed within the middle of the city, while "avoidance" was defined as breeding around the city, but not at its center. Using these criteria, the authors defined 38 species as "successful" and 44 as "unsuccessful" in urban areas.
They performed both species- and family-level analyses; the first looked at whether individual species with larger brains were more tolerant of urban habitats, while the second investigated whether average brain size within phylogenetic families (e.g., "finches," "corvids") predicted success of entire groups of birds within cities. Both analyses controlled for the relationship between brain size and body size (since larger animals tend to have larger brains), as well as accounting for similarities between animals as a result of shared ancestry.
(Male house sparrow, Passer domesticus)
At both the species and family level, larger brain size was strongly positively associated with the ability to put up with the stresses of city life. Dunnocks (Prunella modularis), starlings (Sturnus vulgaris), wrens (Troglodytes troglodytes), and goldcrests (Regulus regulus) are a few of the brainiacs that drove this pattern, along with all 6 tit species surveyed. One surprise was the corvids, extremely intelligent birds that are, as a group, ubiquitous in cities. However, while two-thirds of corvids were classified as urban dwellers, approximately one third (or about 3 species) made the list of urban avoiders.
(Starling, Sturnus vulgaris)
Large brains have frequently been linked with the ability to adapt to new or unpredictable environments, as well as helping determine whether animals will display innovation. Thus, the results from the current study are not entirely surprising, though they are interesting. It's also not surprising to find measurable differences between city birds and their country cousins; other characteristics that are known to vary between these two groups include song type, diet, size, cholesterol, hormone levels, and tolerance of human approach, to name a few.
The authors of the study point out that it might also be interesting to examine relative size of particular brain features, rather than just the whole brain. In birds, for instance, the ability to innovate is associated with the nidopalium/mesopallium complex in the telencephalon; so, birds with a relatively large telencephalon--regardless of total brain size--might be expected to do well in urban environments. However, since the sizes of at least some individual brain components are highly correlated with total brain size, these specific analyses might not add much to the story.
Perhaps even more intriguing is the possibility that life in the city could select for inhabitants with larger brains--and, therefore, presumably more smarts--than their rural relatives. Given enough time, this could drive evolution of subspecies, as well as causing the evolution of new behaviors. However, it remains to be seen just how widespread these patterns are; additional studies are needed in other avian taxa (and, indeed, other types of animals in general) living in different urban areas around the globe.
Maklakov, A.A., Immler, S., Gonzalez-Voyer, A., Rönn, J., and Kolm, N. 2011. Brains and the city: Big-brained passerine birds succeed in urban environments. Biology Letters 7:730-732.
Thanks to the following for providing the images used in this post: