Urban areas are dynamic, heterogeneous habitats that differ dramatically from your traditional “non-urban” habitats. These centers are complex, multi-level habitats that are full of interacting stressors. Additionally, urban habitats are fragmented via impervious surfaces and are subject to noise, chemical, and light pollution alongside human interaction(s). Humans (and their pets) directly and indirectly interact with urban wildlife, inducing stress, increasing mortality, and altering food availability. To survive, wildlife inhabiting these areas must adapt or disperse to avoid these harsh conditions.
Despite urban areas having obstacles for success, many taxa are able to inhabit these areas. The highly fragmented landscape can provide islets of green space for various taxa to survive. However, the alteration in habitat conditions has impacted avifauna dramatically, with some disappearing after intense urbanization. Those that can adapt are able to exploit a new niche with relatively less competition. The urban-dwelling species that survive often show dramatic phenotypic differences (e.g., change in physiology), but the traits that are correlated with survival in cities are not well understood.
The presentation of the cognitive buffer hypothesis in the early 90s stated that large brain size (relative to body size) allows for individuals to respond to new challenges better than those that have a smaller brain, which is essential in unpredictable urban areas. This hypothesis has received mixed support, receiving early support mixed with challenges and eventually reaffirmed in 2019 by Callaghan et al. in 2019.
In their recent publication, Sayol and colleagues explain that large brain size is one of the multiple traits that influence an individual’s ability to survive in urban areas. Another, for example, is being a generalist, when an individual consumes a wide variety of foods. Therefore, brain size can interact with various traits (e.g., life history) and by not considering other factors, we cannot fully understand how these organisms persist in urban areas and become urban dwellers. In this study, Sayol and collaborators combine brain size, life-history, and the ecology of avifauna to understand how these factors influence urban tolerance.
They conducted a global-scale analysis of detailed information on 629 avian species across 27 cities (Figure 1). They approached this analysis in two stages: (1) Estimating the urban tolerance for a species by comparing its abundance in urban and rural areas then conduct a phylogenetic analysis to unearth how traits (brood value, life history) interact with brain size; (2) Conducting a community-level analysis to understand how these traits alter the structure of urban assemblages.
In addition to their finding that brain size is correlated with urban tolerance, this meta-analysis found that brood value (number of annual breeding events) and habitat breadth is also correlated with success in urban areas. Individuals with low brood value and a broad habitat breadth show higher urban tolerance. Further, individuals with higher brood value and small brains were not as abundant in urban areas as individuals with both relatively large brains and a high brood value, showing that brood value and brain size interact to predict an individual’s ability to tolerate urban settings (Figure 2a). When examining the differences between rural and urban dwellers, they found that in urban environments, we see a significant shift towards large brains and lower breeding attempts when compared to their rural counterparts.
We see that brain size is an important predictor for urban tolerance, however, we see adopting alternative strategies such as brood value can allow for individuals to still be persistent in these environments. The authors go on to explain that the adoption of alternative strategies may explain why we see urban tolerance exists in some groups of mammals, such as carnivores, but not others, like ungulates. In this study, Sayol et al. reveal two trait combinations that are correlated with urban tolerance: (1) large brains and high brood values (i.e., fewer breeding attempts over their lifetime) and (2) small brains and low brood values (i.e., many reproductive attempts).
This study confirms that relative brain size is an important factor in succeeding in urban environments, but, it also sheds light on the behavioral traits that are crucial for tolerating the harsh, urban habitat. The authors urge future studies to consider factors of life-history in conjunction with brain size to fully understand how taxa are successful in urban centers. Additionally, they set forth a path for future studies, suggesting to explore how the factors analyzed here (brain size and life history) affect an organism’s response to other anthropogenic threats, such as habitat alterations.
This work is crucial in understanding which taxa are more sensitive to change or threats in the urban environment, influencing conservation efforts. In the age of increasing urbanization, we must seek to understand various taxa’s ability to tolerate urban stress in order to create appropriate environmental/conservation policies.
Read the paper:
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