Table 1.

Hypotheses on traits pre‐adapting species to urban environments

	Hypothesis	Predictions	Variable	Taxa	Rationale
1	Null	Nothing influences ability of species to exploit urban areas	–	E, R, B, C, U, P	Factors other than the biological traits considered (including random chance) actually allow mammals to live in cities
2	Body mass	Small urban dwellers and large urban visitors	Body mass	E, R, B, C, U, P	Small body masses may allow species to hide/nest/roost more easily in wall cracks, underground, small green urban spots, bushes, canopies, etc
					Large body masses, on the other hand, are associated to larger home ranges and higher dispersal abilities (Kelt & Van Vuren 2001; Santini et al. 2013)
3	Diet breadth	Higher diet diversity in urban species	Shannon Index on diet	E, R, C, U, P	Diet diversity makes species more adaptable allowing them to exploit a wider range of resources, therefore making them able to colonise a wide range of habitats (Slatyer et al. 2013)
4	Reproductive timing	Slower and faster reproductive rates in urban species	Weaning age	E, R, B, C, U, P	Weaning age is a proxy of reproductive timing (frequency of reproductive events; Bielby et al. 2007). Species with slow reproductive timing are generally characterised by low mortality rates (Schaffer 1974; Charlesworth 1980; Reznick et al. 1990; Stearns 2000). On the one hand low weaning age may provide faster adaptive responses, increased spread rate and capacity to cope with unpredictable environment (Santini et al. 2016). On the other hand, high weaning age is generally associated to longer parental care that might be necessary to learn how to avoid certain threats (e.g. traffic), the development of a large brain (Gonzalez‐Voyer et al. 2016) and to dispersal abilities (Whitmee & Orme 2012). Therefore, both strategies can potentially advantage urban visitors and dwellers
5	Reproductive output	High reproductive output in urban dwellers and low in urban visitors	Litter size	E, R, B, C, U, P	Litter size is a proxy of reproductive output (investement; Bielby et al. 2007). Species producing large litters generally invest less in each newborn, therefore litter size can represent the balance between number and quality of offspring produced (Schaffer 1974; Charlesworth 1980; Reznick et al. 1990; Stearns 2000). Large litters increase species ability to spread and colonise new environments (Whitmee & Orme 2012; Santini et al. 2016), to cope with unpredictable environments characterised by high mortality rates (e.g. traffic, predation by domestic animals, human persecution)
6	Behavioural flexibility	Higher encephalization in urban species	Brain mass	E, R, B, C, U, P	A large brain for a given body mass is expected to provide adaptive benefits. The cognitive buffer hypothesis states that enhanced encephalization (large brains for a given body mass) provides adaptive benefits such as behavioural flexibility to cope with new conditions. Several papers have shown that birds living in urban environments are characterised by large brains (Maklakov et al. 2011; Fristoe et al. 2017). Others have argued that enhanced encephalisation in terrestrial vertebrates (amphibians, reptiles, birds and mammals) improves their ability to colonise and successfully establish into novel environments (Sol et al. 2002, 2008; Amiel et al. 2011). Only evidence for a small number of species is available for mammals (i.e. Snell‐Rood & Wick 2013)
7	Enhanced flying ability	High aspect ratio in urban species	Aspect ratio	B	Aspect ratio (wing span/wing area) describes wing morphology of bats, i.e. higher values indicate longer, narrower wings, positively correlates with ranging abilities and flight speed, and being associated to species that fly in open spaces or edge habitats (Jung & Kalko 2011). Urban areas are typically open habitats, thus potentially favouring species with higher aspect ratios, both for visitors and dwellers

E = Eulipotyphla, R = Rodents, B = Bats, C = Carnivores, U = Ungulates, P = Primates.