Sex-specific consequences of life in the city

The vast majority of species are excluded from human-dominated landscapes, but some species persist and appear to thrive, despite facing challenges they would not encounter in less disturbed habitat. Corticosteroid (cort) secretion may play a significant role in enabling an animal to cope with urban breeding habitat because it helps mediate physiological and behavioral responses to environmental challenges. Here we present tests of 3 hypotheses relating to cort secretion in urban animals: 1) environmental challenges vary between urban and rural habitat, as reflected in differing levels of cort secretion in animals breeding in these habitat types, 2) there are fitness correlates associated with variation in cort levels within populations, and 3) parasite load is one of the mechanistic links between variation in cort levels and reproductive success. Male white-crowned sparrows (Zonotrichia leucophrys) in urban habitat had significantly higher baseline corticosterone levels than males in rural habitat, whereas female Z. leucophrys' hormone levels did not differ between the 2 habitat types. To assess the fitness correlates of variation in hormone levels, we monitored seasonal reproductive success in one urban population and found that baseline corticosterone levels were predictive of female, but not male, reproductive success: females with higher corticosterone fledged fewer offspring. Data do not suggest a direct relationship between corticosterone levels and fitness and instead indicate an indirect relationship between hormone secretion, territory quality, body condition, and reproductive success. Females with blood parasites had lower heterophil:lymphocyte ratios, lower body condition scores, fewer mates, and fledged fewer offspring than females without parasites. In contrast, parasites did not negatively affect reproductive success in males but were associated with reduced body condition. Corticosterone levels did not differ between birds with and without parasites. In tests of all 3 of our hypotheses, we found sex-specific effects, highlighting the importance of considering sex in investigation of physiological responses to disturbance.