As environments become urbanized, tolerant species become more prevalent. The physiological,
behavioral and life-history mechanisms associated with the success of such species
in urbanized habitats are not well understood, especially in freshwater ecosystems.
Here, we examined the glucocorticoid (GC) profiles, life-history traits, and behavior
of two species of fish across a gradient of urbanization to understand coping capacity
and associated trade-offs. We studied the tolerant live-bearing Western Mosquitofish
(Gambusia affinis) for two years and the slightly less tolerant, egg-laying, Blacktail
Shiner (Cyprinella venusta) for one year. We used a water-borne hormone method to
examine baseline, stress-induced, and recovery cortisol release rates across six streams
with differing degrees of urbanization. We also measured life-history traits related
to reproduction, and for G. affinis, we measured shoaling behavior and individual
activity in a novel arena. Both species showed a trend for reduced stress responsiveness
in more urbanized streams, accompanied by higher reproductive output. Although not
all populations fit this trend, these results suggest that GC suppression may be adaptive
for coping with urban habitats. In G. affinis, GC recovery increased with urbanization,
and individuals with the lowest stress response and highest recovery had the greatest
reproductive allotment, suggesting that rapid return to baseline GC levels is also
an important coping mechanism. In G. affinis, urban populations showed altered life-history
trade-offs whereas behavioral traits did not vary systematically with urbanization.
Thus, these tolerant species of fish may cope with anthropogenically modified streams
by altering their GC profiles and life-history trade-offs. These results contribute
to understanding the mechanisms driving species-specific adaptations and thereby community
structure in freshwater systems associated with land-use converted areas.
