Immunity, reproductive success, and stress in cave- and bridge-roosting Brazilian free-tailed bats (Tadarida brasiliensis): Implications for conservation

As human pressures on wildlife increase, organisms able to acclimate to human-altered environments should have a selective advantage over those unable to do so. The Brazilian free-tailed bat (Tadarida brasiliensis ) is an excellent model for investigating the effects of anthropogenic disturbance because it occupies a variety of roost types, both natural and human-made, including bridges. I hypothesized that bats living in bridges would be in poorer health, due to the putative stressors present at these roosts, compared to bats living in caves. I investigated the effect of roosting ecology on immune system function, reproductive success and stress in T. brasiliensis living in different roost structures. Immune measurements included in vitro bactericidal ability of whole blood and in vivo cell-mediated response to mitogenic challenge. Both colony and roost-type effects were found in response to innate and adaptive immune tests. Bats from cave roosts showed greater adaptive immune responses while bats from bridge roosts invested more heavily into innate immunity. I provide evidence of a negative correlation between innate and adaptive immune responses within individuals. I also studied roost-type differences in reproductive success. Birth size and post-natal growth of pups were examined, as surrogates of reproductive success. Bats born at the bridge site were significantly larger at birth and grew faster than bats born in the cave site, in both years of the study. Several factors including roost-related conditions, climate, and maternal effects, likely account for these findings. Finally, I investigated ecological variation in stress. Stress was assessed by examining roost-type differences in plasma cortisol (stress hormone) levels. The main finding was that bats at bridges had significantly lower baseline levels of plasma cortisol later in the season, during lactation and post-lactation. Overall, bats living in bridges do not appear to be adversely affected by this roost environment and in some measures appear to be in better health than bats living in natural caves. My study represents the first investigation in the field of conservation physiology of the effects of roost choice on health measures within the Chiroptera. These results have implications for the conservation of this and other vulnerable species faced with similar human pressures.