Microclimate and Ammonia Production of an Unusual Littoral Maternity Roost for Mormoopid Bats, Pteronotus spp., in Pacific Coastal Costa Rica

The metabolism of large numbers of bats roosting in confined cave spaces is expected to produce extreme microclimatic conditions (such as extreme heat, CO2 concentrations, ammonia levels). Here, we report a quantitative estimate of 12,661 ± 1,686 Pteronotus (Mormoopidae) individuals in Costa Rica's largest subterranean bat roost. This small cave has a roost area of only ca. 30 m2, equivalent to an average density of 426 bats/m2. Metabolic products of this dense accumulation of bats were expected to produce extreme concentrations of gaseous ammonia. However, atmospheric ammonia concentration in the cave was shown to fall well below modelled expectations, reaching a maximum of 145 ug/m3. We show that most of the ammonia derived from the breakdown of urinary nitrogen must dissolve into guano pore water and wall condensation rather than be released as gaseous ammonia as would occur under drier conditions. Air temperature in the center of the roost rises to ca. 34°C, consistent with our observations in a wide range of bat caves where roost ambient temperature never equals or exceeds bat body temperature. Moreover, the cave microclimate exhibits a circadian rhythm which we attribute to bat movement.