Meeting Abstract
The role of hypoxia tolerance in limiting species elevational distributions is poorly understood. Species distributions are predicted to shift upslope with warming, causing exposure to hypobaric hypoxia. Hummingbirds exhibit exceptional O2 consumption rates which make them particularly susceptible to changes in PO2. The lowland Black-chinned Hummingbird and montane Broad-tailed Hummingbird are elevational replacement species that compete for nectar resources at mid-elevations where they overlap. We have previously shown, using a hypobaric chamber that these two species differ in their behavioral response to low pressure. Competitive dominance shifts from the lowland to montane species under reduced pressure conditions. We captured adult males of both species from the zone of overlap to test the effects of reduced pressure on the physiological response. Specifically, we asked does the hematological response to low pressure differ between high and low elevation species. To quantify the hematological response to hypobaric hypoxia, we acclimatized birds of each species at either the pressure equivalent of 1600 m or 4600 m and then collected blood samples. We measured total hemoglobin concentrations, haematocrit and mean cell volume of red blood cells. Black-chinned Hummingbirds show a decrease in the mean cell volume in response to low pressure that Broad-tailed Hummingbirds do not. The hematological response to hypobaric hypoxia seen in low elevation species, but not in high elevation species, mimics the observed behavioral response. This suggests that differential adaptation to atmospheric pressure contributes to the maintenance of stable elevational replacement distributions in hummingbirds.
