Big mountains; small birds; thin air: Natural history determines variation in hemglobin concentration in Himalayan birds

Background/Question/Methods: Hypobaric hypoxia or the decreasing partial pressure of oxygen with increase in elevation is a global constraint for organisms living at high elevations. Although the relationship between elevation and hypoxia has been established for decades, little is known about how hypoxia might constrain species distribution. Birds are an excellent system to study the effects of hypoxia because they rely on aerobic respiration for thermoregulation and also because flight is extremely energy expensive. Yet there are only a handful of studies on how birds respond to hypoxia with no comparative studies of multiple species along the same elevational gradient. Hemoglobin is a crucial respiratory pigment for oxygen transport in vertebrates. We undertook a comparative study to explore variation in hemoglobin concentration and hematocrit (volume of red blood cells in blood) in 15 species of Himalayan birds along a 2500m gradient. We quantified within and among species variation in hemoglobin concentration and the relationship between hemoglobin concentration and hematocrit. In a linear mixed model framework we incorporated several covariates of natural history to understand the drivers of hemoglobin concentration in these birds. 

Results/Conclusions: We did not find the predicted strong correlation between hemoglobin concentration and hematocrit in all species. Elevational migrants (birds that winter in lowlands and breed in the highlands) showed the predicted positive relationship while resident birds (birds that live year round at the same elevation) did not show any correlation. The linear mixed models also suggest that elevational migrants and resident birds have a functionally different way of regulating hemoglobin concentration. This suggests that natural history plays an important role in how Himalayan birds cope with hypoxia. Our results suggest the importance of taking a comparative community approach while studying the effects of environmental constraints like hypoxia to better understand community level patterns in trait variation.