Human and Hummingbird Hemoglobin Concentrations and MetabolicRhythms at Altitude Determined with Statistical Modeling

Abstract

Hummingbirds show remarkable adaptation to high altitude hypoxia whereas humans are imperfectly adapted to high altitude living. Here we compare hemoglobin levels and metabolism derived from growth rhythms in hummingbirds and humans. To compare growth rhythms, we analyzed growth intervals in hummingbird tail feathers and human growing tissues such as hair. We find that hemoglobin levels were higher in hummingbirds (P<0.001) than in humans, but the influence of altitude on hemoglobin was more pronounced in humans (slope, steeper with increasing altitude, P<0.001), and levels for both taxa converge at extreme elevations. The power spectra from growth intervals in growing tissues which reflect metabolism in both species, were not different (low frequency/high frequency ratios (LF/HF) in the two species) P>0.22 NS. In a comparison among hummingbird species, we found no evidence that metabolic demands (based on power spectra derived from growth intervals) changed with increasing altitude, even while body mass increased significantly (P>0.02). Our index of hummingbird metabolism (spectral LF/HF ratio) was consistent with estimates based on allometric conversion of mass for humans. These results support the notion that hummingbird hemoglobin levels and metabolism are useful models for biologically adaptive strategies to life at high altitude. Humans and hummingbirds exhibit convergent phenotypes for hemoglobin concentration at extreme altitudes. However, whereas human health suffers above 2500 m, hummingbirds are evolutionarily successful and physiologically robust at very high altitudes. Such different outcomes may be in part due to ancient versus recent high altitude colonization, but may also reflect greater altitude-specialization of hummingbird species, fundamental differences between avian and mammalian respiratory systems, or the very different demands of thermoregulation in hummingbirds versus humans.