Hummingbird blood traits track oxygen availability across space and time

Abstract

For endotherms with sustained high metabolism, like hummingbirds, blood-oxygen (O2) carrying capacity should be finely tuned to supply O2 to respiring tissues. Hematological adjustments are expected in response to changes in O2 availability that occur along elevational gradients; however, the rules by which this variation occurs have remained elusive. For example, elevational variation among conspecific individuals may not be comparable to variation among species that have evolved differences in traits, genes, and environmental niches. We sought to elucidate these eco-physiological rules in the hummingbird radiation by analyzing blood traits that affect O2-carrying functions: total hemoglobin concentration ([Hb]), hematocrit (Hct), erythrocyte count (TRBC), mean cell volume (MCV), mean cellular hemoglobin content (MCH), and mean cellular hemoglobin concentration (MCHC). For 1,217 Andean hummingbirds representing 77 species and spanning sea level to 4,578 meters in elevation, we modeled blood traits as responses to environmental conditions and individual and species traits. [Hb] and Hct increased with elevation similarly within species and across the phylogeny-for every 1,000-m ascent, [Hb] increased by one third of a gram per deciliter and Hct increased by one and a half percentage points, corresponding to about three-hundred thousand additional cells per microliter. Hummingbirds adjusted blood cell number and cell size by similar proportions to modulate blood-O2 carrying capacity; however, species at the lowest and highest elevations relied relatively more on adjustments to cell volume, revealing qualitatively different evolutionary responses elicited by moderate versus extreme reductions in O2 availability.

Blood‐ and muscle‐O2 storage capacity in North American diving ducks

“…[Hb] can be increased via changes in red blood cell size, number, or both, and recent work on Andean hummingbirds shows that the adjustment mechanism employed varies with elevation (and thus ambient partial pressure of O 2 ), with lowland species on the higher end of their elevational range and high‐altitude species increasing cell size, while mid‐elevation species increase cell number (Williamson et al 2023). While constant exposure to low ambient O 2 pressure at altitude presents organisms with a different physiological challenge, diving ducks could also be expected to manipulate Hb mass in a similar fashion.…”