Effects of experimental manipulation of hematocrit on avian flight performance in high and low altitude conditions

Abstract

Despite widely held assumptions that hematocrit (Hct) is a key determinant of aerobic capacity and exercise performance, this relationship has not often been tested rigorously in birds and results to date are mixed. Migration in birds involves high-intensity exercise for long durations at various altitudes. Therefore, it provides a good model system to examine the effect of Hct on flight performance and physiological responses of exercise at high altitude. We treated yellow-rumped warblers (Setophaga coronata) with avian erythropoietin (EPO) and anti-EPO to experimentally manipulate Hct and assessed flight performance at low and high altitudes using a hypobaric wind tunnel. We showed that anti-EPOtreated birds had lower Hct than vehicle-and EPO-treated birds posttreatment. Anti-EPO-treated birds also had marginally lower exercise performance at low altitude, committing a higher number of strikes (mistakes) in the first 30 min of flight. However, anti-EPO-treated birds performed significantly better at high altitude, attaining a higher altitude in a ramped altitude challenge to 3000 m equivalent altitude, and with a longer duration of flight at high altitude. Birds exercising at high altitude showed decreased Hct, increased glucose mobilization and decreased antioxidant capacity, regardless of treatment. In summary, we provide experimental evidence that the relationship between Hct and exercise performance is dependent on altitude. Future studies should investigate whether free-living birds adaptively modulate their Hct, potentially through a combination of erythropoiesis and plasma volume regulation (i.e. hemodilution), based on the altitude they fly at during migratory flight.

Haematological traits co-vary with migratory status, altitude and energy expenditure: a phylogenetic, comparative analysis

“…and Clemens found that birds that reside at high altitude year round have high Hct and Hb relative to lowland birds, and that Hb is correlated with altitude in both high altitude residents and elevational migrants 19,21 , despite the fact that Hct and Hb are relatively plastic traits. It is also well known that Hct and Hb is increased prior to and during long distance migration in birds 10,16,22 . Furthermore, Hct and Hb are also highly repeatable within individuals and within species 23,24 .…”

Natural Climbers: Insights from Avian Physiology at High Altitude

“…Species and populations of birds and mammals that are regularly exposed to high altitudes have adaptations to enhance oxygen delivery, including greater HVR and greater oxygen-carrying capacity of the blood (reviewed in Butler, 2010;Scott, 2011;Ivy and Scott, 2014;Scott et al, 2015b). More recent research has shown that high altitude can also act as a strong selection pressure for greater oxidative capacity at the cellular level across both birds (Dawson et al, 2016;Yap et al, 2018;Hao et al, 2019) and mammals (Storz, 2007;McClelland and Scott, 2018). Despite these shared features, the advantages of the avian lung, hemoglobin with a high oxygen affinity and insensitivity to hypocapnia, triumph.…”

“…This appears to be an adaptation necessary to sustain the high oxygen demands of flight (Tucker, 1970), rather than a response to hypobaric hypoxia per se. It is yet to be shown if greater fatty acid metabolism is beneficial at high altitudes, given the higher oxidative cost of metabolizing fatty acids; oxidizing lipids becomes increasingly expensive relative to glycogen with increasing altitude (Melzer, 2011;Yap et al, 2018). Enzymes involved in the metabolism of fatty acids were not found to vary across the locomotor muscles of high-and low-altitude populations of torrent duck (Dawson et al, 2016).…”

Recovery of hematocrit and fat deposits varies by cage size in food‐restricted captive red crossbills (Loxia curvirostra)

“…For example, flight performance was higher in warblers flying at low altitudes when hematocrit was experimentally enhanced but was lower when those same individuals flew at high altitudes, suggesting a cost to manipulating hematocrit (Yap et al, 2018).…”