Lower haematocrit, haemoglobin and red blood cell number in zebra finches acclimated to cold compared to thermoneutral temperature

Abstract

Thermoregulation constitutes an important share of the energy budget of endotherms. Elevated thermoregulatory requirements must be met by oxygen supply through the blood, as heat is produced mainly via aerobic processes. In contrast to mammal studies, it remains unclear whether elevated thermoregulatory needs are followed by changes in haematological variables in birds. We investigated haematocrit (HCT), haemoglobin content per volume of blood (HGB), number of red blood cells (RBCcount), and size of the erythrocytes (RBCarea) in zebra finches Taeniopygia guttata acclimated to either cold or thermoneutral ambient temperatures under laboratory conditions. Seventy‐nine females were maintained for six weeks either in cold (T = +12°C) or thermoneutral (T = +32°C) ambient temperature prior to blood collection. On average, HGB, HCT and RBCcount were significantly lower by about 10% in cold acclimated compared to thermoneutral acclimated birds. Only RBCarea was not different between the two acclimation temperatures. Mean HCT, one of the most commonly measured haematological variable for example was 53 ± 0.9% (LSM ± SEM) in thermoneutral and 49 ± 0.8 % (LSM ± SEM) in cold acclimated zebra finches. On first sight, the observed lower values for three out of the four determined haematological variables in response to acclimation to cold question oxygen supply to be indeed a limiting factor for heat production. However, higher demands of oxygen supply due to increased thermoregulation in birds may instead require specific optimisation of blood viscosity and modulation by other cardiovascular properties. Nucleated red blood cells in birds may pose different strain on blood viscosity compared to non‐nucleated mammalian erythrocytes and explain the contrasting response in haematological variables to temperature acclimation between birds and mammals.

Contrasting response of haematological variables between long-term training and short exercise bouts in zebra finches (Taeniopygia guttata)

“…The study, performed on the same model species as here, suggests different constraints on oxygen supply through blood and showed lower Hct, Hb content and RBC count in birds kept at a cold versus thermoneutral temperature, the opposite response to that in mammals. This discrepancy between birds and mammals might be driven by the presence of a nucleus in avian erythrocytes in contrast to the anucleate mammalian red blood cells (Niedojadlo et al, 2018). The cold-induced changes in haematological variables thus resemble those reported by us for the long-term response to training rather than those regarding short-term exercise bouts and clearly call for more studies on birds under controlled conditions of different energy use or requirements.…”

“…On the other hand, the immediate response to single exercise performance involves either a decrease in Hct or no change, which was observed either after flights up to 1 h or after prolonged flights reaching 22 h in pigeon (Columba livia) and red knot (Calidris canutus) (Butler et al, 1977;Bordel and Haase, 1993;Carmi et al, 1993;Peters et al, 2005;Jenni et al, 2006). Finally, a recent report considering the response of haematological variables to increased energy in birds stands in contrast to the known responses for mammals (Niedojadlo et al, 2018). The study, performed on the same model species as here, suggests different constraints on oxygen supply through blood and showed lower Hct, Hb content and RBC count in birds kept at a cold versus thermoneutral temperature, the opposite response to that in mammals.…”

Re-evaluating model assumptions suggests that Australian birds are more tolerant of heat and aridity than predicted: a response to Conradie et al. (2020)

“…There is also considerable evidence of phenotypic plasticity; zebra finches respond to chronic and acute acclimation and acclimatization to varied T a and water availability by adjusting reproductive, cellular and thermal physiology, including metabolic heat production, evaporative heat loss and/or T b (e.g. Salvante et al , 2007 ; Niedojadlo et al , 2018 ; Cooper et al, 2019 , 2020a , 2020b ; Szafrańska et al , 2020 ; Wojciechowski et al, 2021 ).…”

Variation in erythrocyte morphology in alpine accentors (Prunella collaris Scop.) from Tian Shan, Rila and the High Tatra mountains and effects of molting

“…Carey and Morton (1976) found a trend of increased Hct at a higher elevation in a variety of wild bird species. In an experimental study of zebra finches (Taeniopygia guttata), hemoglobin (Hg), Hct and erythrocyte counts were on average significantly lower in birds acclimated to cold (Niedojadlo et al 2018). Studies comparing the body size of individuals of identical species or close relatives of this species confirm that the weight of individuals increases with increased altitude (Dunson 1965;Carey & Morton 1976;Weinstein et al 1985;Rodríguez-Cajarville et al 2019).…”