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Is erythrocyte size a strategy to avoid hypoxia in Sceloporus torquatus? Field evidence 4 5 Abstract 6 This study examined changes of some different hematological parameters in a reptilian model 7 naturally exposed to altitude-associated hypoxia. Four populations of the Mexican lizard Sceloporus 8 torquatus (Wiegmann 1828) from different altitudes were sampled to evaluate erythrocyte count 9 (Erc), hematocrit (Hct), mean corpuscular hemoglobin concentration (MCHC) and erythrocyte size 10 (Ers). Blood was also assayed to determine hemoglobin ([Hb]), glucose, lactate and electrolytes 11 concentration. Erc was performed using a Neubauer hemocytometer. Hct was calculated as per cent 12 packed cell volume by centrifuging blood samples. [Hb] was determined using a Bausch and Lomb 13 Spectronic Colorimeter. MCHC was calculated with 100 x [Hb]/Hct. Erythrocyte size (Ers) was 14 calculated from blood smear micro-photographs analyzed with the Sigma Scan Pro software. Serum 15 electrolytes (Na + , K + and Ca 2+ ), pH, glucose, and lactate values from blood samples were obtained 16 through a blood electrolyte analyzer. Highland populations of Sceloporus torquatus exhibited a 17 significant increase in erythrocyte count, hematocrit, erythrocyte size, and hemoglobin 18 concentration. In contrast, MCHC showed no correlation with altitude. Additionally, significant 19differences in lactate, sodium, potassium and calcium were observed in highland populations. In 20 general, we found that most hematological parameters were significantly different among lizard 21 populations from different altitudes. This is the first study to report changes in erythrocyte size in 22 relation to altitude, which could be a physiological response to hypoxia. 23 24

High-altitude organisms exhibit hematological adaptations to augment blood transport of oxygen. One common mechanism is through increased values of blood traits such as erythrocyte count, hematocrit, and hemoglobin concentration. However, a positive relationship between altitude and blood traits is not observed in all high-altitude systems. To understand how organisms adapt to high altitudes, it is important to document physiological patterns related to hypoxia gradients from a greater variety of species. Here, we present an extensive hematological description for three populations of Sceloporus grammicus living at 2,500, 3,400, and 4,300 m. We did not find a linear increase with altitude for any of the blood traits we measured. Instead, we found nonlinear relationships between altitude and the blood traits erythrocyte number, erythrocyte size, hematocrit, and hemoglobin concentration. Erythrocyte number and hematocrit leveled off as altitude increased, whereas hemoglobin concentration and erythrocyte size were highest at intermediate altitude. Additionally, lizards from our three study populations are similar in blood pH, serum electrolytes, glucose, and lactate. Given that the highest-altitude population did not show the highest levels of the variables we measured, we suggest these lizards may be using different adaptations to cope with hypoxia than lizards at low or intermediate altitudes. We discuss future directions that research could take to investigate such potential adaptations.