External and internal influences on indices of physiological stress: II. Seasonal and size-related variations in blood composition in free-living lizards, Sceloporus occidentalis.

Seasonal changes in blood composition (plasma osmolality, total plasma protein, hematocrit) in two free-living populations of fence lizards, Sceloporus occidentalis, one living in a very arid environment (Pearblossom, CA) and another in a milder desert (Bend, OR), were analyzed. In this analysis, two features of reptilian physiology and ecology were incorporated: ontogenetic variation in blood composition and seasonal variation in body-size distribution. The population living in the arid environment, but not the mild environment, exhibited significant seasonal changes in blood composition: mean plasma osmolality increased (345 +/- 4 to 356 +/- 3 mOsmol/kg) and mean plasma protein (4.19 +/- 0.20 to 3.34 +/- 0.14 g/dl) and hematocrit decreased (43.8 +/- 0.3 to 31.6 +/- 0.4) from April to August. These changes suggested that the physiological condition of the California population was affected by seasonal changes in the external environment. However, among individuals, a significant portion of the variation in blood composition was associated with an internal factor, body size. Larger lizards had higher osmolality, total protein, and hematocrit than smaller lizards in all seasons. In addition, the mean body size of active lizards decreased from April to August in California, but not Oregon. Because the body-size distribution of active lizards in California shifted between seasons, changes in mean population values of these blood parameters probably do not accurately reflect changes that occur within individuals and therefore may be misleading in assessing the physiological consequences of drought.