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Male growth is best described by a von Bertalanffy growth model and female growth by a logistic growth model. This sexual dimorphism is correlated with greater relative surface activity and lower survivorship of juvenile males when compared to juvenile females. Early-hatching (first clutch) offspring exhibit greater body size and survivorship compared to late-hatching (second clutch) offspring. (Abstract shortened with permission of author.)

Field observations indicated a difference in the ability to locomote at low body temperatures in two closely related species of lizards from very different habitats and with radically different seasonal behavior. I measured the critical thermal minimum (the body temperature at which a cooling lizard just loses the ability to right itself) in both species. The winter-active, montane Sceloporus jarrovi had a significantly lower critical thermal minimum in both summer and winter than the winter-hibernating, lowland S. magister. Critical thermal minima were significantly lower in winter than in summer for both species. To determine a physiological basis for these differences, I examined the activity of myosin ATPase, which plays the limiting role in the velocity of muscle contraction, and the energetics of muscle as reflected by high energy phosphate compounds. Microenvironmental conditions were correlated with behavior, constraints on winter activity, and muscle physiology. Ca²⁺-activated myosin ATPase activity in S. magister of valley bottoms is greater than that in the vertical rock-dwelling S. jarrovi. No seasonal acclimatization occurs in myosin ATPase activity in either species. Changes in the muscle metabolism of hibernating animals has been attributed to the lack of muscular contractions in the dormant animals. I measured levels of phosphorylated compounds in a hindlimb muscle from summer and winter lizards of both species. Significant seasonal changes occur in some of the phosphate compounds in both species even though, within a given season, respective levels of phosphorylated compounds are similar in both species. Phosphorylcreatine and total acid-soluble phosphate levels increased in winter animals of both species. Apparently the high levels of phosphorylcreatine in winter S. magister are not simply due to inactivity; winter-active S. jarrovi contain similar amounts. Seasonal cycling of phosphate compounds may relate more to parathyroid status than to muscle activity. Winter activity in S. jarrovi was site-specific and highly dependent on a favorable microclimate. Winter dormancy in S. magister apparently is not dictated by the severity of the microclimate nor physiological limitations of skeletal muscle, but may be strongly influenced by the thermal inertia of that relatively large species.