Energetics of growth and body size in the lizard Sceloporus undulatus: Implications for geographic variation in life history

The fence lizard, Sceloporus undulatus, provides a unique opportunity to investigate the causes of phenotypic variation, because it ranges over half of North America and life history traits vary by as much as twofold among populations. Sceloporus undulatus exhibits latitudinal patterns of life history that are consistent with those observed in many ectotherms; lizards at low latitudes exhibit fast growth, early maturity and small body size, relative to lizards at high latitudes. The covariation between latitude and life history suggests that the thermal environment is a major cause of life history variation in S. undulatus. I studied the thermoregulatory behavior of lizards in two populations of S. undulatus (New Jersey and South Carolina), and its consequences for the rates of physiological processes related to growth. The thermal sensitivities of metabolizable energy intake and maintenance metabolic rate were quantified over a range of body temperatures experienced by field-active lizards. Despite major differences between the thermal environments of New Jersey (NJ) and South Carolina (SC), lizards in both populations used behavioral thermoregulation to maintain a body temperature that maximized net energy assimilation (= metabolizable energy intake - maintenance metabolism). However, three findings suggest that the annual production budget of a SC lizard is greater than that of a NJ lizard; SC lizards have: (1) a higher metabolizable energy intake at average field body temperature, (2) a lower maintenance metabolic rate at average field body temperature, and (3) a longer daily exposure to preferred body temperature in spring and fall. Thus, SC lizards have a greater potential for growth during the active season than NJ lizards. I argue that latitudinal patterns of growth and body size in S. undulatus are caused by variation in the growth potential of lizards and the seasonality of reproduction