Respiratory Biology during Gravidity in Crotaphytus collaris and Gambelia wislizenii

—During gravidity lizards experience a striking decrease in lung volume as a result of lung compression by eggs growing within the body cavity. In order to understand the effect of this decrease in lung volume on the respiratory biology of gravid egg-laying lizards, we measured changes in total lung volume, resting and postexercise expired volume, minute volume, respiratory frequency, and carbon dioxide production rate during reproduction in the Collared Lizard, Crotaphytus collaris, and the Leopard Lizard, Gambelia wislizenii. We found that compression of the lungs by shelled eggs resulted in an average 48% (range: 26–70%) decrease in total lung volume compared to the same postlaying C. collaris females, and an average 38% (range: 29–46%) decrease in G. wislizenii. CO2 production rates were altered significantly during reproduction in female C. collaris and were 58% higher in females carrying late-stage follicles, compared to after laying. Despite the remarkable reduction in lung volume in both of these species and the increase in CO2 production rates in C. collaris, no ventilation parameters changed over the course of reproduction. The highly distensible body cavities of C. collaris and G. wislizenii appear to be able to accommodate both growing eggs and adequate lung volumes for normal respiratory function during gravidity. The respiratory system functions to match oxygen uptake and carbon dioxide release with the metabolic requirements of an organism. In ectotherms, changes in physical and physiological states, such as temperature, feeding and digestion, and exercise, alter metabolic requirements and result in dramatic compensatory changes in ventilation. This change in ventilation is achieved by altering the amount of air that is inhaled (tidal volume), breathing frequency, or both. For example, postprandial (after eating) increases in metabolic rate in the Savannah Monitor Lizard Varanus exanthematicus resulted in tidal volumes that were almost twice that of resting values, whereas breathing frequency remained about the same as resting (Hicks et al., 2000). In contrast, elevated metabolic rates due to exercise in the same lizard resulted in striking increases in breathing frequency and smaller than resting tidal volumes (Wang et al., 1997). Gravidity has also been shown to increase metabolic rates in lizards (Angilletta and Sears, 2000), but if and how ventilation changes to compensate for increased energetic costs during gravidity in egg-laying lizards is unknown. In fact, most of the literature on the respiratory biology of lizards has focused on males and nonreproductive females (Dawson and Templeton, 1963; Wood et al., 1995; Hicks et al., 2000; Butler et al., 2002), so we know little about the physiological changes that occur during a period that is crucial to the survival of individuals and