ENERGY BUDGETS IN FREE-LIVING GREEN IGUANAS IN A SEASONAL ENVIRONMENT

Using a variety of techniques we estimated energy expenditure and allocation of energy in free-living green iguanas (Iguana iguana) in a seasonal environment on Curacao, Netherlands Antilles. 1) Daily energy expenditure (DEE) was measured by means of the doubly labeled water (DLW) technique, using O-18 and deuterium (H-2). The method was validated in green iguanas at ambient temperature and humidity levels occurring on Curacao. Energy expenditures from the DLW method differed by 2.1 +/- 8.2% compared to respirometry and balance methods. This value falls within the range of deviations found in other validation studies, and indicates that the DLW method is acceptable in green iguanas even at high ambient humidity. 2) Average DEE was 71.7 kJ . kg-1.d-1. There were no significant differences in DEE between males and females, although males tended to have higher metabolic rates during the mating season. Energy expenditure on a yearly basis including clutch production in females did not differ between females and males, indicating comparable annual levels of energy expenditure between the sexes. 3) Temperature dependence of standard metabolic rate (SMR) and resting metabolic rate (RMR) were determined by respirometry experiments. RMR increased with temperature with a Q10 of 2.24. In combination with field body temperatures SMR and RMR of free-living iguanas could be determined. SMR amounted to 15-22% of the DEE. DEE was 1.8-2.8 times RMR. 4) The energy expenditure above RMR was allocated between activities involving locomotion and stationary activities. DEE in combination with behavioral data revealed an indirect measure of the costs linked to locomotion in the field, including climbing (255 kJ.km-1.kg-1). Locomotion with a climbing component imposed six times the costs of horizontal walking. Although time spent locomoting was only a very small fraction of the total time, the costs linked to locomotor activities amounted to almost-equal-to 23% of the daily energy expenditure. Postural adjustment costs were almost-equal-to 33% of the DEE. Locomotion could explain 78% of the observed variation in DEE. If other activities, such as foraging and social activities, were included in multiple regression analyses, 96% of the variation in DEE could be explained. 5) Body condition of males decreased during the mating period (March/April), while most of the decrease in body condition of females occurred during the time of oviposition (May/June). Time spent in social activities was higher during the mating period, especially so in males, but time devoted to other activities (locomotion, foraging) did not differ significantly between the seasons. Comparison between the sexes revealed that males spent more time locomoting than females, throughout the year. 6) Differences in body condition in the course of the year were not due to differences in DEE, but were mainly a result of differences in metabolizable energy intake. Though on a yearly basis energy expenditure was equal in both sexes, energy allocation differed between the sexes. Females devoted almost-equal-to 15% of their annual energy budget to the production of eggs, while males showed heightened social activity during the mating phase and spent twice as much time in locomotor activities than females