Physiological ecology of a tropical dragon, Lophognathus temporalis

Abstract

Lophognathus temporalis is an arboreal lizard from the wet-dry tropics of Australia. During the wet season the field metabolic rate (FMR) of the lizards was 209 kJ kg Ϫ1 d Ϫ1 , but during the dry season FMR was only 62 kJ kg Ϫ1 d Ϫ1 . Similarly, water flux decreased from 73.6 mL kg Ϫ1 d Ϫ1 in the wet season to 18.5 mL kg Ϫ1 d Ϫ1 in the dry season. Body temperatures (Tb) were significantly lower in the dry season, and operative temperatures, calculated by incorporating microclimatic data with characteristics of the lizards, indicated that the seasonal shift was due to changes in thermoregulatory behaviour rather than limitations of the thermal environment. By combining field measurements of Tb and FMR with laboratory measurements of standard metabolic rate over a range of Tb, we were able to subdivide the FMR into its components and to determine which factors contributed to the seasonal reduction in energy expenditure. During the dry season, lizards used 147 kJ kg Ϫ1 d Ϫ1 less energy than during the wet season, and 24% of this decrease was estimated to be due to the passive effects of lower nighttime Tb, 14% was due to the active selection of lower daytime Tb, 27% was due to the physiological shift to lower standard metabolic rates, and 35% was due to reduced activity in the dry season. Although the population size remained relatively constant (107 lizards ha Ϫ1 during the wet season and 125 lizards ha Ϫ1 during the dry season), the population structure changed, reflecting the seasonal patterns of recruitment and mortality. The number of lizards active at any one time was much lower in the dry season, reflecting the lower levels of activity in this season. The energy expenditure of the population of L. temporalis was 612 kJ ha Ϫ1 d Ϫ1 during the wet season and 113 kJ ha Ϫ1 d Ϫ1 during the dry season.

Energetics of bluetongue lizards ( Tiliqua scincoides ) in a seasonal tropical environment

“…The seasonal pattern of low FMR during the dry season is correlated with measures of food availability, as is the case for the other lizard species that have been measured in wet-dry tropics (Christian and Green 1994;Christian et al 1996bChristian et al , 1996cChristian et al , 1996dChristian et al , 1998Christian et al , 1999aChristian et al , 1999b. The calculated indices (AR, %AR, AS, and SusMS) are generally high compared to other species, suggesting that a large proportion of the daily energy budget of this species is expended on activity (however, see an alternative analysis below).…”

“…The daytime T b of bluetongue lizards are several degrees cooler than those of most diurnal lizards, which also reduces TRM. These relatively low T b are due to thermoregulatory behaviour in the wet season, and due to the relatively cool temperatures inside the burrows in the dry season compared to above-ground refugia used by other species in the same region (Christian and Bedford 1995;Christian et al 1996cChristian et al , 1999b. The dry season burrows occupied by bluetongue lizards are more than 3C cooler than the burrows used by Varanus gouldii at the same time of year .…”

“…Isotopic samples were measured at the Division of Wildlife and Ecology, CSIRO, Canberra. The techniques for sample analysis to determine isotopic measurements of FMR and water flux are described in detail elsewhere (Christian and Green 1994;Christian et al , 1996bChristian et al , 1996cChristian et al , 1996dChristian et al , 1998Christian et al , 1999b. Because bluetongue lizards are omnivorous (Greer 1989;Christian, personal observation), the CO 2 values determined by the isotopic technique were converted to units of energy using the energy equivalent of 23.7 kJ l 1 CO 2 , which is halfway between the conversion factors for a herbivorous diet and a diet of insects (Nagy 1982a).…”

“…Animals were kept at each temperature (18, 24, 30 and 36C) in a controlled temperature chamber (Forma Scientific) for 12 h prior to, and during, measurements. Details of the gas analysis system have been provided elsewhere (Christian et al , 1996b(Christian et al , 1996c(Christian et al , 1999b. SMR was measured over 50 min intervals between 0000 and 0600 hours.…”

“…Others are active to some extent, but nevertheless expend less energy in the dry season compared to the wet. This reduction in field metabolic rate (FMR) can be accomplished by physiological mechanisms such as metabolic depression (Christian et al 1996b(Christian et al , 1996c(Christian et al , 1999a(Christian et al , 1999b, and behavioural strategies such as thermoregulation at lower body temperatures (T b ) (Christian and Bedford 1995, Christian et al 1996c, 1999b and reduced activity (Christian et al 1996b(Christian et al , 1996c(Christian et al , 1999b. Determining which strategies are employed by a species is critical to our understanding of the annual or life-time energy budgets of the animals.…”