Carnivorous Lizards Research Articles

Density-dependent foraging strategy of a large carnivorous lizard, the savanna monitor (Varanus albigularis).

Density-dependent foraging strategy of a large carnivorous lizard, the savanna monitor (Varanus albigularis).

Density-dependent foraging strategy of a large carnivorous lizard, the savanna monitor (Varanus albigularis).

Density-dependent foraging strategy of a large carnivorous lizard, the savanna monitor (Varanus albigularis).

Postbite elevation in tongue-flicking rate by an iguanian lizard,Dipsosaurus dorsalis

The herbivorous iguanid lizardDipsosaurus dorsalis exhibited PETF (postbite elevation in tongue-flicking rate), an increase in tongue-flicking rate after experimental removal from the mouth of food that had been bitten. This was demonstrated by a significantly higher tongue-flick rate after having bitten food than in three experimental conditions controlling for responses to the experimental setting, sight of food, and mechanical disturbance caused by the experimental removal of food from a lizard's mouth. As in most other families of lizards, PETF was brief, occurring only during the first minute. Lizards are divided into two major suprafamilial taxa, Iguania and Scleroglossa, consisting of carnivorous species characterized by two major foraging modes, ambush and active, and of herbivores and omnivores. PETF is absent in the two families of carnivorous iguanian lizards studied that are ambush foragers but present in three families of scleroglossan lizards that are active foragers. However, PETF is absent in the two species studied in a scleroglossan family, Gekkonidae, which forages by ambush, and present in an iguanian herbivore, as reported herein. We propose that the presence or absence of PETF, in addition to its phylogenetic determinants, is adaptively adjusted to foraging mode.

Island/mainland body size differences in Australian varanid lizards.

Island varanids seem to be an exception to the rule that territorial vertebrate taxa often become gigantic relative to mainland relatives when on islands, whereas non-territorial species become dwarfed (Case 1978). However, no systematic island/mainland studies have examined the empirical size trends in this group of carnivorous lizards. We perform such an analysis for the Australian region and critically evaluate various selective agents that might be responsible for size changes in several island populations. Insular gigantism occurs at least four times among the island populations examined. The magnitude of size change is positively correlated to prey abundance on the islands (as indirectly measured through a condition index of the lizards, essentially a measure of how fat they arc) and the size of prey: islands with large prey have large varanids and vice versa. Since the island population with the largest size change, the Reevesby Varanus rosenbergi, was introduced less than 100 years ago, these size changes can be quite rapid. This might indicate that selective coefficients are strong; however, we can not exclude the possibility that these size differences have no genetic component and simply reflect environmental differences in growth rate and shifts in age structure between island and mainland locations.

Field energetics of a large carnivorous lizard, Varanus rosenbergi

Field metabolic rates (FMRs; CO2 production) and water influx rates of Varanus rosenbergi were measured seasonally by means of doubly-labelled water (3H218O). Metabolic rates and water influexes were highest in summer (0.18 ml CO2 g-1 h-1 and 17.9 ml H2O kg-1 day-1) and lowest in winter (0.04 ml CO2 g-1 h-1 and 6.5 ml H2O kg-1 day-1). FMRs and water fluxes were intermediate in spring. It is estimated that a 1-kg V. rosenbergi would need to consume 4.7 kg of prey each year to remain in energy balance and that the energy commitment to egg production requires a 40% increase in the food acquisition of a gravid female in late spring and early summer. It is calculated that water influxes in summer are totally provided from food but in other seasons, particularly winter, pulmocutaneous water exchange and/or drinking are significant avenues of water intake.

Energy Expenditure of a Large Herbivorous Lizard

The total daily (24 hr) energy expenditure of the large (mean weight 240 g) herbivorous lizard Egernia cunninghami has been calculated, and partitioned into the daily costs of maintenance, food-gathering, and thermoregulatory activity. Energy expenditure was calculated from equations relating oxygen consumption rate to activity, body weight and body temperatures. Duration of activity was recorded for lizards of known weight observed continuously during daylight hours in the field. The daily cycle of body temperature of each observed lizard was assembled from the normal activity temperatures, heating and cooling rates and retreat temperatures. Although the weight-relative energy costs of maintenance and activity are much smaller for E. cunninghami than the 4 g insectivore Uta stansburiana, the cost per individual is considerably greater for the larger lizard. However, E. cunninghami is active for a much shorter portion of its time abroad (< 8%, < 25 min day-1) than U. stansburiana (30%, 235 min day-1) and uses less of the total daily energy expenditure in activity (10% cf. 40%). The average duration of thermoregulatory movement was 2.6 min for E. cunninghami cf. at least 78 min for U. stansburiana. Little time is spent by E. cunninghami in foraging for the leaves and flowers of legumes; food-gathering effectiveness, the percentage of metabolized energy used for activities other than food-gathering, is estimated between 91 and 95%. It is suggested that the paucity of large carnivorous lizards results from the prohibitive costs of gathering vertebrate prey.

Digestive Efficiency in the Herbivorous Lizard Ctenosaura pectinata

The digestive efficiency of the herbivorous lizard, Ctenosaura pectinata, feeding on a low-fiber plant food, was determined to test the animal's ability to digest non-fibrous plant material. Their digestibility coefficient was 86.3%, which is as high as that of several carnivorous lizards fed insects. The animals were free to regulate their body temperature during the study and usually maintained a daytime body temperature of 37 C. Under these conditions food passage rates were approximately four days and energy was assimilated at the rate of 10.7 cal g-1 day-1. There was no correlation between changes of body weight and efficiency of digestion.

Excretion of urate salts by reptiles

1. 1. Several lizards and snakes and the desert tortoise ( Gopherus agassizii) excrete urinary pellets containing electrolytes (Na, K, NH 4). The low solubility of these cations, together with the high urate content, low chloride content and high urine pH, suggests that most cations are bound as urate salts. 2. 2. The K content of the pellets of herbivorous lizards and Gopherus (3·8 ± 0·12 equiv/kg) is much higher than that of carnivorous lizards (0·88 ± 0·09 equiv/kg) and snakes (1·1 ± 0·13 equiv/kg). Urine pH is also significantly higher in herbivorous reptiles. 3. 3. When maintained on a diet of lettuce, lizards excrete urine of higher pH and pellets of higher K content than when maintained on a diet of mealworms. 4. 4. Fasting significantly decreases urine pH and K content of the urinary pellets of several lizards. 5. 5. The NH 4 content is much higher in the pellets of most snakes (4·1 ± 0·26 equiv/kg) and monitor lizards (4·5 ±0 ·16 equiv/kg) than in most lizards (0·54 ± 0·04 equiv/kg) and boid snakes (0·79 ± 0·03 equiv/kg).