Ecophysiology of Australian Arid-Zone Reptiles

Abstract

Australia’s reptilian fauna is much richer in number of species than that of other desert regions of the world but their ecophysiology has been little studied, and we only have speculations and hypotheses to account for their abundance and diversity. There appears to be no study to date of temperature regulation or osmoregulation of a desert snake, and only a single one of an arid-zone gecko, and most published work has focused on agamid and varanid lizards. A number of agamids have been found to be semelparous, dying after breeding in spring, and thereby avoiding the need for specific adaptations to survive the long, hot and dry summer period. Such specific adaptations have yet to be identified and the inherently low rates of water turnover and resource utilisation of Australian reptiles, coupled with ectothermy and wide thermal safety zones, appear to be the primary reason for their success in arid environments, although they may face future challenges from global warming.

Glossary

Ectotherm

An animal that derives its body heat from an external source, such as the sun.

Eccritic or preferred body temperature (PBT)

The mean body temperature maintained behaviourally by a reptile when placed in a thermal gradient.

Mean body temperature (MBT)

The average body temperature of a reptile when active, which may differ from the PBT because of a lack of thermal resources.

Agamid

A lizard belonging to the family Agamidae, also called dragon lizard, e.g. the ornate dragon lizard, Ctenophorus ornatus.

Varanid

A lizard belonging to the family Varanidae, also called monitor lizards, e.g. the perentie, Varanus giganteus.

Index of thermal exploitation (Ex)

A ratio calculated by dividing the time that a lizard spends within its set-point range by the time available for the animal to exploit this temperature range and describes the thermoregulatory characteristics of ectotherms in a heterogeneous thermal environment.

CTMax

The critical thermal maximum, or highest body temperature, above which the animal is unable to recover.

FMR

Field metabolic rate. The rate of metabolism of a free-ranging animal in its natural habitat measured as either the rate of oxygen consumption or carbon dioxide production, usually with the doubly-labelled water method.

Scincid

A lizard belonging to the family Scincidae, also called skinks, e.g., the bobtail goanna, Tiliqua rugosa.

Vasotocin (AVT, arginine vasotocin)

A peptide hormone released from the posterior lobe of the pituitary gland of reptiles that enhances water reabsorption from the kidney and also lowers the PBT.

Hypernatraemia

An increase in the concentration of sodium ions in the blood above normal hydrated levels.

PT (panting threshold)

The body temperature at which some lizards open their mouth (gape) and increase their respiratory rate (panting).

Metanephric kidney

The structurally more complex kidney of higher vertebrates, which in the case of birds and mammals, is capable of producing an hyperosmotic urine (i.e. one with an osmotic pressure greater than the plasma).

Cephalic salt glands

Specialised glands found in many reptiles that assist in osmoregulation by secreting a concentrated salt solution rich in either sodium or potassium. All are located in the head (nose, eye or tongue) and hence called ‘cephalic’.

Anuric

Ceasing to produce urine; seen in many lizards when feeding on diets rich in salt.

Semelparous

Animals, such as salmon, that die after breeding only once. Named after the unfortunate Semelae, mother of Bacchus, who succumbed after a night of passion with the Greek God Zeus.