Lizards of the savannas

From Tropical Topics newsletter, No. 78 June 2003, produced by Stella Martin at the Queensland Environmental Protection Agency. Download the PDF to read the whole issue. Acknowledgements to Russell Best, Queensland Parks and Wildlife Service and Tony Griffiths, Key Centre for Tropical Wildlife, Charles Darwin University.

Diverse lizards of Oz

Next to birds, lizards are among the most commonly encountered vertebrate animals in the savannas. Indeed, the diversity of lizards in the tropical savannas is particularly rich, exploiting numerous habitats, from woodlands to sandstone escarpments, rainforests and semi-arid zones, many endemic to just small areas.

Evolution of lizards

The combination of poor soils and dry climate is believed to have led to the evolution of plants with tough, thick, chemical-laden leaves—and relatively few plant-eaters. Instead, the vegetation is broken down by termites, which thrive in Australia—providing an abundant food source for reptiles. Just as mangroves don’t need salt but have developed strategies for living where no other plants can compete, so too have reptiles exploited an environment largely uninhabitable for other animals.

The great diversity of reptile species may be due to the long-term stability, and great size, of the Australian continent. Those animals which cope well with conditions have had plenty of time to evolve into different species exploiting different ecological niches. This diversity may even be fostered by shortages. This may favour large numbers of specialist species which have adapted to localised conditions over fewer, more widely distributed, species which might have evolved in a more benign environment.

Efficiency of lizards

Excepting Antarctica, Australia is the driest continent on earth; 75 per cent of it is arid or semi-arid. The climate is erratic, with long, irregular dry and wet cycles determined by the effects of El Nino and La Nina. Successful animals must cope with unreliable rainfall and prolonged periods of shortages, but take advantage of unpredictable times of plenty. In addition, Australia’s soils are also nutrient-poor, with low productivity, and fires are frequent. Energy-efficiency is thus the key to long-term survival.

Mammals and birds generate their own heat from food, using up to 90 per cent of their energy intake for this purpose. However most lizards, instead of consuming food to generate energy, use solar power, basking in sunshine or sleeping in a warm spot to power up their ‘batteries’ for the day’s (or night’s) activities.

Some lizards, such as dragons, are able to raise their ribs and angle their bodies to increase the body surface exposed and can become darker to absorb more heat. This strategy, known as thermoregulating, is not an option for some lizards which live in closed forests. Instead, they thermoregulate passively, or ‘thermoconform’, their body temperature simply conforming to that of the air around them.

Lizards are also efficient users of water. Their dry scaly skin reduces moisture loss, those living in the hottest areas tending to have compact bodies to minimise loss and/or restricting most activities to hours of darkness. In addition, reptiles do not use water to flush nitrogenous wastes from the body as urine, like mammals do, but convert them to a form which can be dumped as solid dung.

Getting away from predators

Geckos, legless lizards and skinks will readily drop their tails when threatened, the wriggling appendage serving as a distraction while the owner makes its getaway. Fracture planes between certain vertebrae at the top of the tail allow it to detach easily when pulled apart by special muscles. This sacrificial offering is easily regrown. Tails of dragons and goannas can also break off, but do not do so as readily and tend not to regrow. All dragons, goannas and Australian geckos lay eggs but some skinks give birth to live young. Experiments with skink eggs suggested that those kept at ‘maternal’ temperatures were larger and fitter than those kept at normal nest temperatures. This may be an important factor in cool, or dry, climates.

Chattering classes

Most lizards are silent, but geckos chatter and squawk, some species making the characteristic and sometimes startling ‘gecko’ call which has given them their name. Legless lizards may make a high-pitched squeak and some skinks produce a soft squeak.