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Lizard suffers world's worst pregnancy

An Australian lizard endures what may be the world's worst pregnancy, scientists say.

Due to anatomical restrictions, pregnant stumpy-tailed lizards must hold a gut-busting infant weighing more than a third as much as an adult, without any swelling or extra body space.

The feat is equivalent to a woman giving birth to a seven-year-old child.

"I can't think of another animal that has such a large gestational load," says Dr Suzy Munns, who recently saw a veritable baby boom with mother stumpies she had collected from the South Australian desert.

Munns, a researcher in the School of Veterinary and Biomedical Sciences at James Cook University, is conducting ongoing studies on how these lizards, which can grow to over 40 centimetres long, survive such a difficult pregnancy.

Munns' findings on the stumpy-tailed lizards, which are also known as sleepy or shingleback lizards, were published recently in the journal Physiological and Biochemical Zoology.

She says stumpies are the largest members of the skink family and can give birth to a single infant that is, on average, 35% of the mother's body weight.

The growing foetus lies on top of the mother's lungs and digestive tract so that, in the latter stages of gestation, the females can no longer move much or eat.

The lizard's abdomen cannot expand because rigid scales cover most of the animal's body.

CT scans reveal that a pregnant stumpy's lungs are compressed, or even collapsed, in certain areas, due to the foetus' weight.

Analysis of the animals' oxygen consumption shows that mothers cannot take in much air, which explains their sedentary behaviour.

Towards the end of the pregnancy, mother stumpies slowly drag themselves between sun and shade, and have difficulty escaping from predators.

Lizard mums put 'all eggs in one basket'

Munns says animals use a variety of strategies to reproduce. Some have a lot of eggs or offspring, many of which die before reaching adulthood. Others, like the stumpy, produce a small number of large babies.

"Stumpies fall into the 'put all your eggs in one basket' scenario: large young, high maternal investment and high individual survival of the young," she says.

"Maternal investment in each baby is high, but the chances of survival for each baby is also high, due to their large size and independence at birth."

One parental perk of the arduous process is that the father does not have to provide care and the mother stumpy offers very little.

Professor Michael Bull, managing editor of the journal Austral Ecology and a researcher from Flinders University, has been studying the behaviour and ecology of these lizards for 25 years. He says Munn's account is accurate.

He says the lizards are also choosy about when they give birth. During drought years, the reptiles often abandon parenthood altogether and "just focus on surviving".

"We think they live for over 50 years, so missing one year of reproduction is not going to make a big difference," Bull says.

"In a way, these lizards are like desert plants, relying on occasional good years to provide pulses of new recruits, but then toughing out the bad years in between."