First warm-blooded lizards switch on mystery heat source at will. Newscientist.com
The first known warm-blooded lizard, the tegu, can heat itself to as much as 10 °C above its surroundings – making it unique among reptiles.
But bizarrely, it only switches on its heating system at certain times of the year.
The discovery may add to the debate about whether dinosaurs were warm- or cold-blooded, or something in between – a bit like these lizards.
It may also provide a clue to how warm-bloodedness, or endothermy, first evolved, says Glenn Tattersall of Brock University, Canada, who co-led the research with Cleo Leite at the Federal University of São Carlos in Brazil.
Unlike mammals and birds – which can heat themselves – reptiles, amphibians and fish generally depend on absorbing heat from their surroundings.
The team studied the Argentinian black and white tegu (Salvator merianae), a 60-90-centimetre-long lizard that lives across much of South America.
As expected, for much of the year it cooled down when the sun set, reaching a low similar to that of its burrow between about 4 and 6am.
The surprise came when the lizards reached the reproductive time of the year, from September to December. During the cold early hours of the morning in that season, their breathing and heart rates rose and their temperatures reached as much as 10 °C above those of their burrows.
The discovery was so unexpected that the scientists took a further three years to confirm it, says Tattersall. “We would expect them to be as cold as they possibly could be at that time,” he says.
Going into overdrive
Even when the scientists removed access to sunshine or food for a few days, the lizards still warmed up before dawn. But how do they do it?
Last year another group reported the first known warm-blooded fish – the opah – which generates heat by the muscular flapping of its fins.
But in the case of the tegu lizard, no one yet knows how it generates its heat. Tattersall believes it secretes a hormone that causes one or more tissues to go into overdrive, possibly those of the liver, heart or muscles, producing heat in the process.
And while there are other reptiles that can stay warmer than the environment, such as female pythons that shiver to heat their eggs, the tegu is unique in the size of the temperature rise, and in the fact that the phenomenon exists in both sexes.
Extra energy needed
How endothermy ever arose is a mystery since it requires so much extra energy and therefore foraging time, making it disadvantageous to take first steps towards it.
One theory, the parental care hypothesis, suggests that there is an advantage to producing extra heat during reproduction, because the heat speeds up the process and provides the energy for a more attentive style of parenting.
Tattersall believes that the tegu may represent such a transition state from cold- to warm-bloodedness.
Stephen Brusatte, palaeontologist at the University of Edinburgh in the UK, agrees. “The fully warm-blooded conditions of birds and mammals may have evolved gradually as their ancestors tinkered with their metabolic systems and went through a hybrid phase like this new lizard shows,” he says.
But Tom Kemp, emeritus research fellow at the University of Oxford, says that warm-bloodedness is such a complex phenomenon that it is too simplistic to think that any one feature or adaptive purpose is alone responsible for its evolution.
Journal reference: Science Advances, DOI: 10.1126/sciadv.1500951