Australia's unique monotreme, the short-beaked echidna, is helping zoologists rethink the evolution of endothermy (producing heat to maintain their body temperature).

The recently retired Zoology Professor at the University of Queensland (UQ), Professor Gordon Grigg, rates the discovery of hibernation in echidnas (Tachyglossus aculeatus) living in the snow of the Australian Alps as one of his greatest achievements in over 30 years of zoological research.

I interviewed Professor Grigg in October 2007, and he described the excitement of observing echidnas hibernating above the snowline of Australia's highest mountain, Mt Kosciuszko ( 2,228 metres above sea level). His paper, The Evolution of Endothermy and Its Diversity in Mammals and Birds, was published in Physiological and Biochemical Zoology in December 2004.

Professor Grigg said the research described in the paper, and the conclusions he and his fellow researchers came to, "are leading to new thinking about how endothermy evolved".

The Question: Do Echidnas Hibernate?

There are only two kinds of monotremes in the world, echidnas and platypuses, and they are found only in Australia, Tasmania, or New Guinea.

“Since birds and mammals evolved from reptiles, they developed endothermy and do not need to rely on behaviour to regulate their body temperature as reptiles do. But what about monotremes?” he said.

Because monotremes lay eggs rather than bearing live young, and some aspects of their skeleton are similar to reptiles', they are considered a more primitive form of mammal. Echidnas were not supposed to hibernate like mammals – so how did they survive in the snow?

Using tiny radio transmitters surgically implanted into the abdomens of several echidnas above and below the snowline in Mt Kosciuszko National Park, Professor Grigg and his colleagues, Lyn Beard and Michael Augee, were able to remotely record and analyze changes in the animals’ temperature over several weeks, during summer and again in winter, when external temperatures fell below zero.

In 1987 they were able to prove that snow echidnas do hibernate. Echidnas mate in mid-winter, so their hibernation occurred in short periods of about two weeks, when their temperature dropped to about 10 degrees Celsius, the ambient temperature of their burrow, interspersed with periods of activity over one or two days when their temperature rose briefly into the low 30s.

This unusual observation enabled Grigg and other zoologist to reconsider theories of how endothermy arose in mammals.

Theories of Endothermy Evolution

"Endothermy has arisen in insects, fishes, reptiles, birds, and mammals, as well as in some plants. However the evolution of endothermy in mammals has attracted the most debate and attention, and it is regarded as one of the most significant transitions in vertebrate evolution," he said.

"Unlike current models for the evolution of endothermy that assume that hibernation and torpor are specialisations arising from homeothermic ancestry, and therefore irrelevant, we consider that they are central."

The sophistication of thermoregulation in reptiles, including brooding endothermy (for example in keeping turtles' eggs at the correct temperature to promote hatching) "suggests an incipient capacity for facultative endothermy in reptiles," he said

"Short-beaked echidnas provide a useful living model of what an (advanced) protoendotherm may have been like."

Thus the evolution of endothermy could be seen as a direct result of the benefits conferred by warmth to the animals, "such as expanding daily activity into the night, higher capacities for sustained activity, higher digestion rates, climatic range expansion, and control over incubation temperature."

Not One Simple Answer

Professor Grigg said this new model means there is not just one answer to the question of how endothermy evolved. "It accommodates diversity and recognizes that there need not be only one specific, detailed model, but that different expressions of extent may have been achieved by different selection pressures in different groups in response to different circumstances, and at different times. "