Staying warm takes an awful lot of effort. Mammals, and other warm-blooded creatures, ramp up their metabolic rates to keep the internal fires burning. However, fires don't rage unless they're well stoked, so mammals have souped-up their cardiac systems and raised their systemic blood pressure for efficient oxygen delivery. However high blood pressures wouldn't suit all systems, so endothermic animals divided their cardiac flow in two, developing two ventricles that separate low-pressure blood directed to the lungs, from the high-pressure systemic flow. On the other hand, ectothermic creatures with their meagre metabolic loads have no need for high blood pressure to keep their bodies fuelled. Instead they retained a partially divided cardiac ventricle that serves both body and lungs with a low pressure blood supply. All that is except for a few particularly active reptiles; their dividing ventricular ridge is much more developed. Knowing that the modified ridge was capable of separating the pulmonary and systemic blood flows in Python molurus, Tobias Wang wondered whether the ridge might produce sufficient separation to allow both halves of the ventricle to generate separate pressures too. Teaming up with Jordi Altimiras, Wilfried Klein and Michael Axelsson, Wang measured the pressure inside a Python molurus' beating heart; the systemic pressure was seven times higher! The ridged ventricle was behaving just like a mammal's divided heart (p. 4241).

Converging at Wang's lab in Denmark, the team fitted young pythons with catheters and waited for them to regain consciousness before measuring the arterial blood pressures. Wang explains that the measurements were quite straightforward, and he was delighted when his suspicions were confirmed; the blood pressure in the systemic artery was seven times greater than the pressure in the pulmonary artery. Next the team inserted catheters into the ventricle, and measured the pressures generated in both halves. The ventricle was behaving as if it had two separate chambers. Although the muscular ridge doesn't entirely separate the ventricle, it somehow allows the heart to deliver blood at high pressure to the systemic system, while protecting the lungs from pressure damage.

But `how' and `why' the python has opted for an endothermic-style heart with an ectothermic life style is a bit of a mystery. Working with Axelsson and Carl Løfmann, Wang has already answered `how', by inserting an angioscope into a perfused python heart to watch the ridge during a contraction cycle. `There is no doubt that the muscular ridge provides a complete separation of the two sides of the heart immediately after contraction has commenced' says Wang.

The `why' question is much more intriguing. Although pythons are well known for their lethargic approach to predation, the reptile's metabolic rate rockets after a feast. Pythons also generate huge amounts of heat by shivering while incubating their eggs. Wang wondered whether these enormous metabolic demands had driven the snake to develop the large pressure-separating ridge? But having failed to find pressure separation in other snakes with high digestive metabolic rates, while identifying pythons that retain high blood pressure even though they abandon their eggs, Wang admits that he isn't sure what is going on. `God knows what it's all about' he says, but is optimistic that looking at even more distantly related python species could eventually solve why python's have divided hearts.