Keeping warm has its advantages. Endotherms, which maintain a constant raised body temperature, keep going no matter how chilly their surroundings. Meanwhile, ectotherms are at the behest of the environment: many have to sit tight until they have absorbed enough warmth to function. But which physiological systems did our ectothermic ancestors have to upgrade for us to benefit from internal central heating? Stanley Hillman from Portland State University, USA, and Michael Hedrick from California State University East Bay, USA, explain that there are two competing theories: that ectotherms increased the number of mitochondria in muscle and modified the energy-producing organelles to upgrade to an endothermic lifestyle; or, our ancestors expanded the cardiovascular system to supply the additional oxygen required to fuel our costly, high-temperature way of life. With evidence stacking up on both sides, Hillman and Hedrick decided to review the differences between the cardiovascular systems of ectotherms and endotherms to find out whether expanding the cardiovascular system allowed endotherms to turn up the thermostat.
First, the duo scoured the literature for cardiovascular measurements from a wide range of exercising animals – from ectothermic fish and amphibians to endothermic birds and mammals – to put their theory to the test. But there was one glaring omission: ‘a gap existed in the data for reptiles for [cardiac] flow and pressure during exercise’, they say, before accepting – after contacting many colleagues – that the measurements have yet to be made. However, despite the setback, they interrogated the data to identify differences between the cardiovascular systems of exercising ectotherms and endotherms to find out just how hard the animals’ hearts can work.
Calculating the vascular conductance, cardiac power and work done per heart beat (stroke work), Hillman and Hedrick found that endotherms’ hearts pumped blood at a higher pressure (17.1 kPa vs 3.3 kPa for the ectotherms) and at a higher heart rate (∼5 beats s–1 compared with the ectotherms’ ∼1 beat s–1), allowing the exercising endotherms to produce higher exercise cardiac power. And when they compared the relative size of the endotherms’ and ectotherms’ hearts, they found that the endotherms’ larger hearts enabled them to increase their blood pressure. They say, ‘A major difference between ectotherms and endotherms is the large increase in blood flow rates’, which is largely due to their higher heart rates and allows endotherms to increase the oxygen supply to the power-hungry muscles that keep them warm. However, the duo also found every mW of cardiac power supports a remarkable 158 mW of aerobic power output (which is only 0.6% of the exercise aerobic energy expense) regardless of the animal's lifestyle, showing that the cost of circulation is low for endo- and ectotherms alike.
Focusing on the cardiac changes that were essential for ectotherms to warm up, Hillman and Hedrick explain that our ancestors had to remove the cardiac shunts – which allow amphibians and reptiles to mix oxygenated and deoxygenated blood – to increase oxygen transport. They also had to increase their heart rates by developing the sarcoplasmic reticulum – which controls the calcium levels in muscle that regulate muscle contraction – to increase the heart rate. And finally, endotherms had to increase their cardiac muscle mass relative to the body mass of similarly sized ectotherms to produce the higher blood flow rates that are essential for a metabolically demanding endothermic lifestyle.
The duo says, ‘These results suggest that a key step in the support of endothermy was the greatly enhanced ability of the cardiovascular system to deliver oxygen which accounted for the approximately ten-fold increase in aerobic scope between endotherms and ectotherms’.