Ask a scientist: What makes us warm-blooded?

You and me baby, we ain't nothing but mammals, but what is it that makes us that way?

Stuff reader Steve Rencen has wondered that exact same thing, which prompted him to ask us:

"What actual mechanism or system in animals, including humans, determines whether we are hot or cold-blooded?"

Professor Bill Davison from Canterbury University's School of Biological Sciences is one of New Zealand's leading minds in this area, and he has lent his wisdom to help us answer Steve's query.

Davison said all vertebrate animals have a mechanism for detecting temperature, "located in the hypothalamus - a region of the brain - with signals reaching it from internal receptors, monitoring the core temperature, and skin receptors".

"Most animals are ectotherms (cold blooded) with their body temperature determined by external temperature, and these animals use behavioural thermoregulation to control their core temperature.

"Two examples are lizards basking in the sun to warm up, and fish in a lake moving up and down the water column to find a preferred temperature."

Davison said mammals and birds were endotherms (warm blooded), "with a high, constant, core temperature fuelled by internal heat sources".

He said it was the hypothalamus in a mammal that acted as a thermostat to regulate temperature.

"When core temperature starts to fall, the posterior region of the hypothalamus signals to the body to start heat generating processes, such as brown fat metabolism and shivering.

"When heat generation increases the temperature beyond a set point, the anterior hypothalamus becomes active to shut down heat production. These two opposing mechanisms keep core temperature in a human constant to within 0.1 degree celcius," Davison said.

But why are animals either hot-blooded or cold-blooded in the first place?

"Cold blooded is the primitive condition and most animals fit into this category by default. Birds and mammals evolved endothermy (warm blooded) independently," Davison said.

"We currently think that endothermy evolved for locomotion purposes. Hot muscles allow an animal to move quickly, and endothermy allows the animal to remain active at night when external temperatures are low.

"Two more warm blooded animals are bumble bees that warm up their flight muscles before takeoff, allowing them to be active on cold mornings, and tunas which have hot swimming muscles."