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The agile wallaby didn't stand a chance when it encountered this python (Source: Paul O'Neill/NT Parks and Wildlife)

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How does a python kill its prey?

Tuesday 13th October 2015 2:45 pm

In the animal world, it’s easy to be a top predator. All you need is eyes on the front of your head, sharp eyes and ears to find your next dinner, lots of muscles on your trunk and limbs, and some weaponry such as big teeth, big claws or big talons.

But what if you have no arms or legs, no ears, no eyelids, and no claws or talons?

In that case, you might be a python. Surprisingly, it was only as recently as 2015 that we discovered how they kill their prey.

First, a few python factoids.

They can go for up to two years without a feed. To cut down on energy usage, they actually shrink their gut down to practically nothing. You see, it takes a lot of energy to run a gut — all those enzymes and liquids to manufacture and recycle.

Pythons can eat an animal that actually weighs more than they do — up to one-and-a-half times their own weight. That means they have to suddenly regrow their gut back into existence. It takes a huge amount of work to do this.

They have to crank up their energy output to up to 45 times normal — and keep it there for a few days. You’ll appreciate how hard this is when you realise that to win a gold medal, an Olympic sprinter will crank up their energy output to 20 times normal — and only for 10 seconds (not 45 times, for three days).

But how do pythons kill their meal?

Well, until 2015, the accepted opinion among the snake scientists, or herpetologists to give them their proper name, was suffocation.

Each time the rat (or whatever their intended meal is) breathes out, it makes its own ribcage a bit smaller. At that exact moment, so went the perceived wisdom, the python would tighten up its grip another notch. After several such tightenings, the rat couldn’t get any air into its lungs and would shortly die of suffocation. There was no real squeezing as such.

But there were a few inconsistencies with the suffocation theory. For example, there was the case of a Malaysian man who had been killed by a python doing its ‘constrictor’ thing, and was being swallowed, head first.

Before the rest of his body had gone in, he had been pulled out of the python. When his corpse was examined at autopsy, he was found to have multiple fractures in his ribs, neck and the rest of his spinal cord. So maybe the python could squeeze with a lot of force…

This was the background to the research by Dr Scott M Boback and colleagues. They wanted to put the crushers to the test.

They anaesthetised a rat, and implanted inside it a whole bunch of measuring devices, using modern technology. They then offered “robo-rat” to their tame and hungry boa constrictor. As per normal, the snake struck the rat’s head to render it unconscious, and immediately coiled its body around the rodent.

As the rat’s blood pressure and heart rate data flooded onto their computer screen, they were astonished to see that within six seconds, the blood pressure dropped down to half. If the rat had been conscious, it would have become unconscious from a lack of blood to the brain. Within 60 seconds, the heart rate dropped from over 400 beats per minute to about half.

You need to realise that the heart is a pump. Like all pumps, it can deliver its liquid load only into a zone of lower pressure. In a regular rat, the background blood pressure in the veins is about 4 mm Hg, while the heart pushes blood out at a pressure of about 82 mm Hg. In this case, the blood can leave the heart and travel, via a slightly complicated pathway, towards the veins.

But once the boa constrictor started to squeeze, the background pressure inside the rat increased to 160 mm Hg within six seconds. There’s no way that a pump with an output pressure of 80 mm Hg can deliver any blood into a zone at a pressure twice as high.

And the analysis of various blood parameters showed that the rat definitely did not die from suffocation.

It’s enough to raise anyone’s blood pressure. Squeezing the truth out of a python’s killer instinct turned out to be heart-stopping …


This blog first appeared on Dr Karl's Great Moments in Science

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