If it's been a while since breakfast and your stomach is rumbling, spare a thought for the snakes that survive for eight or nine months without a meal. During these incredibly long fasting periods, snakes don't have much use for their gut, so they simply shrivel up and shut down their digestive tract to save energy. But as soon as they gobble up an unsuspecting victim, they need to get their digestive system up and running again pretty fast. `All this makes snakes fascinating models to study the physiology of digestion', says Matthias Starck of the University of Munich. He decided to take a closer look at snakes' guts as the animals digested a meal after a period of fasting(p. 881).
Starck explains that mammals simply make new tissue to bulk up their digestive system after a period of starvation. For a while, digestion researchers assumed that snakes use this energetically expensive solution too,but this turned out to be completely wrong. `A much cheaper way to increase the absorptive surface of your gut quickly is to blow it up like a balloon by pumping blood into it' Starck says, `and we suspected that this is what snakes do.'
Teaming up with Christian Wimmer, Starck set out to test this `hydraulic hypothesis'. Using non-invasive ultrasound, the team hoped to correlate changes in blood flow volume with increases in the size of snakes' guts and livers after a meal. They measured the organs of six hungry ball pythons, fed the snakes and watched over the next two days as the snakes' bodies adjusted to the meal. Looking at the ultrasound images, Starck and Wimmer saw that the gut and liver dramatically increased in size after the snakes had devoured a mouse.
But was this enlargement due to increased blood flow? The pair used Doppler ultrasound imaging to measure blood flow volume from the aorta to the gut and from the gut to the liver. Sure enough, they saw that blood flow to and from the gut increased after the meal, accounting for half the organ size increase. The rest of the increase in organ size was still a mystery. But when Starck and Wimmer examined tissue samples taken from the snakes' guts and livers,they noticed something that could explain the rest of the increase: the cells lining the gut and liver were bloated with lipid droplets absorbed during digestion. When they correlated the amount of accumulated fat in the cells with the size of the gut and liver, they found that the lipids accounted for the remaining variation in organ size. So snakes enlarge their digestive tract by pumping more blood to the gut and liver, while fat accumulation in the cells lining the gut further enlarges the absorptive surface as the prey is digested.
Starck reveals that `the hardest part was coping with the incredibly flexible morphology of these creatures.' The problem is that the snakes'bodies expand to make room for their full stomach, which shifts the position of the blood vessels in their gut, so Starck and Wimmer had a hard time keeping track of the moving vessels. But Starck is satisfied that his struggles with the supple snakes have finally paid off: pythons really do use hydraulics to pump up their guts.