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Study shows how alligators digest heavy bones

By Carlos Mayorga

When a raccoon approaches a lake to drink water, it may not be aware of the imminent danger below the water’s surface.

Even in the pitch dark, when an alligator is completely submerged in the water except for its eyes, alligators have freckles on their snouts that can sense ripples in the water and know when a potential meal is sipping at the waters’ banks.

Alligators can be violent, fierce predators that can tear prey into pieces, swallowing whole chunks at a time. In a single meal, these crocodilians can consume prey up to 23 percent of the size of their own body weight, which is comparable to a 130-lb. woman eating a 30-lb. hamburger.

A recent study headed by U biologist Colleen Farmer shows how crocodilians possess the unique ability to bypass blood flowing to the lungs and divert it to the stomach, allowing it to digest large amounts of food at once. Higher levels of blood in the stomach spark an increase in gastric acid, speeding up the alligator’s capacity to digest the strong bones of its prey.

Researchers have known for some time that crocodilians have this ability, but scientists were relatively unaware of how vital the process is to an alligator’s survival until Farmer’s study.

Unlike humans and other animals, crocodilians possess a special vessel called the left aorta that they can choose to activate for digestion.

Using the left aorta allows carbon-dioxide-rich blood to bypass the lungs and go straight to the stomach to aid in the digestive process. The stomach then uses the CO2 in the blood to produce gastric acid and bicarbonate at a rate that is at least 10 times faster than what has been measured in mammals.

“What is intriguing about Colleen’s work is that she is the first to test out this hypothesis,” said James Hicks, editor in chief of the Physiological and Biochemical Zoology, a journal published by the University of Chicago Press, which will publish the study’s findings next month.

In order to understand more about this process, researchers placed a probe around the left aorta in test alligators and studied the animals during fasting and after a meal. Scientists found a correlation in the use of the vessel during digestion, but wanted to prove it further, Farmer said.

“In the next experiment we did, we surgically tied off the vessel so that the alligator couldn’t use the (left) aorta,” Farmer said. “The surgery prevented them from flipping to the vessel (to aid in digestion).”

Researchers used two groups of alligators: one group that was able to use the left aorta and another group that was not. Scientists fed the alligators ox tails and used X-rays to see how rapidly each animal was able to dissolve the food.

Farmer said that they found a strong correlation between the alligators’ ability to use the vessel and the amounts of gastric acid they were able to produce.

“The control group, the alligators that could use the vessel, had maximal rates of gastric acid,” Farmer said. “The alligators had the capacity to rapidly pump gastric acid into their stomachs.”

Although scientists have determined that crocodilians have this ability, researchers are trying to pinpoint the reasons they need it.

One reason to expedite digestion would be to prevent the buildup of bacterial growth, which could make the animal sick, Farmer said.

Another reason could be that crocodilians often burst out of the water to grab their prey and must drag the larger animals into the water in order to drown them. This generates high amounts of poisonous lactic acid in their muscles that could be lethal to the crocodilian unless the acid is pumped into the stomach.

A fast digestive system could also be useful because crocodilians need sunlight for warmth. The creatures are cold-blooded, which requires them to depend on the sun to maintain a warm belly, an element that is essential to producing gastric acid. But for young crocodilians, fast digestion is critical to survival, as more than half become prey to larger animals.

Farmer said she is hoping to secure more funding to take the research further, perhaps exploring the role the vessel plays in the functions of other organs.

“CO2 is important in other digestive functions too,” Farmer said. “Blood supply doesn’t just simply go to the stomach, but other organs like the liver.”

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