Shortcut to big heart: pythons build cardiac muscle in record time.

A Burmese python can boost its heart capacity without working out. When it needs extra pump power to digest something big, its heart can bulk up 40 percent in just 2 days.

"That's the fastest increase ever measured," says James Hicks of the University of California, Irvine, who led the recent python-heart study. "Imagine yourself getting in great cardiac shape just by eating."

The notion that digestion could be an athletic workout for large snakes dates from the 1930s, when measurements showed metabolic jumps after meals. Pythons, which are basically couch potato hunters, provide an extreme example because they find a likely spot and wait for dinner to saunter by. A snake routinely goes for weeks or months without a meal but then lands a big prize. Large Burmese pythons will readily eat pigs and other prey as big as they are. "They're just eating machines," says Hicks.

After its sudden shift from fast to feast, a Burmese python's metabolic rate can rev up by a factor of 40, says Stephen Secor of the University of Alabama at Tuscaloosa. In the mid-1990s, he and Jared Diamond of the University of California, Los Angeles described the python as having to "pay before pumping" because they noticed it had to invest in enlarging its heart and digestive organs before getting the benefits of digestion.

"It's like running a marathon but doing it for 2 weeks," Secor says, referring to the snake's metabolic shift during digestion.

Hicks and his colleagues reexamined the snake's heart growth. They confirmed that the ratio of the heart's wet and dry weights remained roughly the same during fasting and digesting, so the heart was enlarging during digestion not by expanding with fluid but by adding muscle.

The researchers also checked for a compound, a specific messenger RNA, required for the manufacture of heart-muscle fiber. They found an abundance of this RNA in heart samples from digesting snakes but not from fasting snakes, they report in the March 3 Nature.

A python takes about 14 days to digest a meal. Then, as the snake resumes its fast, its heart shrinks back to its former size.

The internal organs of animals that hibernate during winter or go dormant in hot weather also shrink and expand when physiological demands change, as do the organs of some migrating birds.

Pythons could make a good study animal for scientists figuring out the basic mechanisms for heart growth, Hicks says. Hearts enlarge during human development, in athletic training, and in some diseases.

Secor says that he hopes that snake-wary heart researchers won't avoid working with Burmese pythons. "They're pussycats" he says.