SCIENCE LAB SCARING : BOA CONSTRICTOR

Coping With Pressure

They take the breath away, while they keep breathing easily.

To kill its prey, a boa constrictor coils around it, squeezing hard enough to stop the prey's blood from flowing, and then, stretching its jaws open, devours it whole.

''They do this for 10, 15, up to 45 minutes,'' said Elizabeth Brainerd, an evolutionary biologist at Brown University. ''And that takes quite a bit of energy, so they have to be breathing.''

She and her colleagues studied how boa constrictors breathe under such cramped conditions and found that the snakes are able to precisely shift among regions of their rib cages that expand to draw air into their lungs.

''Basically snakes are all ribs,'' Dr. Brainered said. To breathe, a snake slowly expands a section of its rib cage, creating a pressure change that pulls air in.

When resting, boa constrictors breathe using ribs near the upper third of their lungs. But when a blood pressure cuff was wrapped around those ribs, a specific set of ribs further down began expanding to draw in air.

''The snake just turns off a section of the rib cage and then turns on another section,'' said John Capano, an author of the study, who added that as soon as the cuff was removed, the ribs normally used to breathe during the rest immediately re-engaged.

The researchers believe this ability to modulate rib engagement emerged while or before snakes evolved the ability to constrict, and maybe ahead of their ability to eat large prey. [ Sam Jones ]