The boa constrictor can move different parts of its ribcage independently,
allowing it to breathe even while it crushes prey using its body.
The mammals, reptiles and birds that this snake hunts can survive for
several minutes as the constrictor squeezes the life out of them using the
upper third of its body. But that part of its body also contains the lungs,
so at first glance this might suggest these snakes risk suffocating
themselves while killing prey.
John Capano at Brown University, Rhode Island, and his colleagues have
worked out how the snakes cope. Capano says they draw air into their
constricted lungs by selectively rotating ribs farther down the body.
“They seem to have very refined control over which segment of their body
they actually are ventilating with,” he says. “This explains how it is that
they’re using a ribcage to ‘kill’ another ribcage, but that they themselves
are fine.”
Snakes lack diaphragms, which means they have to move their ribs in order to
inhale and exhale. While they have very long lungs that extend beyond the
bodily region used for constricting, the bottom part of the lungs is
“literally just like a balloon” and can’t actually respire, says Capano.
Like many biologists, Capano and his colleagues wondered if this so-called
“saccular” lung region played a role in helping boas breathe during the
relatively long constriction process. They placed a human blood pressure
cuff over different parts of three boa constrictors (Boa constrictor) and
observed the body movement of each during hissing – which requires deep
breathing.
X-ray videos revealed that when any one section of ribs was constricted by
the cuff, the snakes moved other ribs elsewhere in the body to compensate.
In particular, when the cuff covered the region over the upper lungs, the
snakes would start moving the ribs around the saccular lung region. And when
the higher ribs weren’t constricted, the saccular region ribs stayed still,
says Capano.
The researchers realised that when the upper lung region was constricted,
the snakes were using these lower ribs to inflate and deflate the saccular
lung area, says Capano. This would draw air through the upper lung region,
where oxygen could exchange with carbon dioxide so that true respiration
could continue to occur.
This activity was a “wilful” choice by the snakes, he adds. Using a
technique to study muscle activity revealed that the boas can control which
rib muscles to move.
“These snakes can very discreetly shift which subset of ribs they are using
to ventilate, at will,” Capano says. “That means turning on two sets of
ribs, or turning on nothing, or turning on another set in the hind and then
immediately switching it back to the front.”
Reference:
John G. Capano, Scott M. Boback et. al, Modular lung ventilation in Boa
constrictor, J Exp Biol (2022) 225 (6): jeb243119.
DOI: 10.1242/jeb.243119
Tags:
Plants & Animals