We showed with the weights that they can adjust how much they activate those muscles to meet whatever conditions require, Uriona says. In other words, the gators exert fine control over the muscles used to shift their lungs for aquatic maneuvers.
Gators on an Evolutionary Roll
Until now, researchers assumed alligators land-dwelling ancestors developed the diaphragmaticus muscle to help them breathe while running.
It may be that instead of these muscles arising for breathing, they arose for moving around in the water and later were co-opted for breathing, Uriona says.
He says natural selection would favor the development of muscles to move the lungs for quiet maneuvers in water because animals with such ability can easily move around in the water, either trying to sneak up and eat something, or avoid being eaten.
Uriona says the diaphragmaticus likely began as part of the rectus abdominis muscle and evolved into a separate muscle.
If the diaphragm-like muscle evolved to help breathing, two steps were required. First, the diaphragmaticus connection to the breastbone had to be replaced by a connection to the liver. Second, separate nerve circuits had to evolve to control the diaphragmaticus and abs since one is used to inhale and the other to exhale.
If, instead, the diaphragmaticus first evolved to help ancient crocs maneuver in water, only the first step would have been required. When the diaphragm-like muscle and other muscles later evolved to assist breathing, then the second step was needed.
Uriona and Farmer say it is simpler evolutionarily for one step to occur at a time, making it more logical that the diaphragm evolved first when early crocodilians took to the water, and began to play a role in breathing only later.
|Contact: Lee Siegel|
University of Utah