Wednesday, July 29, 2009
Hear Like a Moth, Eat Like a Bat
I have been giving some thought lately to the ancient and intimate linkage between survival and the ability to perceive events in the environment. Organisms throw as many resources at the problem as they have to.
We mammals need our extravagance of levers and multiple membranes because the mammalian inner ear is liquid filled and must be stimulated by more force than most sounds in air convey. It is liquid filled, evolutionists theorize, because the predecessors of mammals evolved in the ocean, and a liquid sensing organ is appropriate for sensing signals coming from a liquid environment. Fish out of water, we haven’t managed to get rid of the liquid, and so need to make accommodations.
It’s not that way for insects. With no aquatic ancestry to overcome, their hearing organs have a remarkably straightforward construction. Most have a simple membrane, much like the mammalian eardrum, to which the auditory nerves attach more or less directly. Such basic structures have their limitations, but in general insects manage just fine without hammers and anvils.
Some nocturnal moths, for example, survive as well as they do because they have evolved ears that are sensitive only to the range of high frequencies used by insectivorous bats. They cannot hear one another, they cannot hear the rustle of leaves or the mosquito-slapping of the graduate students sent to observe whether or not they are caught – but they can hear the bats, and that’s what keeps them alive. The lifecycle of moths is short enough that evolutionary pressure can be seen at work. Take away bats, and the number of non-hearing moths increases; restore them and it goes down.
We mammals need our extravagance of levers and multiple membranes because the mammalian inner ear is liquid filled and must be stimulated by more force than most sounds in air convey. It is liquid filled, evolutionists theorize, because the predecessors of mammals evolved in the ocean, and a liquid sensing organ is appropriate for sensing signals coming from a liquid environment. Fish out of water, we haven’t managed to get rid of the liquid, and so need to make accommodations.
It’s not that way for insects. With no aquatic ancestry to overcome, their hearing organs have a remarkably straightforward construction. Most have a simple membrane, much like the mammalian eardrum, to which the auditory nerves attach more or less directly. Such basic structures have their limitations, but in general insects manage just fine without hammers and anvils.
Some nocturnal moths, for example, survive as well as they do because they have evolved ears that are sensitive only to the range of high frequencies used by insectivorous bats. They cannot hear one another, they cannot hear the rustle of leaves or the mosquito-slapping of the graduate students sent to observe whether or not they are caught – but they can hear the bats, and that’s what keeps them alive. The lifecycle of moths is short enough that evolutionary pressure can be seen at work. Take away bats, and the number of non-hearing moths increases; restore them and it goes down.
