The Universal Sense

Free The Universal Sense by Seth Horowitz

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Authors: Seth Horowitz
point still sticks with me and drives my interests. It’s led to everything from hauling a hundred pounds of recording gear into mosquito- and snapping-turtle-infested swamps to gene-screening injured frogs to try to identify the molecular basis for their ability to regrow their brains. I’ve had my hand stuck in the mouth of a male bullfrog intent on swallowing me whole,and had to hit the guano-covered floor of a bat-infested attic as nursing mother bats dive-bombed me with their babies hanging from their nipples. According to my doctor, I have developed the world’s only recorded allergy to bullfrog urine.
    One of the things that fascinated me about frogs is that, as amphibians, they are representative of some of the earliest forms that successfully ventured forth out of the water and onto the land. The fossil record shows that anatomically modern-looking frogs have been around for over 300 million years. This has created a perception that frogs are “simple” or primitive organisms and that by examining them we can learn only the basics of hearing. As an example of this, for many years, frog hearing was thought of as a simple mating call detector—that is, it was narrowly tuned to hear only the sounds of its fellow frogs. At first thought this seems to make sense—if your social behavior is dependent on calling and hearing other frogs of your species, why waste brain resources on extraneous noise? But frogs, like all animals, are not machines designed for a specific task but complex organisms in a complex ecology. To quote Dick Fay again: “The problem with hearing only your own species is that you’ll probably get eaten by the first predator that makes noise outside your calling range.” And as anyone who has tried to sneak up on a bunch of frogs knows, Fay is right—a chorus of bullfrogs filling the night with their low-pitched calls at a headache-inducing 100 dB will suddenly silence themselves with the first footstep within 20 meters. Their calls may include audible frequencies from 200 to 4,000 Hz, but they can detect ground-borne seismic vibrations orders of magnitude lower in pitch and amplitude than our ears can pick up.
    But as with fish, you can’t just lump frogs together. Frogs are a highly diverse order: some totally aquatic, some mostlyterrestrial; some able to sit on the tip of your finger, others over 8 pounds and a foot long. One thing that does unite them is that in all known species, their social behavior and survival are dependent on their hearing. In fact, the presence of an obvious tympanic membrane (homologous with the human eardrum or tympanum) was one of the earlier ways that scientists used to differentiate frogs from toads, and its relative size compared to the eye is still how you tell the boy frogs from the girl frogs (males’ eardrums get much larger than their eyes, whereas female frogs’ ears are more petite). However, as with many things in the classification of animals by their external characteristics rather than their genetic relatedness, this turned out to be a problem, as some frogs have completely internal ears. This is one reason that Xenopus laevis , an aquatic frog, was initially called the African aquatic toad: being totally aquatic, they have evolved internal tympanic disks to allow them to hear each other calling in the muddy ponds that make up their natural habitat in Africa.
    Xenopus laevis has been the darling of a lot of different types of research since the 1930s. Its eggs are very permissive structures that will express proteins transplanted from other species to create functional structures—for example, DNA that codes for pieces of cells such as neuronal ion channels will undergo translation in an X. laevis egg or oocyte, creating an egg with neuronal ion channels in it, allowing researchers to carry out precise studies of neuronal kinetics or pharmacodynamics not otherwise possible. Its tadpoles develop in transparent external eggs, allowing a great deal

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