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Having survived for over 200 million years, some turtle species are now
threatened with extinction because they get caught in fishing nets. Martin
Lenhardt at the Virginia Institute of Marine Science has worked with turtles
accidentally netted in Chesapeake Bay, and found a way to save them (WO
01/17119). Turtles have an air-filled middle ear that resonates at a frequency
that changes with age and the depth of water. Lenhardt discovered that if boats
emit a 100-kilohertz ultrasound signal, the shells of nearby turtles convert it
to a lower frequency, between 200 hertz and 15 kilohertz. This makes the beasts’
middle ears resonate, scaring them into swimming away or diving to safety.