Many organisms have hair sensory cells that detect air or fluid movement. Terrestrial (land dwelling) vertebrates have these hair sensory cells concentrated in two ears, one on either side of the head. Thus, we say that vertebrates with ears 'hear' sounds. In the vertebrate ear, sound waves hitting the ear cause movement of the fluid in a chamber housing the sensory hair cells. The movement of the fluid causes the fine hairs to bend and receptors in each hair cell send this information on for processing. As it is difficult to see at night, nocturnal animals communicate mainly by sound and their sense of hearing is well developed. Exercises 4.1 and 4.2 demonstrate how bats and spiders, respectively use the vibrational sense to find food. Exercise 4.3 and 4.4 examine the calls that male frogs use to find mates.

4.1. Bat Echolocation (K-12)

Bats use high sound pitches that are at the upper limit of human hearing. They actually produce ultrasonic calls that they send out into the night air in search of flying insects. When these sound waves hit a flying object, the signal bounces back just as a ball you have thrown at a wall comes back towards you. Receptors in the bat’s ears are tuned to screening the ultrasonic playbacks to detect potential prey and repeated calling permits the foraging bat to locate these prey.

• Choose an individual who will be the bat in this exercise. This person should put on the blindfold that can be found in the shoebox with the nose sticker on it.
• Five additional students should be selected as moths, the favored prey of bats.
• In a place free of furniture and other objects that our bat might trip on, the rest of the class should form a circle around the bat and moths.

The goal of the exercise is for the bat to locate all of the moths by echolocation.

• The bat does this by calling out 'BAT'.
• The moths must reply each time they hear the word bat by calling out 'MOTH'.
• The bat needs to move towards the call of the nearest moth.
• The moth is captured if the bat touches it with his/her hand.
• At this point, the moth has to exit the circle.
• You might time this exercise to see if some students are better than others in using echolocation.
• Be sure to place the blindfold back in the appropriate box at the end of this exercise.

Exercise 4.2 The vibration sense of spiders (K-12)

Spiders also use the sense of hearing though the spider’s ears are located on its legs the term vibrational sense is used rather than hearing that is restricted to animals with ears. Hair cells in the joints of the legs detect the change in the position of the leg. As a spider sits on its web, an insect hitting the web causes the silk strands to move which in turn causes the spider’s foot and leg to move. There are sensory hairs located in the joints of the leg that are disturbed by the leg movement. These hairs send the vibratory information on for processing. As in bats, a spider’s hair cells are tuned to particular vibration patterns. Information on the type and size of insect hitting the web is obtained through this sense. Most spiders can see no more than a couple of centimeters distant (there are 2.54 cm/1 inch). Thus the vibration sense is its main sense.

• Divide the class into groups of 6 individuals. Each group should take a clear plastic box with the spider web sticker on it.
• One person in each group will be the spider and the other five, insects.
• First unwind the fishing lines wrapped around the wooden black in the box.
• The designated spider should hold the wood block in one hand.
• Each insect should then take one of the monofilament lines and move away from the spider.
• The spider will then position his/her other hand on the top of the wood block such that one finger is resting on each of the five lines.
• Place the blindfold over the eyes of the spider
• Check that each of the moths backs up until all lines are taut.
• The teacher or some selected student should tap one of the moths, who will pluck his/her thread.
• When the spider detects the pluck, it should pull on this line
Did the spider find the correct prey?
• Switch off with different individuals serving as spider and moths.

Exercise 4.3. What kind of Frog is it? You can tell from calls males make. (3-12)

Animals use the vibration sense to detect prey and to avoid predators. They also communicate through the production of sounds and the processing of these sounds. People talk and others hear what they are saying (listen). Like birds, male frogs and toads sing to attract females to them. It is important to to both sexes that they locate only individuals of the same type or species, so each species has a unique song or call. However, the calls of closely related species are more similar to one another than are other frog or toad calls. In Tennessee, we have three major groups of frogs, the 'true' frogs, the 'toads', and the 'tree' frogs (see figures below). In this exercise, you will learn the differences in calls among the three major frog groups and will then be asked to assign the calls that you hear to the correct group.

• Take out a sheet paper and make a list of numbers from 1-11.
• Find the CD for Unit 10 with the frog picture on it and open track 1 for the introduction to Exercise 4.3 What kind of frog is it?
• Follow your guide through the exercise.
• Track presents the unknown calls
• On track 3, the guide will play the calls again, telling you what animal produced it.
• You may also check your answers under Exercise 4.3 in the answer booklet

Exercise 4.4. Frog call patterns put to paper: Reading Sonograms (6-12)

It is easy enough to listen to songs made by various frog and toad species and detect differences among them. Biologists, however, need to be able to measure the differences (quantify them) and determine the extent to which these calls vary. We cannot simply take out a ruler and measure the differences in calls as we might do say with the length of a leg or the height on an individual. Nor can we take out a color chart and assign a color shade to it as we might do for eye or skin color. One of the most basic techniques biologists use to analyze non-visual traits is to present the information graphically, a form that can be measured visually. Sounds can be converted to sound spectrograms or sonograms in which they are laid out in two dimensions (time and frequency or pitch) as shown on the following graph of the chickadee's song.

In this exercise, you will learn how to read sonograms. Your challenge in the end will be to identify the sonogram that belongs to each of the mystery calls made by various true frog, tree frog and toad species.

• Find the CD for Unit 10 with the frog picture on it and open track 4 for an introduction to Exercise 4.4 Frog Sonograms
• Follow along with the sonograms shown below as instructions are given to you under track 5. After you learn about the translation of songs into graphs, you will be able to place each call with its graphical representation.

The Challenge: Place each of the following frog species (1-5) played on track 7 with the graphical representation of its call (drawings A-E) below
• Listen to instructions for this exercise on track 6
• Go to track 7 to listen to frog and toad calls
• Match these calls to the sonograms below

Call 1 = Mountain Chorus Frog
Call 2 = Southern Chorus Frog
Call 3 = American Toad
Call 4 = Bull Frog
Call 5 = Southern Leopard Toad

• Check your answers under Exercise 4.4 in the answer booklet.

Links:
Hoy,R, Popper, A.N. & R.R. Fay eds 1998. Comparative Hearing: Insects. Spring Handbook of Auditory Research. Springer NY
http://www.som.tulane.edu/ferminlab/Hearing.html
http://faculty.washington.edu/chudler/hearing.html
http://www.cln.org/themes/hearing.html