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Insect and Frog Acoustics
Introduction:
Our goal as a class is to “hear” what they (the animals we focus on) hear. They
use organs located on the antennae or at the base of their legs (depending on
the species of Orthoptera) or tympanic membranes (our frogs) but the principle of
changing sound waves to nervous energy is the same. Every organ of sound
reception does not have the same range of abilities. They hear things
(frequencies or tones or other aspects of sound) that we cannot and visa versa.
In general, they have smaller brains (no offense) so the last thing they need is to
use limited neurons processing sounds that have no relevance to them. The
relevant sounds usually elicit very specific responses (ex. Flee if the sound of an
approaching predator is perceived).
We will use the technology today to focus our huge brains on the subset of
sounds that are relevant to our subjects. We will further focus on sounds that
function in intra-specific communication. Male crickets and frogs sing; females
do not. All of the crickets or frogs of one species can hear their species’ song.
Would you expect the males and females to have different responses to hearing
the male song? If your answer is “yes” we are on our way.
Taking on the roles of our animals:
As individuals, we have selfish goals (in addition to the class goal). Everyone
wants to maximize their genetic representation in the next generation of crickets
and frogs. How you get your genes into the next generation depends on whether
you are male or female. Half the class will take the role of males and half,
females. Even though we have a 1:1 sex ratio (one male per one female) there is
conflict. Females can only lay the number of eggs they can fit in their abdomens
(limited) while males can make “unlimited” sperm and are only held back from
having vast numbers of offspring by their access to eggs. This means that
females “choose” males and males compete with other males to be chosen (two
types of sexual selection to be discussed in lecture). In crickets and frogs the
song is an important part of the choosing and competing.
Crickets sing by rubbing their wings together while our frogs produce songs in
ways somewhat similar to our song production. The resulting songs have a
frequency and amplitude that we can record and measure and which can be
displayed visually. The receivers (male and female conspecifics) also perceive
these characteristics of the song and for them, there is much information in the
characteristics.
Procedures:
You may think you know your gender but for purposes of today’s lab you will be
assigned one.
Females-you know how many eggs you can make because we will tell you.
Determining how many of these eggs will carry your genes successfully depends
on your choice of sperm donor. You don’t want to saddle your cricketlets with
low quality genes. How will you decide which are the best donors? Hint: look at
the title of today’s lab and think how song might be a good gene indicator.
Frame your ideas as a testable hypothesis and get ready to make your choice.
You will choose 5 “surrogates” from the crickobator. Weigh 25 females and
choose the 5 largest.
Male Singers-if you have big sound and the energy to keep it going you sing.
Your big sound will come from crickets (unless you have stridulatory organs
strapped to your “wings”). Collect individual male songs. As you record the
songs you need the weight and general condition of each singing male. Can
you hear any differences in the male songs? Now use Audacity to tell the
difference. Can you tell big males from smaller males? Is there any difference in
the conditions of your males (broken legs or antennae)? What would you use as
an indicator of successful song?
The Arena
When females are ready to choose and male singers have collected their songs,
we are ready for the arena. Everyone assemble in the lab. Singing males set up
territories from which to project. Females start in one area. How will you prevent
escapes? How will you score choices? Each patch keep track of your crickets.
Keep a jar and a piece of paper handy in case your cricket gets off track.
Data Collection
On the board, create a table that addresses the goals (focus on the class goal).
Can you predict female choice based on song characteristics? Which
characteristic is most predictive? Are the “most valuable” females any more or
less predictable than the less valuable? Discuss your conclusions with the class
and compile a class conclusion.
H. Carl Gerhardt and Franz Huber. 2002. Acoustic Communication in Insects and
Anurans: Common Problems and Diverse Solutions. University of Chicago Press.
Chicago. 531 pp.