Biology Reference
In-Depth Information
Large defender
Small defender
Fig. 14.3
Medium size males
are more likely to
attack silenced
males when the
high-pitched croak
of a smaller male
was played.
However, croaks
cannot be the only
assessment cue
because for either
croak pitch there
are fewer attacks
at large defenders.
The strength of a
defender's kick
may also be
important. From
Davies and Halliday
(1978). Photo
© Jurgen Freund/
naturepl.com
10
0
High
Deep
High
Deep
Pitch of broadcast croaks
How does the attacking male assess the size and fighting ability of his rival? Whenever a
male attacked a pair, the mating male always called. In this and many other species of
frogs and toads, the pitch of a male's croak is closely related to his body size - the larger the
male, the larger the vocal cords and so the deeper the croak. Consequently, croak could be
a reliable index of size, and hence fighting ability. Davies and Halliday (1978) tested this
idea with an experiment in which they allowed medium-sized males to attack either small
or large paired males, which were silenced by means of a rubber band placed behind their
arms and passing through their mouths like a horse's bit. During the attack, they used a
loudspeaker next to the mating pair, to broadcasted tape-recorded croaks of either a large
or a small male. For both sizes of defender they found that there were fewer attacks when the
deep croaks of a large male were played than when the high-pitched croaks of a small male
were broadcast (Fig. 14.3). Therefore, croak pitch is used to assess a rival's body size, which
is a good predictor of fighting ability because large males are more difficult to displace.
The pitch of a
croak is a reliable
index of body size
Males are more
likely to try and
displace males
when they are
played a high-
pitched croak
