Biology Reference
In-Depth Information
Fig. 14.20
Photograph of a mantis shrimp
Gonodactylus bredini
. Photo © Jurgen
Freund/naturepl.com
heterochaelis
, has shown that larger individuals are more likely to win fights; they also
have larger forelimbs, so the size of forelimbs appears to be a reliable index of fighting
ability (Hughes, 1996).
However, in another species,
Gonodactylus bredini
, this threat signal is sometimes used
in a dishonest way. Every two months, a mantis shrimp moults its hard exoskeleton and
has to spend three days as a soft, impotent, warrior before the new skeleton hardens
up. During the moult, individuals still use the threat to deter intruders, even though
it is no longer an honest signalling of fighting ability (Steger & Caldwell, 1983).
Furthermore, newly moulted individuals are more likely to display when facing smaller
opponents, and hence when the opponent is more likely to back down (Adams &
Caldwell, 1990). Bluffing against larger individuals can be more costly, because if the
opponent does not back down and instead attacks, there is a high likelihood that the soft
shrimp will be killed. This dishonest signal is relatively rare because only a small
proportion of any individuals are moulting at any one time (<15%), ensuring that
forelimb size is, on average, a relatively reliable index of fighting ability.
Size of forelimbs
appears to be a
reliable index of
fighting ability in
mantis shrimps …
… but moulting
mantis shrimps
bluff
Summary
Signals are acts or structures that alter the behaviour of others, which have evolved
because of that effect, and which are effective because the receiver's response has also
evolved. This distinguishes signals from cues, which are a feature of the world that can
be used as a guide to future action. So, for example, body size is a cue, but when a spider
vibrates a web to give information about its body size, this is a signal.
The major problem of signalling is to explain what keeps signals reliable or honest.
We have distinguished three ways in which this can occur:
