Biology Reference
In-Depth Information
of interest to the receiver, about the state or future intentions of the signaller, and this
information must be correct often enough for the receiver to be selected to respond to it.
The problem from an evolutionary perspective is what keeps a signal honest, such that
the receiver benefits by responding to it? An honest signal would be one which conveys
correct or useful information. For an alarm call, this would be information about a
predator. For a displaying male, this would be information about their genetic or
phenotypic quality as a mate. The problem is that senders could do better if they could lie
or exaggerate their signals, so we must ask what prevents this and keeps signals honest?
What stops a male peacock from producing such a large and impressive tail that all females
choose to mate with it, or why don't all insects signal that they are poisonous to their
vertebrate predators? If individuals did lie and exaggerate, then their signals would not
carry useful information, there would be no advantage in paying attention to such signals
and so the communication system would break down.
Consequently, the central problem of communication is what maintains the honesty
of signals? Before we can address this question, we must first distinguish between the
different types of interaction and communication.
A signal is honest
if, on average, it
conveys correct or
useful
information
What maintains
the honesty of
signals?
The types of communication
It is useful to distinguish between two types of interaction: cues and signals (Maynard
Smith & Harper, 2003). A cue is when the receiver uses some feature of the sender
to guide their behaviour, but this feature has not evolved for that purpose. An
example of a cue is when a mosquito that is searching for a mammal to bite will fly
up wind if it detects carbon dioxide. In this case, the carbon dioxide acts a cue to the
mosquito (indicating the presence of a source of blood), but mammals do not
produce carbon dioxide to signal their presence to mosquitoes (they would rather
not be bitten!).
Signals are acts or structures produced by the sender that alter the behaviour of the
receiver; they have evolved because of that effect and are effective because the receiver's
response has evolved (Maynard Smith & Harper, 2003). This definition may seem
convoluted but it has two important consequences. Firstly, because the receiver has
evolved to respond to the signal, the response must, on average, benefit the receiver.
Otherwise the receiver would be selected to not respond to the signal. The term 'on
average' is key here because some individuals can produce deceptive signals, which are
successful because they exploit the honest signalling of others; we shall return to this
later in the chapter.
The second consequence of this definition is that, because the signal has evolved
owing to its effect on others, this distinguishes a signal from a cue. Consider contests
between funnel-web spiders,
Agelenopsis aperta
, over web sites (Fig. 14.1). Susan
Riechert (1978, 1984) found that when spiders differed in weight, the smaller spider
retreated rather than fighting over the site. She confirmed this experimentally by
gluing a flattened piece of lead shot to the back of some spiders and showing that
this made larger spiders retreat from them. If the spiders were able to directly assess
weight, independent of the actions of the other spider, then weight would just be a
cue of size and fighting ability. This is because weight did not evolve to signal fighting
ability - larger spiders just have an advantage in fights. However, spiders actually
signal their size by vibrating the web. This act of vibrating the web would be
A cue is a feature
of the world that
can be used as a
guide to future
action
A signal is an act
or structure that
alters the
behaviour of
another organism,
which evolved
because of that
effect and which
is effective
because the
receiver's response
has also evolved.
On average, the
response to a
signal provides a
benefit to the
receiver
