Biology Reference
In-Depth Information
a signal if it has evolved because it influences the
behaviour of another spider by providing information
about weight. The key point here is that the signal
must be able to evolve independently of the feature of
the signaller about which it conveys information -
spiders can vary if and how they vibrate a web
irrespective of their weight and, hence, fighting ability.
In contrast, fighting ability is tightly linked to weight.
In the rest of this chapter we will focus on signals,
discussing how they can evolve and the form that they
take. The major problem that we must consider is what
keeps signals reliable or honest, such that receivers
will be selected to respond to them? Put another way,
what stops senders exaggerating their signals to
deceive the receivers to their own advantage?
Signals must be
able to evolve
independently of
the feature they
carry information
about
The problem of signal
reliability
Fig. 14.1
The North
American funnel-web
spider,
Agelenopsis aperta
.
Photo © Visuals Unlimited.
Consider an extreme signal such as the tail of
the male Indian peafowl
Pavo cristatus
. On page 199
we described how females choose to mate with the
males that have the most eyespots in their tail, and that the number of eyespots is a
reliable signal of the genetic quality of males. However, this raises the problem of why
don't lower quality males just put more eyespots in their train, to deceive females into
thinking they are higher quality, and therefore obtain more mates? In this section we
shall discuss three hypothetical answers to this problem, following the conceptual
framework laid out by John Maynard Smith and David Harper (2003). Our aim is not to
argue which one is correct, but rather to use the peafowl as a means to describing the
three possible ways that the honesty of signal can be maintained.
One possibility is that the size of a tail is constrained by the size and, therefore, quality
of a male, such that poor quality males just aren't big enough to be able to carry extra
eyespots. In this case, the number of eyespots is a reliable signal because it cannot be
faked - only high quality males are able to carry extra eyespots. Such a signal is termed
an
index
, which is formally defined as a signal whose intensity is causally related to the
quality being signalled and which cannot be faked.
Another possibility is that poor quality males can make the extra eyespots but that it
would be very costly for them to do so. Perhaps if they invest resources into building extra
eyespots, then they would have reduced resources to put into other actions, such as immune
function, which would lead to them being plagued by parasites. In this case, the number of
eyespots is a reliable signal because whilst it could be faked, the cost of doing so makes it
relatively inefficient for low quality males (i.e. cost of producing a larger signal would
outweigh the benefit of increased mating success). In contrast, high quality males have
sufficient resources that the benefit of making extra eyespots (increased mating success)
outweighs the cost. Such a signal is termed a
handicap
, which is formally defined as a signal
whose reliability is ensured because it is costly to produce or has costly consequences.
An index is a
signal that cannot
be faked
A handicap is a
signal that is
costly to fake
