Biology Reference
In-Depth Information
Zahavi (1975) proposed that such
displays would be favoured precisely
because they are costly, and that it is
the cost that makes them reliable. He
called this the handicap principle.
Zahavi's idea was extremely
controversial throughout the 1970s
and 1980s. Indeed, one important
theoretical paper was simply titled 'The
handicap mechanism of sexual selection
does not work' (Kirkpatrick, 1986).
Much discussion was in the context of
sexual selection, and whether males
produced costly ornaments to signal
quality to females. It was argued that if a
female mated with a high quality male
who produced costly ornaments, then
their offspring would gain the benefit of
good genes from their father, but that
this would be negated by the cost of
then producing ornaments themselves.
Consequently, there would be no fitness
advantage to either producing the
ornament or basing mate choice upon it
(Maynard Smith, 1976b).
Alan Grafen (1990a, 1990b) solved
this debate with a decisive pair of
theoretical papers. He showed that the
handicap principle could work but
that it would only do so when the
fitness cost of producing the costly
ornament was greater for low quality
males. The idea here is that both
high and low quality males could
produce costly ornaments, but that
the ornament would be so costly for
low quality males that the cost would
outweigh the benefit. In contrast, with high quality males the benefit would outweigh
the cost, so only high quality males would be selected to produce the ornament
(Fig. 14.9).
C
B-C > 0
B
B-C < 0
Male quality
Fig. 14.9
The handicap principle and
sexual selection. The benefit (
B
) of
producing a costly signal, such as the
increased mating success from having an
ornament, is assumed to be roughly
equal for all males. The cost (
C
) of
producing the costly ornament is
assumed to be lower for higher quality
males, because they are in better
condition and have additional resources
to invest in ornament production. In this
case, the benefit of producing the
ornament only outweighs the cost (
B
−
C
> 0) for high quality males, so only high
quality males are selected to produce the
ornament, making the ornament a
reliable signal of male quality. More
generally, the handicap principle requires
that the cost to benefit ratio is lower for
individuals giving stronger signals.
Costly signals are
reliable if the
relative cost of
signalling is
greater for lower
quality individuals
Stalk-eyed flies
In Chapter 7, we described how in the stalk-eyed fly,
Teleopsis dalmanni,
females choose
to mate with males with relatively larger eye spans. How can we distinguish between
male eye span being an index or a handicap? A clear prediction of the handicap model
The handicap
principle predicts
strong condition
dependence
