5.2. Sexual Selection
Although the focus on the role of long-distance communication in speciation continues (e.g., Howard et al. 1998), even more interest has centered
on its function in sexual selection (Andersson 1994). Although speciation
is the process that gives us diversity through an increase in species numbers,
sexual selection is a process that enhances diversity within species. It is
primarily this process that makes males and females of the same species
look so different from one another—humans included. It is also the process
that makes some animals look so attractive and stunning, sound so charming, and behave in manners that are sometimes amusing to us.
Sexual selection favors traits that enhance reproductive success by
enhancing one’s ability to acquire mates. This can be achieved by a male
becoming more attractive to a female, and often, but certainly not in all
cases, this judgment of attractiveness is influenced by the male’s longdistance signal.A large number of studies of acoustically advertising animals
shows that receivers’ preferences for long-distance signals will not only
guide them to males of their own species but often to a subset of males
within the species (Ryan and Keddy-Hector 1992; Andersson 1994). Thus, it
is preferable to refer to these signals as mate-recognition signals rather than
species-recognition signals or sexually selected signals (Ryan and Rand
1993). Sexual selection generated by these receiver preferences can cause
the elaboration of mate-recognition signals. This has been most clearly seen
in the elaborate plumage of many male birds but is no less extreme when
one examines the variety of sounds that males use to coax a female into
mating.The power of sexual selection is documented not only in the extreme
signals that males have evolved but in the cost incurred to exhibit them.
Metabolic rate can increase tenfold during calling in some frogs (Section 2
of this chapter; Wells 2001), and crickets (Cade 1975) and frogs (Tuttle and
Ryan 1981) can attract deadly acoustically orienting parasitoids and frogeating bats when signaling for females (Section 4 of this chapter).
Much of sexual selection involves communication. One is interested in a
dyadic interaction between signaler and receiver, how signal variation is
perceived and acted on by the receiver, and the reproductive consequences
of such actions for both pairs of the communicating dyad. As with speciation theory, a major issue is how signal and receiver are functionally linked.
Behavioral studies of sexual selection have tended to concentrate on
what information the signal conveys to the receiver about the sender. Some
of this information might be useful in allowing a female to determine
whether a male is of the appropriate species or is in control of resources
critical to the female’s immediate reproductive success. In such cases, selection should favor receiver preference for the signal variant that guides the
female to the mate that maximizes her reproductive success. In other cases,
it is suggested that females are not as interested in maximizing the number
of offspring they birth but instead choose males whose signals suggest that
5. Selection on Signals
245
Although the focus on the role of long-distance communication in speciation continues (e.g., Howard et al. 1998), even more interest has centered
on its function in sexual selection (Andersson 1994). Although speciation
is the process that gives us diversity through an increase in species numbers,
sexual selection is a process that enhances diversity within species. It is
primarily this process that makes males and females of the same species
look so different from one another—humans included. It is also the process
that makes some animals look so attractive and stunning, sound so charming, and behave in manners that are sometimes amusing to us.
Sexual selection favors traits that enhance reproductive success by
enhancing one’s ability to acquire mates. This can be achieved by a male
becoming more attractive to a female, and often, but certainly not in all
cases, this judgment of attractiveness is influenced by the male’s longdistance signal.A large number of studies of acoustically advertising animals
shows that receivers’ preferences for long-distance signals will not only
guide them to males of their own species but often to a subset of males
within the species (Ryan and Keddy-Hector 1992; Andersson 1994). Thus, it
is preferable to refer to these signals as mate-recognition signals rather than
species-recognition signals or sexually selected signals (Ryan and Rand
1993). Sexual selection generated by these receiver preferences can cause
the elaboration of mate-recognition signals. This has been most clearly seen
in the elaborate plumage of many male birds but is no less extreme when
one examines the variety of sounds that males use to coax a female into
mating.The power of sexual selection is documented not only in the extreme
signals that males have evolved but in the cost incurred to exhibit them.
Metabolic rate can increase tenfold during calling in some frogs (Section 2
of this chapter; Wells 2001), and crickets (Cade 1975) and frogs (Tuttle and
Ryan 1981) can attract deadly acoustically orienting parasitoids and frogeating bats when signaling for females (Section 4 of this chapter).
Much of sexual selection involves communication. One is interested in a
dyadic interaction between signaler and receiver, how signal variation is
perceived and acted on by the receiver, and the reproductive consequences
of such actions for both pairs of the communicating dyad. As with speciation theory, a major issue is how signal and receiver are functionally linked.
Behavioral studies of sexual selection have tended to concentrate on
what information the signal conveys to the receiver about the sender. Some
of this information might be useful in allowing a female to determine
whether a male is of the appropriate species or is in control of resources
critical to the female’s immediate reproductive success. In such cases, selection should favor receiver preference for the signal variant that guides the
female to the mate that maximizes her reproductive success. In other cases,
it is suggested that females are not as interested in maximizing the number
of offspring they birth but instead choose males whose signals suggest that
5. Selection on Signals
245