life-history parameters assumed to be predictive of higher survivorship
were measured. The offspring of males with long calls performed significantly better than or not significantly differently from those of males with
short calls. Although the data are too sparse to make any sweeping generalizations, the studies of gray tree frogs offer some of the best support for
the notion that females can assess variation in long-distance acoustic signals
to influence the genetic quality of their young.
6.3. Sensory Exploitation
Plants have evolved suites of adaptations to attract pollinators, much as
males often use long-distance signals to attract females. In many cases, the
attractions are mutualistic. The pollinator is attracted by various signals of
the plant and inadvertently pollinates the plant while harvesting nectar. A
number of plants, however, do not produce nectar. Orchids, for example,
mimic the pheromones or the general body outline of insects; insects then
attempt to mate with the flower, pollinating it during their mistaken sexual
foray (Piji and Dodson 1966).
Although these interactions do not involve acoustic signals, they illustrate
a relevant point. Congruence between the signal and receiver need not
involve the sorts of genetic relationships within a genome envisioned in
pleiotropy and linkage disequilibrium. Such phenomena cannot occur
between species. In the orchid example, it appears that plants have evolved
signals that exploit the insect’s responses to conspecific sexual signals. The
response of the insect to the orchid is not adaptive for the insect, but we
assume that there is a net benefit to the receiver’s biases; that is, the sum
of costs and benefits of responding to plants and to sexually receptive conspecifics. Recent studies in sexual selection and communication have suggested that males evolve signals to exploit response biases of the females’
receiver system (recently reviewed in Christy 1995; Endler and Basolo 1998;
Ryan 1998, 1999).
6.3.1. Response Biases
Williams (1966) made a crucial point in distinguishing between an evolved
function and incidental consequence in evolution. In the context of animal
communication, the response properties of the receiver can be under selection to recognize the signal of a conspecific; if so, conspecific recognition is an
evolved function. Depending on the recognition strategy, however, other
stimuli might elicit strong receiver responses. For example, consider a
receiver in an acoustic environment in which there is only one heterospecific
using acoustic signals for mate recognition and its signal is much shorter in
duration than the conspecific signal. A simple and effective recognition
strategy would be to respond only to signals above a certain threshold in
duration; this strategy, too, is an evolved function of the receiver. It seems that
recognition strategies often involve such simple generalizations, as opposed
5. Selection on Signals
255
were measured. The offspring of males with long calls performed significantly better than or not significantly differently from those of males with
short calls. Although the data are too sparse to make any sweeping generalizations, the studies of gray tree frogs offer some of the best support for
the notion that females can assess variation in long-distance acoustic signals
to influence the genetic quality of their young.
6.3. Sensory Exploitation
Plants have evolved suites of adaptations to attract pollinators, much as
males often use long-distance signals to attract females. In many cases, the
attractions are mutualistic. The pollinator is attracted by various signals of
the plant and inadvertently pollinates the plant while harvesting nectar. A
number of plants, however, do not produce nectar. Orchids, for example,
mimic the pheromones or the general body outline of insects; insects then
attempt to mate with the flower, pollinating it during their mistaken sexual
foray (Piji and Dodson 1966).
Although these interactions do not involve acoustic signals, they illustrate
a relevant point. Congruence between the signal and receiver need not
involve the sorts of genetic relationships within a genome envisioned in
pleiotropy and linkage disequilibrium. Such phenomena cannot occur
between species. In the orchid example, it appears that plants have evolved
signals that exploit the insect’s responses to conspecific sexual signals. The
response of the insect to the orchid is not adaptive for the insect, but we
assume that there is a net benefit to the receiver’s biases; that is, the sum
of costs and benefits of responding to plants and to sexually receptive conspecifics. Recent studies in sexual selection and communication have suggested that males evolve signals to exploit response biases of the females’
receiver system (recently reviewed in Christy 1995; Endler and Basolo 1998;
Ryan 1998, 1999).
6.3.1. Response Biases
Williams (1966) made a crucial point in distinguishing between an evolved
function and incidental consequence in evolution. In the context of animal
communication, the response properties of the receiver can be under selection to recognize the signal of a conspecific; if so, conspecific recognition is an
evolved function. Depending on the recognition strategy, however, other
stimuli might elicit strong receiver responses. For example, consider a
receiver in an acoustic environment in which there is only one heterospecific
using acoustic signals for mate recognition and its signal is much shorter in
duration than the conspecific signal. A simple and effective recognition
strategy would be to respond only to signals above a certain threshold in
duration; this strategy, too, is an evolved function of the receiver. It seems that
recognition strategies often involve such simple generalizations, as opposed
5. Selection on Signals
255