have slightly different mating calls. One subspecies, A. c. blanchardi, lives
in open habitats in central Texas. The second subspecies, A. c. crepitans,
resides in forested habitats to the east. For both subspecies, calls transmitted through an open environment experience less degradation than calls
transmitted through forest. However, regardless of the environment, the
calls of A. c. crepitans are always less subject to degradation than the calls
of A. c. blanchardi. In addition, the difference in call degradation between
the open and forested habitats is less for A. c. crepitans than for A. c. blanchardi (Ryan et al. 1990). Ryan et al. (1990) suggested that the calls of the
forest subspecies, A. c. crepitans, have evolved in response to relatively
strong selection for transmission efficiency in forest and thus transmit with
less degradation in both environments. They further suggested that the calls
of A. c. blanchardi, which lives in more open habitats, have not been under
as strong environmental selection and have instead been more influenced
by selection in other contexts.
Another convincing argument in favor of Morton’s hypothesis comes
from comparisons of short- and long-distance communication signals within
species, especially in primates. As we have been discussing, long-distance
communication signals should be under strong selection for increased
transmission efficacy. But signals used over shorter ranges should not be
under as strong selection and in some cases may be selected to degrade
rapidly over distance in order to reduce eavesdropping by predators and
other unintended receivers (Endler 1992). As predicted, the whoopgobble
of mangabeys, a call used for intergroup communication, transmits farther
and with less attenuation than calls used for intragroup communication.
Three other primate species exhibit similar differences in attenuation
between calls used for long- and short-distance communication (Waser and
Waser 1977).
3.3. Environmental Selection on Signaling Behavior
For a given signal, both the location from which it is broadcast and the time
of day during which it is transmitted can influence the amount of degradation it experiences. In addition to the temporal and spectral properties of
acoustic signals, selection can also act on the behavior of the signaler.
3.3.1. Caller Height
Sounds transmitted from above the ground almost always experience
less degradation than sounds transmitted near the ground (Marten and
Marler 1977; Waser and Waser 1977; Henwood and Fabrick 1979; Brenowitz
et al. 1984; Mathevon et al. 1996). Senders of long-distance communication
signals can thus increase propagation distance simply by calling from above
the ground. Likewise, the receivers of the signals can benefit from being
above the ground regardless of the position of the sender (Dabelsteen et
al. 1993). Birds are the most obvious example of how sound transmission
5. Selection on Signals
237
in open habitats in central Texas. The second subspecies, A. c. crepitans,
resides in forested habitats to the east. For both subspecies, calls transmitted through an open environment experience less degradation than calls
transmitted through forest. However, regardless of the environment, the
calls of A. c. crepitans are always less subject to degradation than the calls
of A. c. blanchardi. In addition, the difference in call degradation between
the open and forested habitats is less for A. c. crepitans than for A. c. blanchardi (Ryan et al. 1990). Ryan et al. (1990) suggested that the calls of the
forest subspecies, A. c. crepitans, have evolved in response to relatively
strong selection for transmission efficiency in forest and thus transmit with
less degradation in both environments. They further suggested that the calls
of A. c. blanchardi, which lives in more open habitats, have not been under
as strong environmental selection and have instead been more influenced
by selection in other contexts.
Another convincing argument in favor of Morton’s hypothesis comes
from comparisons of short- and long-distance communication signals within
species, especially in primates. As we have been discussing, long-distance
communication signals should be under strong selection for increased
transmission efficacy. But signals used over shorter ranges should not be
under as strong selection and in some cases may be selected to degrade
rapidly over distance in order to reduce eavesdropping by predators and
other unintended receivers (Endler 1992). As predicted, the whoopgobble
of mangabeys, a call used for intergroup communication, transmits farther
and with less attenuation than calls used for intragroup communication.
Three other primate species exhibit similar differences in attenuation
between calls used for long- and short-distance communication (Waser and
Waser 1977).
3.3. Environmental Selection on Signaling Behavior
For a given signal, both the location from which it is broadcast and the time
of day during which it is transmitted can influence the amount of degradation it experiences. In addition to the temporal and spectral properties of
acoustic signals, selection can also act on the behavior of the signaler.
3.3.1. Caller Height
Sounds transmitted from above the ground almost always experience
less degradation than sounds transmitted near the ground (Marten and
Marler 1977; Waser and Waser 1977; Henwood and Fabrick 1979; Brenowitz
et al. 1984; Mathevon et al. 1996). Senders of long-distance communication
signals can thus increase propagation distance simply by calling from above
the ground. Likewise, the receivers of the signals can benefit from being
above the ground regardless of the position of the sender (Dabelsteen et
al. 1993). Birds are the most obvious example of how sound transmission
5. Selection on Signals
237