4. Acoustic Communication in Whales and Dolphins
195
interactions with other whales. The sex of a singing whale can be determined
by underwater photography of the genital slit (Glockner 1983) or by taking
a small tissue sample for genetic determination of sex (Baker et al. 1991;
Palsbl/lll et al. 1992). These analyses revealed that almost all singing
humpbacks are lone adult males.
Extensive research on the interactions between singing humpbacks and
other whales on the breeding ground suggest that song is a reproductive
advertisement display with functions both in male-male competition (intrasexual selection) and female choice (intersexual selection). Observations of
interactions between humpbacks emphasize the role of song in mediating
interactions between males, but females may also monitor, approach, and
join with singers (Tyack 1981). The acoustic structure of humpback song
clearly has attributes associated with both intra- and intersexual selection,
and is more comple~ than would be expected for a signal used only for
male-male interactions (Helweg et al. 1992; Bradbury and Vehrencamp
1 9 9 8 ) . _
A variety of results have encouraged biologists to suggest that humpback
song plays a role in male-male competition:
• Song appears to act as a spacing mechanism, maintaining distance
between singers (Tyack 1981; Frankel et al. 1995; Helweg et al. 1992).
• No known females have been attracted to playbacks of song (Tyack 1983;
Mobley et al. 1988).
• Aggressive interactions (particularly between singers and known males)
are much more commonly observed than sexual interactions (particularly
between singers and known females) (Tyack 1981,1982; Darling 1983).
These behavioral observations are clearly consistent with the idea that song
plays a role in mediating male-male interactions.
Evidence that humpback song appears to be used in male-male interactions does not mean that it is not also used by females to select a mate. Both
intra- and intersexual selection can operate at the same time on the same
display (Catchpole 1982; Bradbury and Vehrencamp 1998). In many species,
aggressive male-male interactions are much more common and obvious
than male-female interactions. Just because the responses of male humpbacks to song are seen more frequently than those of females does not mean
that the subtler and rarer responses of females to singers are not biologically significant. Female humpbacks have been observed to join with singers
(Medrano et al. 1994), and after they join, behavior associated with sexual
activity in other species of whale has been observed (Tyack 1981). Many
features of humpback song are consistent with a role in intersexual selection, and these features differ from those suggested for intrasexual selection. Intersexual selection may favor female-attracting songs to be long,
complex, and continuous, while intrasexual competition would be likely to
favor songs that are shorter, simpler, and sung with gaps of silence, so that
a singer could listen for rival males (Catchpole and Slater 1995; Bradbury
195
interactions with other whales. The sex of a singing whale can be determined
by underwater photography of the genital slit (Glockner 1983) or by taking
a small tissue sample for genetic determination of sex (Baker et al. 1991;
Palsbl/lll et al. 1992). These analyses revealed that almost all singing
humpbacks are lone adult males.
Extensive research on the interactions between singing humpbacks and
other whales on the breeding ground suggest that song is a reproductive
advertisement display with functions both in male-male competition (intrasexual selection) and female choice (intersexual selection). Observations of
interactions between humpbacks emphasize the role of song in mediating
interactions between males, but females may also monitor, approach, and
join with singers (Tyack 1981). The acoustic structure of humpback song
clearly has attributes associated with both intra- and intersexual selection,
and is more comple~ than would be expected for a signal used only for
male-male interactions (Helweg et al. 1992; Bradbury and Vehrencamp
1 9 9 8 ) . _
A variety of results have encouraged biologists to suggest that humpback
song plays a role in male-male competition:
• Song appears to act as a spacing mechanism, maintaining distance
between singers (Tyack 1981; Frankel et al. 1995; Helweg et al. 1992).
• No known females have been attracted to playbacks of song (Tyack 1983;
Mobley et al. 1988).
• Aggressive interactions (particularly between singers and known males)
are much more commonly observed than sexual interactions (particularly
between singers and known females) (Tyack 1981,1982; Darling 1983).
These behavioral observations are clearly consistent with the idea that song
plays a role in mediating male-male interactions.
Evidence that humpback song appears to be used in male-male interactions does not mean that it is not also used by females to select a mate. Both
intra- and intersexual selection can operate at the same time on the same
display (Catchpole 1982; Bradbury and Vehrencamp 1998). In many species,
aggressive male-male interactions are much more common and obvious
than male-female interactions. Just because the responses of male humpbacks to song are seen more frequently than those of females does not mean
that the subtler and rarer responses of females to singers are not biologically significant. Female humpbacks have been observed to join with singers
(Medrano et al. 1994), and after they join, behavior associated with sexual
activity in other species of whale has been observed (Tyack 1981). Many
features of humpback song are consistent with a role in intersexual selection, and these features differ from those suggested for intrasexual selection. Intersexual selection may favor female-attracting songs to be long,
complex, and continuous, while intrasexual competition would be likely to
favor songs that are shorter, simpler, and sung with gaps of silence, so that
a singer could listen for rival males (Catchpole and Slater 1995; Bradbury