74
extends to higher frequencies. These comparisons show that the sound characteristics of a single species may span a wide range of frequencies and are not made by a
single sound production mechanism or behavior. Thus the acoustic repertoire for
some species covers a wide range of frequencies as the result of the production of
several sound types (as in C. unimaculatus and C. multicinctus ), whereas the number and frequency range of sounds produced by other species may be more
limited.
3.3 Evolution of Sound Production in Butterfl yfi shes
The evolutionary interpretations of the diversity of acoustic behaviors and sound
production mechanisms in butterfl yfi shes are in the very early phases of analysis,
but some patterns are beginning to emerge. Characters defi ned by acoustic behaviors that are mapped onto a molecular phylogeny of butterfl yfi shes shows that the
head bob and body motion behaviors may be shared among some members of the
bannerfi sh and Chaetodon clades, whereas the tail slap acoustic behavior may be a
character only of Chaetodon (Fig. 7 , Tricas and Boyle 2015a ). The prominent head
bob acoustic behavior in Forcipiger , which is driven by several muscles and a ventral linkage between the head and pectoral girdle (see Sect. 3.1 ), is not yet known in
Hemitaurichthys or Heniochus (see Sect. 3.1 ). The head bob behavior in C. unimaculatus (Clade 2) and C. auriga (Clade 4) includes a prominent and active protrusion of the jaws, whereas C. kleinii (Clade 2) only demonstrates the prominent
jaw protrusion and not the head bob. The apparent variation in the occurrence of the
head bob and associated jaw actions among Chaeotodon species leaves the question
of the ancestral character state of the head bob behavior open to interpretation (Fig.
8a ). The head bob behavior is apparently lacking in both C. multicinctus (also Clade
2) and in C. ornatissimus (Clade 3). In addition, preliminary EMG experiments on
C. multicinctus demonstrate activity of the ventral portion of the anterior epaxial
muscles lateral to the anterior swim bladder horns during the production of the body
motion sound (Boyle and Tricas, unpublished data). This muscle activity pattern
appears to be most similar to that observed for the nearby region of the hypaxial
muscles in both Hemitaurichthys (Boyle and Tricas 2010 ) and Heniochus
(Parmentier et al. 2011 ). Further kinematic, anatomical, and electromyographic
analyses are needed to test the hypotheses that the head bob sound is a shared trait
nested deep within the butterfl yfi sh phylogeny, that it was lost at least once in the
bannerfi shes and twice in Chaetodon , or that it has evolved in parallel in these
groups.
In contrast to the head bob behavior, the most distinctive acoustic behavior in
Chaetodon is the tail slap, which is not yet reported in the bannerfi shes. Statistical
models that use currently available data to reconstruct the ancestral character states
for the tail slap behavior show that this character may have existed in the ancestors
of the Chaetodon clade (Tricas and Boyle 2015a ). However, the existence of the tail
slap behavior in the common ancestor of both the bannerfi shes and Chaetodon canT.C. Tricas and J.F. Webb
extends to higher frequencies. These comparisons show that the sound characteristics of a single species may span a wide range of frequencies and are not made by a
single sound production mechanism or behavior. Thus the acoustic repertoire for
some species covers a wide range of frequencies as the result of the production of
several sound types (as in C. unimaculatus and C. multicinctus ), whereas the number and frequency range of sounds produced by other species may be more
limited.
3.3 Evolution of Sound Production in Butterfl yfi shes
The evolutionary interpretations of the diversity of acoustic behaviors and sound
production mechanisms in butterfl yfi shes are in the very early phases of analysis,
but some patterns are beginning to emerge. Characters defi ned by acoustic behaviors that are mapped onto a molecular phylogeny of butterfl yfi shes shows that the
head bob and body motion behaviors may be shared among some members of the
bannerfi sh and Chaetodon clades, whereas the tail slap acoustic behavior may be a
character only of Chaetodon (Fig. 7 , Tricas and Boyle 2015a ). The prominent head
bob acoustic behavior in Forcipiger , which is driven by several muscles and a ventral linkage between the head and pectoral girdle (see Sect. 3.1 ), is not yet known in
Hemitaurichthys or Heniochus (see Sect. 3.1 ). The head bob behavior in C. unimaculatus (Clade 2) and C. auriga (Clade 4) includes a prominent and active protrusion of the jaws, whereas C. kleinii (Clade 2) only demonstrates the prominent
jaw protrusion and not the head bob. The apparent variation in the occurrence of the
head bob and associated jaw actions among Chaeotodon species leaves the question
of the ancestral character state of the head bob behavior open to interpretation (Fig.
8a ). The head bob behavior is apparently lacking in both C. multicinctus (also Clade
2) and in C. ornatissimus (Clade 3). In addition, preliminary EMG experiments on
C. multicinctus demonstrate activity of the ventral portion of the anterior epaxial
muscles lateral to the anterior swim bladder horns during the production of the body
motion sound (Boyle and Tricas, unpublished data). This muscle activity pattern
appears to be most similar to that observed for the nearby region of the hypaxial
muscles in both Hemitaurichthys (Boyle and Tricas 2010 ) and Heniochus
(Parmentier et al. 2011 ). Further kinematic, anatomical, and electromyographic
analyses are needed to test the hypotheses that the head bob sound is a shared trait
nested deep within the butterfl yfi sh phylogeny, that it was lost at least once in the
bannerfi shes and twice in Chaetodon , or that it has evolved in parallel in these
groups.
In contrast to the head bob behavior, the most distinctive acoustic behavior in
Chaetodon is the tail slap, which is not yet reported in the bannerfi shes. Statistical
models that use currently available data to reconstruct the ancestral character states
for the tail slap behavior show that this character may have existed in the ancestors
of the Chaetodon clade (Tricas and Boyle 2015a ). However, the existence of the tail
slap behavior in the common ancestor of both the bannerfi shes and Chaetodon canT.C. Tricas and J.F. Webb