444
sound propagation because the speed of sound varies with substrate type (e.g., the
speed of sound is faster in rock versus sand or clay, etc.) which in turn will affect
the cutoff frequency of sound propagation (Bass and Clark 2003 ; Rogers and Cox
1988 ). In terms of signal perception by the receiver, the harmonics of the mate call
may also infl uence the detection of the signal when females are near the source of
the advertisement call. McKibben and Bass ( 2001b ) showed that encoding of a
tone at or near the fundamental frequency of the mate call was improved, at the
level of the saccular afferents, when tones similar to harmonics were added to the
presentation stimuli.
Recently Coffi n et al. ( 2012 ) showed that midshipman females also exhibit seasonal changes in saccular hair cell densities that were concurrent with changes in
sensitivity of the saccule. This increase in hair cell density was only observed in the
saccule of reproductive females and not in the other inner ear end organs (i.e., lagena
and utricle) of reproductive females. Also, the seasonal change in saccular hair cell
density was not dependent on body size because the observed increase in hair cell
density was consistent across the size range of fi sh sampled. In addition, Coffi n
et al. ( 2012 ) also reported an increase in the number of small, potentially immature
saccular hair cell bundles in summer reproductive females. In sum, the seasonal
increase in saccular hair cell density coupled with an increase in smaller hair cell
bundles in reproductive females was concurrent with a dramatic increase in the
magnitude of the recorded evoked saccular potentials and corresponded with an
increase in saccular sensitivity. Thus, the saccular-specifi c changes in hair cell density and morphology likely contribute to the seasonal enhancement of female midshipman hearing for the detection of male advertisement calls.
2.2 Steroid-Dependent Plasticity of the Midshipman
Auditory Sense
The steroid hormones testosterone, 11-ketotestosterone and estrogen (17β-estradiol)
are known to fl uctuate seasonally in male and female plainfi n midshipman with
their reproductive cycle (Sisneros et al. 2004b ). The seasonal variation of these
steroid hormone levels is consistent with changes in midshipman reproductive
behavior and biology (Brantley et al. 1993 ; Knapp et al. 1999 ; Sisneros et al. 2004b ).
Sisneros et al. ( 2004b ) defi ned four time periods that corresponded with the annual
reproductive cycle of the plainfi n midshipman that included pre-nesting, nesting,
post-nesting, and non-reproductive periods. The pre-nesting period occurs in the
spring months of March and April when type I males and females exhibit gonadal
recrudescence of the testes and ovaries, respectively, with type I males exhibiting a
gradual increase in plasma levels of testosterone (T) and 11-ketotesosterone (11KT) while females exhibit a brief spike of 17β-estradiol (E 2 ) and T plasma levels in
April, which is approximately 1 month before to the start of summer breeding season. The nesting period occurs in late-spring and summer during the months of May
through August when type I males with an intermediate gonadal somatic index
P.M. Forlano and J.A. Sisneros
sound propagation because the speed of sound varies with substrate type (e.g., the
speed of sound is faster in rock versus sand or clay, etc.) which in turn will affect
the cutoff frequency of sound propagation (Bass and Clark 2003 ; Rogers and Cox
1988 ). In terms of signal perception by the receiver, the harmonics of the mate call
may also infl uence the detection of the signal when females are near the source of
the advertisement call. McKibben and Bass ( 2001b ) showed that encoding of a
tone at or near the fundamental frequency of the mate call was improved, at the
level of the saccular afferents, when tones similar to harmonics were added to the
presentation stimuli.
Recently Coffi n et al. ( 2012 ) showed that midshipman females also exhibit seasonal changes in saccular hair cell densities that were concurrent with changes in
sensitivity of the saccule. This increase in hair cell density was only observed in the
saccule of reproductive females and not in the other inner ear end organs (i.e., lagena
and utricle) of reproductive females. Also, the seasonal change in saccular hair cell
density was not dependent on body size because the observed increase in hair cell
density was consistent across the size range of fi sh sampled. In addition, Coffi n
et al. ( 2012 ) also reported an increase in the number of small, potentially immature
saccular hair cell bundles in summer reproductive females. In sum, the seasonal
increase in saccular hair cell density coupled with an increase in smaller hair cell
bundles in reproductive females was concurrent with a dramatic increase in the
magnitude of the recorded evoked saccular potentials and corresponded with an
increase in saccular sensitivity. Thus, the saccular-specifi c changes in hair cell density and morphology likely contribute to the seasonal enhancement of female midshipman hearing for the detection of male advertisement calls.
2.2 Steroid-Dependent Plasticity of the Midshipman
Auditory Sense
The steroid hormones testosterone, 11-ketotestosterone and estrogen (17β-estradiol)
are known to fl uctuate seasonally in male and female plainfi n midshipman with
their reproductive cycle (Sisneros et al. 2004b ). The seasonal variation of these
steroid hormone levels is consistent with changes in midshipman reproductive
behavior and biology (Brantley et al. 1993 ; Knapp et al. 1999 ; Sisneros et al. 2004b ).
Sisneros et al. ( 2004b ) defi ned four time periods that corresponded with the annual
reproductive cycle of the plainfi n midshipman that included pre-nesting, nesting,
post-nesting, and non-reproductive periods. The pre-nesting period occurs in the
spring months of March and April when type I males and females exhibit gonadal
recrudescence of the testes and ovaries, respectively, with type I males exhibiting a
gradual increase in plasma levels of testosterone (T) and 11-ketotesosterone (11KT) while females exhibit a brief spike of 17β-estradiol (E 2 ) and T plasma levels in
April, which is approximately 1 month before to the start of summer breeding season. The nesting period occurs in late-spring and summer during the months of May
through August when type I males with an intermediate gonadal somatic index
P.M. Forlano and J.A. Sisneros