258
Marine Mammal Physiology: Requisites for Ocean Living
particularly surprising given the lack of echolocation abilities in both groups (Popov and
Supin 1990; Mann et al. 2005). Underwater hearing threshold measurements with manatees have demonstrated low-frequency hearing limits near a few hundred hertz, with
high-frequency hearing up to approximately 50–100 kHz (Klishin et al. 1990; Popov and
Supin 1990; Gerstein et al. 1999; Mann et al. 2005; Gaspard et al. 2012). Like the pinniped
species that have been examined, manatees appear to be better at detecting tonal acoustic
stimuli in noise than terrestrial mammals (Gaspard et al. 2012). Together, these findings
demonstrate that although sirenians have an auditory system that is adapted for a fully
aquatic lifestyle, the lack of specializations seen in odontocetes is most likely a result of the
absence of echolocation in this phylogenetic group.
11.3 Responses to environmental noise
Most marine mammals rely on sound to exploit their environment and engage in interactions that support life-history functions. Since the beginning of the industrial age,
there has been a continuing growth in ocean noise. Increases in ocean noise have the
potential to impact the ability of marine mammals to detect and identify important
acoustic information, a process known as masking wherein the presence of one sound
impedes the detection of another. The rate of increase in ocean noise dwarfs the evolutionary time scale over which marine mammals have evolved to deal with natural ocean
noise conditions, suggesting that their ability to compensate for increased ocean noise
is limited by the inherent plasticity of their sound production and reception systems.
Human contributions to ocean noise result from shipping, commercial and recreational
boating, military activities, green energy construction and operations (e.g., wind turbines), and seismic exploration. Of these, commercial shipping is likely the most persistent noise source in the ocean as commercial shipping continues 24 hours a day and
it is distributed throughout the world’s oceans. Because of the low-frequency nature
of shipping noise (generally in the hundreds of hertz or less), mysticete whales are the
most likely to be impacted by commercial shipping, particularly in regions with very
active shipping lanes (Clark et al. 2009; Hatch et al. 2012). Nevertheless, there are many
different opportunities for high-level acoustic activity resulting from human sources to
impact sound reception in marine mammals and consideration should not be limited
to mysticetes. Neither should the question be constrained to anthropogenic sound.
Biological sources significantly contribute to ocean noise in some systems (e.g., snapping shrimp) and even marine mammals themselves may be significant contributors to
ocean noise (e.g., song competition among whales). This then raises the question: How
does a marine mammal respond when ocean noise interferes with its ability to capitalize on acoustic cues?
When in the presence of excessive noise, communication signals may be masked by
the noise excess. In such a situation, there is a poor signal-to-noise ratio and the potential
for a receiver (e.g., a listening whale) to detect and interpret the signal is reduced. When
marine mammals face conditions in which noise limits their ability to communicate, they
may change the characteristics of the signals they produce. In some instances, both the
amplitude of the signal and the rate at which it is produced may be increased, a phenomenon known as the Lombard effect. This phenomenon, which is named for the man
who discovered it, is found to occur in humans and other mammals, as well as in birds
(Brumm and Zollinger 2013). Elevations in signal amplitude are produced in northern
right whales and killer whales in response to increased ambient noise (Holt et al. 2008;
Parks et al. 2011). Increases in the rate of signal production and amplitude, as well as shifts
Marine Mammal Physiology: Requisites for Ocean Living
particularly surprising given the lack of echolocation abilities in both groups (Popov and
Supin 1990; Mann et al. 2005). Underwater hearing threshold measurements with manatees have demonstrated low-frequency hearing limits near a few hundred hertz, with
high-frequency hearing up to approximately 50–100 kHz (Klishin et al. 1990; Popov and
Supin 1990; Gerstein et al. 1999; Mann et al. 2005; Gaspard et al. 2012). Like the pinniped
species that have been examined, manatees appear to be better at detecting tonal acoustic
stimuli in noise than terrestrial mammals (Gaspard et al. 2012). Together, these findings
demonstrate that although sirenians have an auditory system that is adapted for a fully
aquatic lifestyle, the lack of specializations seen in odontocetes is most likely a result of the
absence of echolocation in this phylogenetic group.
11.3 Responses to environmental noise
Most marine mammals rely on sound to exploit their environment and engage in interactions that support life-history functions. Since the beginning of the industrial age,
there has been a continuing growth in ocean noise. Increases in ocean noise have the
potential to impact the ability of marine mammals to detect and identify important
acoustic information, a process known as masking wherein the presence of one sound
impedes the detection of another. The rate of increase in ocean noise dwarfs the evolutionary time scale over which marine mammals have evolved to deal with natural ocean
noise conditions, suggesting that their ability to compensate for increased ocean noise
is limited by the inherent plasticity of their sound production and reception systems.
Human contributions to ocean noise result from shipping, commercial and recreational
boating, military activities, green energy construction and operations (e.g., wind turbines), and seismic exploration. Of these, commercial shipping is likely the most persistent noise source in the ocean as commercial shipping continues 24 hours a day and
it is distributed throughout the world’s oceans. Because of the low-frequency nature
of shipping noise (generally in the hundreds of hertz or less), mysticete whales are the
most likely to be impacted by commercial shipping, particularly in regions with very
active shipping lanes (Clark et al. 2009; Hatch et al. 2012). Nevertheless, there are many
different opportunities for high-level acoustic activity resulting from human sources to
impact sound reception in marine mammals and consideration should not be limited
to mysticetes. Neither should the question be constrained to anthropogenic sound.
Biological sources significantly contribute to ocean noise in some systems (e.g., snapping shrimp) and even marine mammals themselves may be significant contributors to
ocean noise (e.g., song competition among whales). This then raises the question: How
does a marine mammal respond when ocean noise interferes with its ability to capitalize on acoustic cues?
When in the presence of excessive noise, communication signals may be masked by
the noise excess. In such a situation, there is a poor signal-to-noise ratio and the potential
for a receiver (e.g., a listening whale) to detect and interpret the signal is reduced. When
marine mammals face conditions in which noise limits their ability to communicate, they
may change the characteristics of the signals they produce. In some instances, both the
amplitude of the signal and the rate at which it is produced may be increased, a phenomenon known as the Lombard effect. This phenomenon, which is named for the man
who discovered it, is found to occur in humans and other mammals, as well as in birds
(Brumm and Zollinger 2013). Elevations in signal amplitude are produced in northern
right whales and killer whales in response to increased ambient noise (Holt et al. 2008;
Parks et al. 2011). Increases in the rate of signal production and amplitude, as well as shifts