273
Chapter twelve: Visual and hydrodynamic flow perception
tasks that can most likely be fulfilled in very turbid waters, or if they can use their eyes
for more sophisticated visual tasks. Generally, all marine mammals might sometimes
face the problem that vision does not provide any reliable data about the environment,
the question arises which alternative sense marine mammals could rely on to replace or
complement vision in dark and murky waters?
Please note that, beyond vision, the focus is laid on hydrodynamic flow perception
in this chapter as marine mammal acoustics is elaborated profoundly and extensively in
Chapter 11. Readers interested in chemoreception in marine mammals are relegated to
existing reviews covering this field of research (see for example, Dehnhardt 2002).
Marine mammals are foraging in the benthic (at the sea bottom) as well as the pelagic
(open water) zone. At the sea bottom, prey items might be hard to detect visually as many
benthic fish are hiding in the mud or are cryptic. If vision is additionally impaired due to
low light levels and turbidity, marine mammals have to adopt alternative strategies for
the detection of prey. When hunting in the benthic zone, harbor seals were observed to
cruise over or even dig into the sea bottom looking for prey (Bowen et al. 2002), and also
dolphins (Rossbach and Herzing 1997; Smolker et al. 1997; de Gurjao et al. 2003; Mann et al.
2008), walruses (Odobenidae; cited after Kastelein and van Gaalen 1988; Kastelein et al.
1990), and sea otters (Enhydra) (Hines and Loughlin 1980) seem to have specific benthic
feeding tactics. In these conditions, marine mammals might come in direct contact with
the prey and could even actively touch the prey item. Active touch is mediated by claws or
by vibrissae in the facial region (for a review, see for example, Dehnhardt 2002; Dehnhardt
and Mauck 2008; Dehnhardt et al. 2014). Almost all marine mammals possess facial vibrissae (see Section 12.2.3). In a number of marine mammal species, these vibrissae are able to
identify objects and specific object parameters (see Section 12.2.4) and could consequently
be used during benthic foraging. However, how can marine mammals also detect and
hunt prey in the pelagic zone with their vibrissae?
Every fish that is swimming through the water is generating water disturbances
and this way leaves a so-called hydrodynamic trail behind itself (Figure 12.2c,d; Hanke
et al. 2000; Hanke and Bleckmann 2004). On the basis of the finding that harbor seals
are sensitive to vibrations when a rod generating the vibrations was directly in contact
with the vibrissae (Renouf 1979; Mills and Renouf 1986), a hydrodynamic function of
the vibrissae of harbor seals was established when a visually and acoustically masked
seal was asked to sense dipole water movements generated by a sinusoidally oscillating sphere (Figure 12.2a; Dehnhardt et al. 1998). Hydrodynamic dipole stimuli are best
suited for the physiological characterization of the sensory system in terms of detection
and/or difference thresholds (see Section 12.3.2). The seal achieved the lowest detection
threshold for dipole water movements with velocities of 245 µm/s at 50 Hz (Figure 12.2b;
Dehnhardt et al. 1998), which compares well to the sensitivity of the lateral line of some
decapods and marine teleosts (Bleckmann 1994). The detection thresholds of a California
sea lion (Zalophus californianus) measured at 20 and 30 Hz was even lower than the highest
sensitivity of the harbor seal (Dehnhardt and Mauck 2008), and the manatee’s (Trichechus
manatus latirostris) exceptional sensitivity reached 1 µm/s at 150 Hz (Gaspard et al. 2013).
Thus, the vibrissae of the marine mammals tested so far indeed function as very sensitive hydrodynamic receptors.
However, to show that an animal can indeed use a certain type of sensory information as suggested by psychophysical studies, experiments designed in a sensory ecology
approach are required. If a seal wants to use its vibrissae for prey tracking, it has to be
able to not only detect hydrodynamic events but also to actively follow hydrodynamic
trails (Figure 12.2c,d). The ability of harbor seals to track such hydrodynamic trails was
Chapter twelve: Visual and hydrodynamic flow perception
tasks that can most likely be fulfilled in very turbid waters, or if they can use their eyes
for more sophisticated visual tasks. Generally, all marine mammals might sometimes
face the problem that vision does not provide any reliable data about the environment,
the question arises which alternative sense marine mammals could rely on to replace or
complement vision in dark and murky waters?
Please note that, beyond vision, the focus is laid on hydrodynamic flow perception
in this chapter as marine mammal acoustics is elaborated profoundly and extensively in
Chapter 11. Readers interested in chemoreception in marine mammals are relegated to
existing reviews covering this field of research (see for example, Dehnhardt 2002).
Marine mammals are foraging in the benthic (at the sea bottom) as well as the pelagic
(open water) zone. At the sea bottom, prey items might be hard to detect visually as many
benthic fish are hiding in the mud or are cryptic. If vision is additionally impaired due to
low light levels and turbidity, marine mammals have to adopt alternative strategies for
the detection of prey. When hunting in the benthic zone, harbor seals were observed to
cruise over or even dig into the sea bottom looking for prey (Bowen et al. 2002), and also
dolphins (Rossbach and Herzing 1997; Smolker et al. 1997; de Gurjao et al. 2003; Mann et al.
2008), walruses (Odobenidae; cited after Kastelein and van Gaalen 1988; Kastelein et al.
1990), and sea otters (Enhydra) (Hines and Loughlin 1980) seem to have specific benthic
feeding tactics. In these conditions, marine mammals might come in direct contact with
the prey and could even actively touch the prey item. Active touch is mediated by claws or
by vibrissae in the facial region (for a review, see for example, Dehnhardt 2002; Dehnhardt
and Mauck 2008; Dehnhardt et al. 2014). Almost all marine mammals possess facial vibrissae (see Section 12.2.3). In a number of marine mammal species, these vibrissae are able to
identify objects and specific object parameters (see Section 12.2.4) and could consequently
be used during benthic foraging. However, how can marine mammals also detect and
hunt prey in the pelagic zone with their vibrissae?
Every fish that is swimming through the water is generating water disturbances
and this way leaves a so-called hydrodynamic trail behind itself (Figure 12.2c,d; Hanke
et al. 2000; Hanke and Bleckmann 2004). On the basis of the finding that harbor seals
are sensitive to vibrations when a rod generating the vibrations was directly in contact
with the vibrissae (Renouf 1979; Mills and Renouf 1986), a hydrodynamic function of
the vibrissae of harbor seals was established when a visually and acoustically masked
seal was asked to sense dipole water movements generated by a sinusoidally oscillating sphere (Figure 12.2a; Dehnhardt et al. 1998). Hydrodynamic dipole stimuli are best
suited for the physiological characterization of the sensory system in terms of detection
and/or difference thresholds (see Section 12.3.2). The seal achieved the lowest detection
threshold for dipole water movements with velocities of 245 µm/s at 50 Hz (Figure 12.2b;
Dehnhardt et al. 1998), which compares well to the sensitivity of the lateral line of some
decapods and marine teleosts (Bleckmann 1994). The detection thresholds of a California
sea lion (Zalophus californianus) measured at 20 and 30 Hz was even lower than the highest
sensitivity of the harbor seal (Dehnhardt and Mauck 2008), and the manatee’s (Trichechus
manatus latirostris) exceptional sensitivity reached 1 µm/s at 150 Hz (Gaspard et al. 2013).
Thus, the vibrissae of the marine mammals tested so far indeed function as very sensitive hydrodynamic receptors.
However, to show that an animal can indeed use a certain type of sensory information as suggested by psychophysical studies, experiments designed in a sensory ecology
approach are required. If a seal wants to use its vibrissae for prey tracking, it has to be
able to not only detect hydrodynamic events but also to actively follow hydrodynamic
trails (Figure 12.2c,d). The ability of harbor seals to track such hydrodynamic trails was