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Chapter twelve: Visual and hydrodynamic flow perception
investigated in a number of experiments. Hydrodynamic trails up to 40 m length were
generated mainly by remote-controlled miniature submarines (Dehnhardt et al. 2001;
Wieskotten et al. 2010a), but additionally by conspecifics (Schulte-Pelkum et al. 2007) or
artificial fish (Kilian 2010) for comparison. All experiments revealed the seal’s extraordinary trail-following abilities. They could not only follow the trails with high precision irrespective of trail generator, but even did so when they were allowed to follow the trail after
the trail had aged by more than 20 s (Wieskotten et al. 2010a). Mimicking burst-and-glide
swimming of some fish (Videler and Weihs 1982; Blake 1983; Hinch et al. 2002; Standen
et al. 2004), the seals even performed well when the trail consisted of a phase in which the
submarine was actively swimming and a phase in which it was only gliding (Wieskotten
et al. 2010a). However, if the seal was unsuccessful in following this kind of trail, it lost the
trail mainly in the transition zone between active propelled and gliding phase indicating that burst-and-glide swimming might be an effective antipredator strategy. In 2011,
a comparative hydrodynamic trail-following study was conducted with a California sea
lion (Gläser et al. 2011). The sea lion followed the trails successfully, however, its performance broke down rapidly when a delay was introduced between trail generation and
start of trail-following. These differences might originate in the different morphology of
the vibrissae of seals and sea lions (see Section 12.2.4).
Before actually starting a trail-following event, it is essential to know, for example,
in which direction the fish that had generated the trail is swimming. Additionally, if a
trail follower was able to analyze a hydrodynamic trail in detail, reading a generator’s
features such as size or form out of the trail, it could optimize foraging and could only
go for the best prey item or it could also decide to swim into the opposite direction if the
trail had been generated by a predator. These suggestions are based on the fact that the
hydrodynamic trails generated by different fish species differ due to the fish’s different
size, shape, or swimming style (Hanke and Bleckmann 2004). Wieskotten et al. (2010b,
2011) approached these questions with harbor seals and generated hydrodynamic trails
by a multitude of paddles. All these experiments were conducted in a box which enabled
the generation of well-defined and measurable hydrodynamic events in calm waters. The
seal indicated movement direction with high precision and with a contact time of vibrissae with the hydrodynamic trail of less than 0.5 s (Wieskotten et al. 2010b). Again, the seal
was reliably answering even when the hydrodynamic event was up to 35 seconds old.
Additionally, the seal discriminated objects differing in size and shape with high precision (Wieskotten et al. 2011). In conclusion, harbor seals can gain a multitude of information from reading a hydrodynamic trail.
With particle image velocimetry (PIV) (see Section 12.3.3), the seal’s behavior during
hydrodynamic perception as well as the generated trails were measured and analyzed
which revealed various parameters that the seals could have potentially used to make their
decisions. Among those, the vortices included in the trails seem to provide powerful information. This finding led to a series of hydrodynamic experiments in which it was and
still is investigated which kind of information harbor seals can gain from single vortices
(Dehnhardt et al. 2014). The results indicate that harbor seals are able to assess the travel
direction of a single vortex ring irrespective of position of stimulation at the vibrissae, ipsior contralateral vibrissal pad. Furthermore, when presented with two vortex rings in succession they can judge these vortices on the basis of size (Krüger et al. 2014). In conclusion,
the harbor seal’s documented sensitivity to hydrodynamic events allows the seal to detect
and follow prey even in dark and murky waters. Moreover, when encountering a hydrodynamic event, seals can interpret the events with high precision for example, comparable to
human trackers reading and interpreting the traces left behind by animals or other humans.
Chapter twelve: Visual and hydrodynamic flow perception
investigated in a number of experiments. Hydrodynamic trails up to 40 m length were
generated mainly by remote-controlled miniature submarines (Dehnhardt et al. 2001;
Wieskotten et al. 2010a), but additionally by conspecifics (Schulte-Pelkum et al. 2007) or
artificial fish (Kilian 2010) for comparison. All experiments revealed the seal’s extraordinary trail-following abilities. They could not only follow the trails with high precision irrespective of trail generator, but even did so when they were allowed to follow the trail after
the trail had aged by more than 20 s (Wieskotten et al. 2010a). Mimicking burst-and-glide
swimming of some fish (Videler and Weihs 1982; Blake 1983; Hinch et al. 2002; Standen
et al. 2004), the seals even performed well when the trail consisted of a phase in which the
submarine was actively swimming and a phase in which it was only gliding (Wieskotten
et al. 2010a). However, if the seal was unsuccessful in following this kind of trail, it lost the
trail mainly in the transition zone between active propelled and gliding phase indicating that burst-and-glide swimming might be an effective antipredator strategy. In 2011,
a comparative hydrodynamic trail-following study was conducted with a California sea
lion (Gläser et al. 2011). The sea lion followed the trails successfully, however, its performance broke down rapidly when a delay was introduced between trail generation and
start of trail-following. These differences might originate in the different morphology of
the vibrissae of seals and sea lions (see Section 12.2.4).
Before actually starting a trail-following event, it is essential to know, for example,
in which direction the fish that had generated the trail is swimming. Additionally, if a
trail follower was able to analyze a hydrodynamic trail in detail, reading a generator’s
features such as size or form out of the trail, it could optimize foraging and could only
go for the best prey item or it could also decide to swim into the opposite direction if the
trail had been generated by a predator. These suggestions are based on the fact that the
hydrodynamic trails generated by different fish species differ due to the fish’s different
size, shape, or swimming style (Hanke and Bleckmann 2004). Wieskotten et al. (2010b,
2011) approached these questions with harbor seals and generated hydrodynamic trails
by a multitude of paddles. All these experiments were conducted in a box which enabled
the generation of well-defined and measurable hydrodynamic events in calm waters. The
seal indicated movement direction with high precision and with a contact time of vibrissae with the hydrodynamic trail of less than 0.5 s (Wieskotten et al. 2010b). Again, the seal
was reliably answering even when the hydrodynamic event was up to 35 seconds old.
Additionally, the seal discriminated objects differing in size and shape with high precision (Wieskotten et al. 2011). In conclusion, harbor seals can gain a multitude of information from reading a hydrodynamic trail.
With particle image velocimetry (PIV) (see Section 12.3.3), the seal’s behavior during
hydrodynamic perception as well as the generated trails were measured and analyzed
which revealed various parameters that the seals could have potentially used to make their
decisions. Among those, the vortices included in the trails seem to provide powerful information. This finding led to a series of hydrodynamic experiments in which it was and
still is investigated which kind of information harbor seals can gain from single vortices
(Dehnhardt et al. 2014). The results indicate that harbor seals are able to assess the travel
direction of a single vortex ring irrespective of position of stimulation at the vibrissae, ipsior contralateral vibrissal pad. Furthermore, when presented with two vortex rings in succession they can judge these vortices on the basis of size (Krüger et al. 2014). In conclusion,
the harbor seal’s documented sensitivity to hydrodynamic events allows the seal to detect
and follow prey even in dark and murky waters. Moreover, when encountering a hydrodynamic event, seals can interpret the events with high precision for example, comparable to
human trackers reading and interpreting the traces left behind by animals or other humans.