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Marine Mammal Physiology: Requisites for Ocean Living
(Regan and Gray 2000), estimate its heading, thus the direction it is traveling to (Warren
and Hannon 1988), to locomote precisely toward a goal (Gibson 1950; Cutting et al.
1992; Warren et al. 2001) or to assess the distance that it has just traveled (Lappe et al.
2007). After the documentation of motion vision (Hanke et al. 2008) and high motion
sensitivity in harbor seals (Weiffen et al. 2014; see Section 12.2.2), optic flow perception
was recently investigated for the first time in a marine mammal (Gläser et al. 2014). In
the optic flow experiment, a harbor seal was presented with an underwater optic flow
projection which simulated a forward movement through a cloud of particles represented by 500 white dots moving out of the FOE toward the observer on straight paths
(Figure 12.1). The FOE could be on a numerous positions which were, however, covered
by a mask (Figure 12.1b, inset) in order to force the animal to rely on the global optic flow
instead of on local motion signals at the FOE. Thus, the FOE and its vicinity were not visible to the seal. Additionally, a cross was superimposed on the flow pattern (Figure 12.1b,
inset). It could either directly mark the FOE or could deviate from the FOE by a preset
angle of deviation. In the first experimental condition, the seal correctly responded if it
touched the cross on the projection; in the second condition, it was required to turn away
from the projection in order to receive a reward. By varying the angle of deviation, the
50% threshold (see Section 12.3.2) was estimated. The seal quickly found access to the
complex optic flow stimuli and was directly going with the flow indicating that the flow
pattern created an illusion of self-motion. The seal could still detect a deviation of 0.6°
between cross and FOE (50% threshold) (Figure 12.1b) which is comparable to the best
performances published so far in literature which was documented for monkeys (Britten
and van Wezel 1998, 2002; Gu et al. 2010) and humans (Warren and Hannon 1988; Warren
et al. 1988). With this sensitivity to optic flow, harbor seals can most likely rely on optic
flow information for goal-directed locomotion or assessing parameters important for
path integration such as traveled distance. In conclusion, with the documented sensitivity
to optic flow fields, harbor seals and maybe also other marine mammals can probably
benefit from particle load in the water contrary to the notion that particles only impede
underwater vision. Thus researchers will be forced to rethink visual orientation on the
basis of these findings, and numerous research questions will probably entail this first
experiment on optic flow perception (see Section 12.4).
However, under some environmental conditions, vision might indeed be almost
impossible and constantly impaired in contrast to the situations outlined so far in
which particle load changes temporally and spatially. River dolphins (Platanistidae and
Lipotidae) inhabiting riverine habitats with extreme amounts of particles constantly dissolved in the water. As expected, these species have reduced eyes (Herald et al. 1969;
Pilleri 1974). In the most extreme case, in the Indus River dolphin (Platanista gangetica), the
eyes are very small and immobile with only a small eyelid opening, the lens is lacking,
the ciliary body is atrophied, and the tapetum is almost absent. The optic nerves only contain low fiber numbers as also found in the Amazon River dolphin (Inia geoffrensis) that
only possesses approximately 15,000 axons in the optic nerve (Morgane and Jacobs 1972;
Mass and Supin 1989). This highly contrasts with, for example, the bottlenose dolphin
(Tursiops truncatus) with its well-developed eyes from which optical information within
160,000–180,000 axons is transmitted to the brain (Morgane and Jacobs 1972; Dawson et al.
1982). On the other hand, Waller observed behaviors in Platanista (Waller 1983) comparable to visually guided behavior generally found in dolphins such as visual inspection
and a well-developed retina (Waller 1982). In conclusion, it remains to be investigated
if the eyes of the river dolphins only function to detect and gather light (Herald et al.
1969) and determine the direction of light (Herald et al. 1969; Pilleri 1974), the only visual
Marine Mammal Physiology: Requisites for Ocean Living
(Regan and Gray 2000), estimate its heading, thus the direction it is traveling to (Warren
and Hannon 1988), to locomote precisely toward a goal (Gibson 1950; Cutting et al.
1992; Warren et al. 2001) or to assess the distance that it has just traveled (Lappe et al.
2007). After the documentation of motion vision (Hanke et al. 2008) and high motion
sensitivity in harbor seals (Weiffen et al. 2014; see Section 12.2.2), optic flow perception
was recently investigated for the first time in a marine mammal (Gläser et al. 2014). In
the optic flow experiment, a harbor seal was presented with an underwater optic flow
projection which simulated a forward movement through a cloud of particles represented by 500 white dots moving out of the FOE toward the observer on straight paths
(Figure 12.1). The FOE could be on a numerous positions which were, however, covered
by a mask (Figure 12.1b, inset) in order to force the animal to rely on the global optic flow
instead of on local motion signals at the FOE. Thus, the FOE and its vicinity were not visible to the seal. Additionally, a cross was superimposed on the flow pattern (Figure 12.1b,
inset). It could either directly mark the FOE or could deviate from the FOE by a preset
angle of deviation. In the first experimental condition, the seal correctly responded if it
touched the cross on the projection; in the second condition, it was required to turn away
from the projection in order to receive a reward. By varying the angle of deviation, the
50% threshold (see Section 12.3.2) was estimated. The seal quickly found access to the
complex optic flow stimuli and was directly going with the flow indicating that the flow
pattern created an illusion of self-motion. The seal could still detect a deviation of 0.6°
between cross and FOE (50% threshold) (Figure 12.1b) which is comparable to the best
performances published so far in literature which was documented for monkeys (Britten
and van Wezel 1998, 2002; Gu et al. 2010) and humans (Warren and Hannon 1988; Warren
et al. 1988). With this sensitivity to optic flow, harbor seals can most likely rely on optic
flow information for goal-directed locomotion or assessing parameters important for
path integration such as traveled distance. In conclusion, with the documented sensitivity
to optic flow fields, harbor seals and maybe also other marine mammals can probably
benefit from particle load in the water contrary to the notion that particles only impede
underwater vision. Thus researchers will be forced to rethink visual orientation on the
basis of these findings, and numerous research questions will probably entail this first
experiment on optic flow perception (see Section 12.4).
However, under some environmental conditions, vision might indeed be almost
impossible and constantly impaired in contrast to the situations outlined so far in
which particle load changes temporally and spatially. River dolphins (Platanistidae and
Lipotidae) inhabiting riverine habitats with extreme amounts of particles constantly dissolved in the water. As expected, these species have reduced eyes (Herald et al. 1969;
Pilleri 1974). In the most extreme case, in the Indus River dolphin (Platanista gangetica), the
eyes are very small and immobile with only a small eyelid opening, the lens is lacking,
the ciliary body is atrophied, and the tapetum is almost absent. The optic nerves only contain low fiber numbers as also found in the Amazon River dolphin (Inia geoffrensis) that
only possesses approximately 15,000 axons in the optic nerve (Morgane and Jacobs 1972;
Mass and Supin 1989). This highly contrasts with, for example, the bottlenose dolphin
(Tursiops truncatus) with its well-developed eyes from which optical information within
160,000–180,000 axons is transmitted to the brain (Morgane and Jacobs 1972; Dawson et al.
1982). On the other hand, Waller observed behaviors in Platanista (Waller 1983) comparable to visually guided behavior generally found in dolphins such as visual inspection
and a well-developed retina (Waller 1982). In conclusion, it remains to be investigated
if the eyes of the river dolphins only function to detect and gather light (Herald et al.
1969) and determine the direction of light (Herald et al. 1969; Pilleri 1974), the only visual