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Chapter twelve: Visual and hydrodynamic flow perception
12.2.3 The vibrissal system: Adaptations to the aquatic medium
Vibrissae in the facial region can be found in many marine mammals. In sirenians,
vibrissae additionally cover the whole body (Bryden et al. 1978; Reep et al. 2002, 2011),
and in baleen whales, vibrissae can also be found around the blowholes (Japha 1907,
1912; Nakai and Shida 1948; Ling 1977). Toothed whales possess vibrissae on the jaw,
however, they lose them already prenatally or shortly after birth (Ling 1977). Amazon
River dolphins are an exception as, in this species, adults still have bristles on the jaw
(Kükenthal 1909; Layne and Caldwell 1964). However, these hairs more resemble guard
hairs that are found in the body fur of mammals. The river dolphin’s bristles might also
be richly innervated but lack the follicle-sinus complex of vibrissae. In some odontocetes,
the follicle crypts that remain after the vibrissae are lost are large in diameter, and
recent investigations in the Guiana dolphin (Sotalia guianensis) have described these
empty crypts as functional electrosensitive organs (Mauck et al. 2000; Czech-Damal
et al. 2011).
Vibrissal hairs are anchored in follicles. The whole unit is usually referred to as
vibrissal follicle-sinus complex (F-SC; Rice et al. 1986). Due to this feature, vibrissae are
also called sinus-hairs. Detailed anatomical investigations of the F-SCs of marine mammals are available for some pinnipeds (Stevens et al. 1973; Hyvärinen and Katajisto 1984;
Hyvärinen 1989; Marshall et al. 2006; Hyvärinen et al. 2009) and sirenians (Reep et al.
2001) and revealed specific adaptations which can be linked to their function in the
aquatic environment. In comparison to terrestrial species such as the cat (Felis catus; Ebara
et al. 2002), the mystacial F-SCs of pinnipeds are much larger as their F-SC possess an
additional blood sinus, the upper cavernous sinus. Its length accounts for 60% of the total
F-SC length. The existence of an additional cavernous sinus probably leads to elevated
surface temperatures measured at the vibrissae of, for example, harbor seals (Mauck et al.
2000) and maybe also in Cape fur seals (Arctocephallus pusillus pusillus) (Erdsack et al.
2014). This way the mechanoreceptors at the ring sinus are thermally shielded from low
external temperatures, and, consequently, the vibrissae can retain high tactile sensitivity
even in cold waters (Dehnhardt et al. 1998). In cold waters, low-melting-point monoenoic fatty acids in the adipose tissue around the vibrissae render the tissue very flexible
ensuring high vibrissal mobility (Käkelä and Hyvärinen 1993, 1996). Furthermore, the
innervation of the F-SCs of pinnipeds and sirenians by the deep vibrissal nerve is much
higher in comparison to terrestrial species which might point to the significance of the
vibrissal system in the aquatic medium. In ringed, bearded seals (Erignathus barbatus) and
northern elephant seals, for example, 1000–1600 axons transmit the sensory information
from one F-SC to the brain (Hyvärinen and Katajisto 1984; Hyvärinen 1989; Marshall et al.
2006; Hyvärinen et al. 2009; McGovern et al. 2015), thus the innervation density of these
pinnipeds is 10 times higher as in cats or rats (Ebara et al. 2002).
12.2.4 Basis for hydrodynamic flow perception
Hydrodynamic flow perception investigated in harbor seals, the California sea lion, and
the Florida manatee was preceded by a number of studies examining the haptic abilities
of pinnipeds and sireneans whereas comparable studies in cetaceans are still missing.
Dykes (1975) first concluded from his single unit recordings from the infraorbital branch
of the Nervus trigeminus of harbor seals and gray seals (Halichoerus grypus) that the
vibrissal function is active touch. Research on the haptic abilities of the walrus (Odobenus
rosmarus divergens; Kastelein and van Gaalen 1988; Kastelein et al. 1990), the California
Chapter twelve: Visual and hydrodynamic flow perception
12.2.3 The vibrissal system: Adaptations to the aquatic medium
Vibrissae in the facial region can be found in many marine mammals. In sirenians,
vibrissae additionally cover the whole body (Bryden et al. 1978; Reep et al. 2002, 2011),
and in baleen whales, vibrissae can also be found around the blowholes (Japha 1907,
1912; Nakai and Shida 1948; Ling 1977). Toothed whales possess vibrissae on the jaw,
however, they lose them already prenatally or shortly after birth (Ling 1977). Amazon
River dolphins are an exception as, in this species, adults still have bristles on the jaw
(Kükenthal 1909; Layne and Caldwell 1964). However, these hairs more resemble guard
hairs that are found in the body fur of mammals. The river dolphin’s bristles might also
be richly innervated but lack the follicle-sinus complex of vibrissae. In some odontocetes,
the follicle crypts that remain after the vibrissae are lost are large in diameter, and
recent investigations in the Guiana dolphin (Sotalia guianensis) have described these
empty crypts as functional electrosensitive organs (Mauck et al. 2000; Czech-Damal
et al. 2011).
Vibrissal hairs are anchored in follicles. The whole unit is usually referred to as
vibrissal follicle-sinus complex (F-SC; Rice et al. 1986). Due to this feature, vibrissae are
also called sinus-hairs. Detailed anatomical investigations of the F-SCs of marine mammals are available for some pinnipeds (Stevens et al. 1973; Hyvärinen and Katajisto 1984;
Hyvärinen 1989; Marshall et al. 2006; Hyvärinen et al. 2009) and sirenians (Reep et al.
2001) and revealed specific adaptations which can be linked to their function in the
aquatic environment. In comparison to terrestrial species such as the cat (Felis catus; Ebara
et al. 2002), the mystacial F-SCs of pinnipeds are much larger as their F-SC possess an
additional blood sinus, the upper cavernous sinus. Its length accounts for 60% of the total
F-SC length. The existence of an additional cavernous sinus probably leads to elevated
surface temperatures measured at the vibrissae of, for example, harbor seals (Mauck et al.
2000) and maybe also in Cape fur seals (Arctocephallus pusillus pusillus) (Erdsack et al.
2014). This way the mechanoreceptors at the ring sinus are thermally shielded from low
external temperatures, and, consequently, the vibrissae can retain high tactile sensitivity
even in cold waters (Dehnhardt et al. 1998). In cold waters, low-melting-point monoenoic fatty acids in the adipose tissue around the vibrissae render the tissue very flexible
ensuring high vibrissal mobility (Käkelä and Hyvärinen 1993, 1996). Furthermore, the
innervation of the F-SCs of pinnipeds and sirenians by the deep vibrissal nerve is much
higher in comparison to terrestrial species which might point to the significance of the
vibrissal system in the aquatic medium. In ringed, bearded seals (Erignathus barbatus) and
northern elephant seals, for example, 1000–1600 axons transmit the sensory information
from one F-SC to the brain (Hyvärinen and Katajisto 1984; Hyvärinen 1989; Marshall et al.
2006; Hyvärinen et al. 2009; McGovern et al. 2015), thus the innervation density of these
pinnipeds is 10 times higher as in cats or rats (Ebara et al. 2002).
12.2.4 Basis for hydrodynamic flow perception
Hydrodynamic flow perception investigated in harbor seals, the California sea lion, and
the Florida manatee was preceded by a number of studies examining the haptic abilities
of pinnipeds and sireneans whereas comparable studies in cetaceans are still missing.
Dykes (1975) first concluded from his single unit recordings from the infraorbital branch
of the Nervus trigeminus of harbor seals and gray seals (Halichoerus grypus) that the
vibrissal function is active touch. Research on the haptic abilities of the walrus (Odobenus
rosmarus divergens; Kastelein and van Gaalen 1988; Kastelein et al. 1990), the California