THE ANATOMICAL RECORD 00:00–00 (2015)
Sensory Hairs in the Bowhead Whale,
Balaena mysticetus (Cetacea, Mammalia)
SUMMER E. DRAKE,1,2 SAMUEL D. CRISH,3 JOHN C. GEORGE,4
RAPHAELLA STIMMELMAYR,4 AND J.G.M. THEWISSEN1*
1
Department of Anatomy and Neurobiology, Northeast Ohio Medical University,
Rootstown, Ohio
2
School of Biomedical Sciences, Kent State University, Kent, Ohio
3
Department of Pharmaceutical Sciences, Northeast Ohio Medical University, Rootstown,
Ohio
4
Department of Wildlife Management, North Slope Borough, Barrow, Alaska
ABSTRACT
We studied the histology and morphometrics of the hairs of bowhead
whales (Balaena mysticetus). These whales are hairless except for two
patches of more than 300 hairs on the rostral tip of the lower lip and
chin, the rostral tip of the upper lip, and a bilateral row of approximately
ten hairs caudal to the blowhole. Histological data indicate that hairs in
all three of these areas are vibrissae: they show an outermost connective
tissue capsule, a circumferential blood sinus system surrounding the hair
shaft, and dense innervation to the follicle. Morphometric data were collected on hair diameters, epidermal recess diameters, hair follicle length,
and external hair lengths. The main difference between the hairs in the
different regions is that blowhole hairs have larger diameters than the
hairs in the chin and rostrum regions. We speculate that the hair shaft
thickness patterns in bowheads reflect functional specializations. Anat
C 2015 Wiley Periodicals, Inc.
Rec, 00:000–000, 2015. V
Key words: hair; vibrissa; whale; Cetacea; anatomy
The presence of hair is a defining characteristic of
mammals. However, there are some orders that are
nearly devoid of hair. In these species, specialized hairs
with a mechanoreceptive function are commonly prominent. The mechanoreceptive function of hairs has been
studied extensively in subterranean mammals, including
the nearly hairless naked mole-rat (Heterocephalus glabus), which utilizes approximately 40 tactile face and
body hairs to assist in mechanosensory-guided orientation (Crish et al., 2003). Park et al. (2003) studied the
anatomy of vibrissae, guard hairs, and body hair in several rodents, using a series of antibody stain to
distinguish innervation differences. Crish et al. (2003)
demonstrated the importance of these hairs in naked
mole-rat mechanoresponse. Rice et al. (1986, 1993, 1997)
studied facial vibrissae in rats in detail, identifying specific functions for distinct nerve bundles. Medium to
large sized myelinated axons run to the follicle-sinus
complex (F-SC), enabling tactile sensitivity.
Among marine mammals, hair is abundant in pinnipeds, but is greatly reduced in sirenians (Reep et al.,
C 2015 WILEY PERIODICALS, INC.
V
2001, 2002, 2011) and nearly absent in cetaceans (Kellogg, 1928; Ling, 1977, Berta et al., 2015). Hair in sirenians is found both on the face and body, with the hairs
on the face being 30 times as dense as hairs on the rest
of the body (Marshall et al., 1998a, 1998b). These tactile
hairs serve a specialized mechanoreceptive role (Reep
et al., 2001, 2002, 2011; Dehnhardt and Mauck, 2008;
Abbreviations used: F-SC 5 follicle-sinus complex
Grant sponsor: Kent State University School of Biomedical
Science, NEOMED, the NSB-DWM, the NSB-Shell Baseline
Research Program.
*Correspondence to: J.G.M. Thewissen, Department of Anatomy
and Neurobiology, Northeast Ohio Medical University, Rootstown,
OH 44272. E-mail: thewisse@neomed.edu
Received 18 August 2014; Revised 4 March 2015; Accepted 4
March 2015.
DOI 10.1002/ar.23163
Published online 00 Month 2015 in Wiley Online Library
(wileyonlinelibrary.com).
2
DRAKE ET AL.
Fig. 1. (A) Head of bowhead whale fetus (2007B16F) with regions
with hair marked by yellow polygons. (B) Patch of adult bowhead skin
with hairs emerging from epidermal recesses. (C) Photo of living bowhead whale while at the surface swimming away from camera. Left
and right blowholes are located on an elevated area of the head, and
hairs are implanted in a left and right arch posterior to this. Photo by
Olga Shpak. (D) Close-up of paired blowhole postmortally, rostral to
left (2013B1). Arrows indicate position of all blowhole hairs, but skin
tears are postmortem artifacts.
Sarko et al., 2011) that is complementary to their other
senses, and these hairs are actually used in food manipulation (Marshall et al., 1998a,b, 2003). For aquatic
mammals that live in marine or riverine environments
with reduced visibility, vibrissae or vibrissal crypts provide an important supplement to vision for sensing the
environment (Dehnhardt et al., 1999).
In species where the distribution of hair is uneven
over the body, hairs are highest in density in locations
where stimulus detection is most likely, such as around
the eyes, chin, neck, and wrists (Sarko et al., 2011).
Facial hair is more common in mysticetes than in odontocetes, although the number of hairs and their spatial
distribution differs greatly between different species
(Yablokov and Klevezal, 1964). Slijper (1962) reports 50–
60 hairs on the face of rorquals (Balaenopteridae),
implanted in three rows on the head. The same author
reports the bowhead whale (Balaena mysticetus) as the
most hairy cetacean with about 250 “bristles” on the tips
of the lower and upper jaw and caudal to the blowhole
(Fig. 1A). The chin of the bowhead whale is the most forward projecting part of its body and, if the hairs serve a
mechanoreceptive function, this function could account
for the high density of hairs found there (Japha, 1910).
Hairs on chin and rostrum are dispersed over a large
area (Fig. 1B), and, in addition, bowheads have a single
bilateral, rostrally concave row of approximately 10
hairs just caudal to the blowhole (Tomilin, 1957; Haldiman et al., 1981; Henry et al., 1983; Fig. 1C), the only
species to be described that possesses hair in this position. Naked molerats have rows of sensory hairs separated by expanses of naked skin on the lateral side of
their trunk and abdomen, similar to the hair arrays of
bowhead whales near their blowhole (Park et al., 2003).
Bowheads have a paired blowhole, located on an elevation called a crown (Eschricht and Reinhardt, 1866),
positioned one fourth of the body length caudal to its
rostral tip. The bilateral rows of hairs are located just
caudal to the crown (Fig. 1C,D). Bowhead eyes are
located low on the sides of their head, just above the
temporomandibular joint (Tomilin, 1957; Fig. 1A). Traditional Eskimo knowledge has long understood that bowhead whales have large blind spots, due to the eyes’
lateral placement, and this has been important in
SENSORY HAIRS IN THE BOWHEAD WHALE
approaching the whale when hunting. This is reiterated
by Slijper (1962), who postulated that much of a whale’s
surrounding environment is beyond its field of vision,
including the area directly in front of, beneath, and
above the animal.
Although K€
ukenthal (1893) stated that mysticete hair
was vestigial, modern authors disagree. Nakai and
Shida (1948) postulated that baleen whales use vibrissae
as water current receptors, aiding in navigating the
open sea, and somehow in the location of food. Upon
viewing whales feeding, Yablokov and Klevezal (1964)
felt that vibrissae were serving as “closest range
receptors” that are sensitive to direct contact from
objects and less likely to be used in orientation. Bowhead whales are skim feeders, meaning that they swim
at the water’s surface and passively allow their prey to
filter through their baleen in a somewhat continuous
manner. However, rorquals are lunge feeders and eat in
discrete lunges, taking in a large amount of water and
prey at one time, then filtering large increments at a
time (Croll et al., 2002). Due to the large amount of time
that bowheads spend at the interface between the water
and the air, it is possible that the hair caudal to the
blowhole is relaying spatial information about the environmental interface, and the chin and rostral hairs could
be sensing the presence of prey. Herman and Tavolga
(1980) suggested a correlation between the persistence
of vibrissae into adulthood and the slimness and length
of the snout in the different families of mysticetes,
reflecting a difference in feeding behavior. However,
they stopped short of speculating the mechanism for this
association.
Japha (1910) described blue whale (Balaenoptera musculus) hairs as vibrissal due to their stiffness and presence of a sinus complex. Mercado (2014) described hairs
implanted on the tubercles on the humpback whale
(Megaptera novaeangliae) head and proposed a sensory
function for these, and Berta et al (2015) assumed the
same for gray whales (Eschrichtius robustus). Haldiman
et al. (1986) discussed the hairs in the bowhead and
identified them as vibrissae based on the increased hair
follicle diameter and the presence of nerves and blood
sinuses within the wall of the follicle, however, they did
not describe these observations with figures or quantitatively. Haldiman and Tarpley (1993) speculated that the
hairs are tactile, due to the presence of innervated sensory follicles, as described by Nakai and Shida (1948) in
sei whales (Balaenoptera borealis). These observations
are consistent with statements by Slijper (1962), Ling
(1977), and Sokolov (1982) about hair function in other
mysticetes.
Bowhead whale integument is similar to skin in other
mammals, although there are well understood differences (Nakai and Shida, 1958; Haldiman et al., 1981; Haldiman and Tarpley, 1993). The external skin is generally
black in pigmentation, but it displays areas of white
skin on the eyelids, flipper insertions, genitoanal area,
and the chin and occasionally the rostrum (Haldiman
and Tarpley, 1993). White spots on the lower jaw are
often arranged around a hair in their center (Haldiman
et al., 1981).
Odontocetes are naked except for a few hairs
implanted on the face of fetuses of most species (Japha,
1910; Ling, 1977), such as the phocoenid Phocoena phocoena, the delphinids Delphinus delphis, and Sotalia
3
guianensis (Dehnhardt and Mauck, 2008), the kogiid
Kogia breviceps, the platanistid Platanista gangetica
(Norman and Fraser, 1948) and the iniid Inia geoffrensis (Dehnhardt and Mauck, 2008). In these species
(Tomilin, 1957; Sterba et al., 2000), there is a row of
fewer than ten hairs on either side of the rostrum (Thewissen and Heyning, 2007), and these hairs are usually
lost soon postpartum (Nakai and Shida, 1948; Sokolov,
1982), although there are reports on captive individuals
where hairs persist and may serve a mechanoreceptive
function (Sylvestre, 1985). Norman and Fraser (1948)
reported that the Ganges dolphin, Platanista gangetica,
retains facial vibrissae throughout life. In the dolphin
Sotalia, hair follicles also remain into adulthood and
possess a sinus system (Mauck et al., 2000), and these
are thought to be electroreceptors due to high temperature output and innervation (Mauck et al., 2000; Wilkens and Hofmann, 2008; Czech-Damal et al., 2012). It
is worth noting that the narwhal, Monodon monoceros,
and beluga, Delphinapterus leucas, never have hairs,
even in fetuses (Slijper, 1962; Herman and Tavolga,
1980; Sokolov, 1982). In spite of a consistent view that
whale hairs are important in environmental detection
(Japha, 1910), documentation and analysis of precise
function has been weak. It is possible that the hairs
could serve a role as water flow detectors. It is necessary for whales to surface to breathe, the blowhole
hairs could be functioning to relay the approach of the
water’s surface, especially since this is an Arctic species
known to migrate under sea ice (George et al., 1989).
The hairs on the chin and rostrum do not breach the
water level, and could serve a mechanoreceptive role,
different from the blowhole hairs. The importance of
mechanoreception has been shown in seals and muskrats, as this sense can supplement or substitute for
other senses (Dehnhardt et al., 1998; Dehnhardt and
Mauck, 2008).
Purves (1967) speculated that mysticetes may possess
the ability to smell the wind in search of plankton. This
hypothesis assumes new importance in light of the
recent finding that the sense of olfaction of bowheads is
better developed than in most other cetaceans (Thewissen et al., 2011), which has long been part of traditional
Eskimo knowledge. It is suggested that bowheads may
detect clouds of krill by the specific airborne odor of
dimethyl sulfide that is released when krill feeds on
phytoplankton (Dacey and Wakeham, 1986; Savoca and
Nevitt, 2014). These odors could be transported by the
wind across the ocean surface and give a bowhead whale
downwind cues to a potential food source (Hagelin et al.,
2012). However, olfaction is not a directional sense.
Therefore, the whale would need to ascertain the wind
direction in order to locate the food, difficult for a mostly
submerged animal. It is here that the vibrissae could
play an important role, relaying the wind direction to
the whale.
Here, we are testing the hypotheses by Tomilin
(1957), Henry et al. (1983), Haldiman et al. (1986), and
Haldiman and Tarpley (1993) that the bowhead’s hairs
are vibrissae. We also hypothesize that hairs near the
blowhole, rostrum, and chin have different functions. In
particular, we propose that the hairs on the blowhole differ from the hairs on the chin and rostrum and will use
morphometrics on the hair, hair follicle, and epidermal
recess, to test this.
4
DRAKE ET AL.
Fig. 2. (A) Close-up of bowhead chin hair (2012B7.20c). (B) Closeup of hairs and epidermal recesses (2012B7.21). White bars indicate
how hair thickness and epidermal recess diameters were determined.
We consider all keratinous filamentous structures in
mammals as hair, and this includes vibrissae. Vibrissae
are a distinct type of hair in mammals and are characterized by a series of morphological specializations (Burgess
and Perl, 1973; Marshall et al., 2006). Vibrissae are
thicker and stiffer than pelagic hairs and display a F-SC
consisting of a dermal papilla wrapped in a dense connective tissue capsule, surrounded by a prominent circumferential blood sinus complex, and with dense innervation
(Reep et al., 2001). A vibrissal hair is associated with displacement detection (Burgess and Perl, 1973), and it conducts external stimuli down the shaft to transmit
vibrotactile information from the surrounding environment to these receptors at the base of the F-SC (Burgess
and Perl, 1973; Gottschaldt et al., 1973; Dykes, 1975;
Halata, 1975).
of the village where it was caught (e.g., B for Barrow),
and a serial number (e.g., 10, the tenth whale caught in
a year). An F is appended to indicate that the specimen
was a fetus.
For histological study, samples were twice rinsed in
1% phosphate buffered saline, and extraneous fat and
skin were trimmed away from the follicle. Follicles were
dehydrated and embedded in paraffin, cut at 10 micrometers thickness on a microtome, in a plane perpendicular to the skin’s surface, then mounted on slides (Fisher
Scientific Fisherbrand Colorfrost Plus). Slides were
deparaffinized and stained with hematoxylin and eosin,
or with Lapham’s Stain (Lapham et al., 1964). Lapham’s
stain is of particular use in visualizing myelin and glia
associated with neurons.
Hair diameters were measured on preserved samples
under a Zeiss SteREO DiscoveryV8 microscope using an
AxioCam MRc camera and AxioVision 4.8.1 11-2009 software (Fig. 2A). The diameter of each hair was measured
at the point where it emerged from the epidermis. The
diameters of the epidermal recesses were measured as
shown in Fig. 2B. The total length of the hairs, from their
epidermal base to the tip, was measured using calipers,
although it is possible that some some hairs were damaged after death. The dermal length of the hair follicle
was measured, with calipers, before histological staining.
Data were analyzed using Systat 11, using a one way
ANOVA with covariate for the hair thickness, epidermal
recess, and hair length data sets. A one way ANOVA
was used for the follicle data set. A Tukey post-hoc analysis was utilized for all tests.
RESULTS
MATERIALS AND METHODS
Bowhead hair and skin samples were obtained from
six adult bowhead whales harvested as part of the
I~
nupiat subsistence hunt in Barrow, Alaska. This
occurred under the supervision of the North Slope Borough, Department of Wildlife Management, and with
permission from the whaling captains, the Barrow
Whaling Captains’ Association and the Alaska Eskimo
Whaling Commission under federal permit NOAANMFS 814-1899-01. Whales are captured in the ocean
and towed back to stable ground (ice or land) where the
subsistence harvest takes place there. At the harvest
site, skin sections of epidermal and dermal layers with
hairs and follicles are excised from the tip of the lower
(chin) and upper (rostrum) jaw and the region of the
blowhole. Samples were preserved in 10% paraformaldehyde solution for several weeks. In the laboratory,
samples were subsampled and analyzed. We did not use
frozen samples. We also report length of the studied
whales, which is coarsely correlated with age (Lubetkin
et al., 2008). The six adult specimens include: 2012B7
(8.99 meters, female), 2012B9 (8.79 m, unknown gender
due to position of whale on the ice,), 2012B18 (9.4 m,
female), 2012B16 (10.31 m, male), 2013B1 (16.46 m,
female), and 2013B8 (6.78 m, female). In addition to
the postnatal specimens, we also studied the hair of a
bowhead whale fetus (40.3 cm total length, 2000B3F).
Samples are part of the collection of the North Slope
Borough, Department of Wildlife Management in Barrow, Alaska. Specimen numbers are indicated by the
year that the whale was caught (e.g., 2000), the initial
Histology
Mammalian hair forms as an epidermal proliferation
that protrudes into the underlying dermis as the epithelial sheet of the hair shaft. This fact can be well appreciated in the developing hair of a bowhead fetus (Fig. 3G).
In the postnatal individuals, bowhead hairs have a thick
connective tissue capsule (Fig. 3A–C), along their entire
extent from base of the epidermis to hair follicle. This
capsule is distinct from the surrounding adipose tissue
and loose connective tissue (Fig. 3A–C). The hair follicle
displays a dermal papilla (Fig. 3B) and a root sheath
(Fig. 3B,F). We did not observe any smooth or striated
muscle tissue near the hair follicle or any associated
glandular structures.
Inside the dense capsule, surrounding the hair follicle are endothelially lined spaces with some erythrocytes (Fig. 3C, F, and H). These venous sinuses extend
mostly along the deeper half of the length of the follicle, and are scarce more superficially. They are not
one continuous space, and instead they are divided by
trabeculae (Fig. 3C, H, and J). These sinuses also do
not fully surround the follicle at all the levels of the
follicle. At the level of the papilla, approximately half
of the circumference of the follicle is surrounded by
vessels (Fig. 3J), whereas at higher levels (Fig. 3D)
less than one quarter of the follicle is invested with a
sinus. Small corpuscles are embedded in the dense connective tissue capsule especially near its base (Fig. 3C,
E, F, and I), and a large nerve enters the capsule near
its midpoint (Fig. 3C). We identify this nerve as the
deep vibrissal nerve.
Fig. 3. Histological sections of bowhead vibrissae. (A) Full longitudinal
sections through two rostrum hairs, one with papilla, in rectangle
(2011B8, slide 33, H&E). (B) Enlarged view of hair papilla, shown in rectangle of A. (C) Full longitudinal section through one rostum hair
(2012B18.70, slide 32, Lapham’s stain). (D) Transverse section of chin
hair near the middle of the follicle (2013B1, slide 20, H&E). (E) Enlarged
view of middle part of hair follicle, shown in rectangle in C. (F) Transverse
section of chin hair through papilla (2013B1, sl. 46, Lapham’s stain). (G)
Hair of chin region of bowhead fetus (2000B3F, slide 26, H&E), approximately 4 months of gestation. (H) Detail of longitudinal section of chin
hair showing venous sinuses and erythrocytes (2013B18.70, slide 32,
203, Lapham’s stain). (I) Detail of transverse section of chin hair, showing corpuscle (2013B1.A, slide 46, 403, Lapham’s stain). (J) Detail of
papilla area, showing distribution of sinuses (2013B1.A, slide 50, H&E).
6
DRAKE ET AL.
Fig. 4. Box plots of thickness of hairs (A), diameter of epidermal recess (B), follicle length (C), and
length of protruding part of hair (D) of bowhead whales. Horizontal line is median, box indicates first and
third quartile, and vertical line (also called whiskers) show the range of values that fall within 1.5 Hspreads
of the box. Numbers indicate number of hairs measured.
The hair follicle of the fetal bowhead is surrounded by
mesenchyme, and the dense connective tissue capsule is
in the process of forming (Fig. 3G). At this stage there is
no ring sinus surrounding the hair follicle, and the
innervation seen in adults is not observed.
Morphometrics
Hairs in all areas of the bowhead whale head are
implanted singly, or occasionally paired, in epidermal
recesses, and recesses are widely separated from their
neighbors (Figs. 1B and 2B). The hairs are straight or
slightly curved, do not have appreciable variation in
thickness along their shaft, do not curl, and display no
pigmentation. Aside from the circular epidermal
recesses, the skin is smooth, lacking depressions (Fig.
1B). Nakai and Shida (1948) accurately described the
recesses as funnel shaped, and similar structures have
been observed in other mysticetes (Mercado, 2014).
There is a considerable difference among the thickness (diameter) of hairs in different regions of the whale
(Fig. 4A). The hairs surrounding the blowhole are
SENSORY HAIRS IN THE BOWHEAD WHALE
significantly thicker at their base than the hairs on the
rostrum (P < 0.001) and the chin (P < 0.001). Chin and
rostrum hairs are not statistically different in thickness
(P 5 0.986). There is no difference among the diameter of
the epidermal recesses in the different regions (Fig. 4B),
and the size of these recesses varies considerably.
The dermal portion of the hair follicle is longer in the
region of the blowhole and chin than in the region of the
rostrum ((P < 0.001; Fig. 4C). The blowhole follicles are
not significantly different than the chin follicles
(P 5 0.083). In bowheads studied by Haldiman and Tarpley (1993) the total hair follicle length was approximately 50–60 mm, whereas our samples ranged from
21 mm to 42 mm. There is no difference among the three
regions in the lengths of the exposed part of the hair
shaft, external to the epidermis (Fig. 4D).
DISCUSSION
While cetaceans had ancestors that were generalized
land mammals with bodies covered by hair (Chen et al.,
2013), all modern cetaceans are hairless or nearly so.
Fossil evidence does not pinpoint exactly when hair
became sparse on the bodies of cetaceans (Gatesy and
O’Leary, 2001), but it has been suggested that the
mechanosense of the snout was important in the earliest
cetaceans (Thewissen and Nummela, 2008). Bowhead
whales are the hairiest of modern cetaceans, and the
purpose of this paper is to determine whether bowhead
hair displays the characteristics of vibrissae, and
whether there are regional differences between the
hairs. Reep et al. (2001) identified three histological
characteristics of vibrissae: dense connective tissue capsule, circumferential blood ring sinus, and extensive sensory innervation. Our work indicates that all three of
these characteristics are found in bowhead hair, supporting the inferences from Haldiman et al. (1986) and Haldiman and Tarpley (1993) for bowheads, and consistent
with similar suggestions in other mysticetes (Japha,
1910; Lillie, 1910; Nakai and Shida, 1948; Slijper, 1962;
Yablokov and Klevezal, 1964; Ling, 1977; Sokolov, 1982;
Berta et al., 2015). The venous sinuses associated with
the the hairs in bowhead whales cover neither the full
circumference of the follicle, not its full length. Certainly, they are smaller than in some other mysticeters,
such as the gray whale (Berta et al., 2015). Given that
bowhead live in near freezing water their entire life, and
that their skin is replete with adaptations related to conserving heat (reviewed by Ford et al., 2013), it is not
surprising that blood flow close to the body surface is
minimal. The presence of small corpuscles that resemble
sensory receptors near the hair papilla (Fig. 3I) also suggests a specialized function for these hairs. These corpuscles resemble those described by Berta et al. (2015)
in gray whales, although their identification as Herbst
corpuscles seems premature.
Our measurements show that, in most characteristics,
hairs are variable and that there are no consistent differences in hairs between the different anatomical
regions. This is true for epidermal recess diameter, follicle length, and external hair length (Fig. 4). The exception is hair thickness: the hairs posterior to the blowhole
are thicker than those in the other areas (Fig. 4A). It is
possible that the regional thickness variation is an adaptation for specific functions of these hair patches, but it
7
is also possible that there are regional differences in
hair thickness across the face in cetacean ancestors and
that these differences have been retained. The closest
relatives of modern cetaceans are artiodactyls (Thewissen et al. 2001). While many artiodactyls have vibrissae
on the rostrum and tip of the mandible (Pocock, 1914), it
is not obvious to which area the hairs posterior to the
blowhole in whales are homologous. Comparisons of
regional differences in hair thickness across the face
have been carried out by Yanli et al. (1998) for carnivores, perissodactyls, rodents, and primates, and there
are no regional trends in these groups.
On the other hand, variability in hair shaft diameter
is known to occur in mammals with hair patches that
have specialized functions. Reep et al. (2001) studied
hair diameters in various regions of the manatee face,
and they found a distinct characteristic range in hair
diameter for each of the facial regions. The hairs that
correspond to mystacial and mental hairs are used more
often for grasping during the feeding process, and the
bristle-like hairs of the oral disk are used for tactile
exploration.
Regional variation in hair shaft diameters also occurs
in bearded seals (Erignathus barbatus; Marshall et al.,
2006). Vibrissae are well studied in earless seals (Phocidae), which use them to sense waves in the water
(Dehnhardt et al., 1998). In eared seals (Otariidae), walruses (Odobenidae), and some earless seals, the diameter
of the vibrissae varies along the hair shaft, with thicker
and thinner regions alternating in a sinusoidal profile
(Watkins and Wartzok, 1985; Hyv€
arinen, 1989; Dehnhardt and Kaminski, 1995). The morphometrics of the
beaded vibrissae differ among species, but crests and
troughs of the hair shaft may increase sensitivity of
wave reception or reduce drag and signal interference
(Ginter et al., 2010, 2012).
We did not observe striated muscular tissue associated
with the hair follicles, suggesting that hairs are not
mobile, unlike the active whisking virbirissae are not
under active muscle control of land mammals (Williams
and Kramer, 2010), the protracting vibrissae of pinnipeds (Yablokov and Klevezal, 1964) and the highly
mobile sirenian vibrissae (Reep et al., 2001).
Our data are most consistent with the hypothesis that
bowhead hairs are a passive receptive system that detects
flow or substrate interfaces (such as water-air, water-ice),
not unlike the lateral line system of fish or the mechanoreceptive organs in the skin of Crocodilia (Soares, 2002).
Indeed, among mammals, there are parallels with the tactile hairs of fossorial rodents (Crish et al., 2003; Park
et al., 2003). We interpret the differences in shaft thickness for hairs from different areas as suggestive of different functions for these hairs. Thewissen et al. (2011)
found bowheads have a good sense of smell, and it is possible that the hairs associated with the blowhole determine
air flow and thus wind direction, since olfaction, by itself,
is not a directional sense. In contrast, vibrissae near rostrum and mandible may detect water flow related to feeding, or the presence of hard structures, such as ice.
ACKNOWLEDGEMENTS
The authors thank the whaling captains of Barrow,
Alaska, and the Alaska Eskimo Whaling Commission for
allowing sampling of whales, and Robert Suydam, Todd
8
DRAKE ET AL.
Sformo, Leslie Pierce, Cyd Hanns, and Brian Person of
the North Slope Borough, Department of Wildlife Management for assistance in sampling. They are grateful to
Denise McBurney and Sharon Usip for their support in
the research laboratory. They thank Olga Shpak for
allowing us to use her photo (Fig. 1C) and Michael
Macrander for comments on the manuscript. Research
reported here was done in partial fulfillment of the conditions for a M.S. degree for S.E. Drake, who thanks
Chris Vinyard and Samuel Crish for their help.
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