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MARINE MAMMAL SCIENCE, (MONTH 2019)
© 2019 SOCIETY FOR MARINE MAMMALOGY
DOI: 10.1111/MMS.12575
Long in the tooth: Biological observations from at-sea
sightings of strap-toothed beaked whales
(Mesoplodon layardii)
ROBERT L. PITMAN ,
1
Antarctic Ecosystem Research Division, Southwest
Fisheries Science Center, National Marine Fisheries Service, NOAA, 8901 La
Jolla Shores Drive, La Jolla, California 92037, U.S.A.; JOHN TOTTERDELL,
CETREC (Cetacean Research), PO Box 140, Exmouth, Western Australia 6707,
Australia; REBECCA WELLARD,Centre for Marine Science & Technology and
Project O.R.C.A (Orca Research and Conservation Australia), Curtin University,
GPO Box U1987, Perth, Western Australia 6845, Australia; PIERCE CULLEN ,
Project O.R.C.A (Orca Research and Conservation Australia), Curtin University,
GPO Box U1987, Perth, Western Australia 6845, Australia; MARIJKE DE BOER,
Seven Seas Marine Consultancy, PO Box 11422, Amsterdam, The Netherlands.
Among the 15 currently recognized species of mesoplodont beaked
whales (Ziphiidae, Mesoplodon spp.), only adult males have functional
teeth, and these have been reduced to a single, enlarged tooth emerging
from each side of the lower jaw. It has long been inferred that males use
their teeth as tusks to aggressively vie for access to breeding females
because only adult males acquire extensive tooth-rake marks on their
bodies (Kellogg 1940, McCann 1974, Heyning 1984, MacLeod 1998).
This repurposing of mesoplodont teeth has led to some remarkable,
species-specific variations in tooth size, shape, and placement. For
example, with the possible exception of the spiraled tusk of the narwhal
(Monodon monoceros), the strap-toothed beaked whale (Mesoplodon
layardii) has perhaps the most bizarre teeth in the animal kingdom. As
the males mature, a single, strap-like tooth emerges from the middle of
each lower jaw, and, as it grows, it curves up and back at a ca. 45
1
Corresponding author (e-mail: robert.pitman@noaa.gov).
MARINE MAMMAL SCIENCE, 9999(9999): 1–20 (2019)
© 2019 Society for Marine Mammalogy
1
angle, over the top of the rostrum. Full-grown teeth measure up to
34 cm in length (Best 2007) and often meet or overlap on top of the ros-
trum (Fig. 1a). Despite being by far the largest tooth of any beaked
whale, only the small, up-turned denticle at the tip of the tooth (Fig. 1b)
makes contact with opponents.
It would seem unavoidable that tooth development in male
M. layardii would restrict jaw movement and impair feeding. However,
Sekiguchi et al. (1996) reported that although an adult female and an
immature male M. layardii could both open their mouths 6.5 cm at the
tip, two adult males could open their mouths only 3.2 and 4.0 cm,
respectively, but both appeared to have been feeding normally prior to
stranding. Beaked whales suck in their prey, and a reduced aperture at
the tip should allow them to be more forceful and directed with their
suction feeding (Heyning and Mead 1996). Thus, male M. layardii may
not be as handicapped in their foraging as they might otherwise
appear to be.
M. layardii is also unique among mesoplodonts (and perhaps all
cetaceans) in having a definitive adult color pattern that is entirely dif-
ferent from its juvenile coloration (cf. Fig. 2, 3). Although tooth erup-
tion in male mesoplodonts generally coincides with the onset of
adulthood (Mead 1989), teeth in M. layardii begin to erupt while males
are still in their juvenile coloration (see below). Therefore, for the pur-
poses of this note, we will refer to the different color pattern stages of
M. layardii as “juvenile”(largely uniform gray), “adult”(definitive
black and white pattern), or “subadult”(transitioning between juvenile
and adult). These terms are only meant to be descriptive, with no
implications about the physical, social, or sexual maturity of the indi-
viduals. Further study with fresh-stranded animals will be necessary to
determine how maturity correlates with the tooth and color pattern
development in this species.
The combination of a distinctive, black and white adult color pattern
and the unique dentition of males make adult M. layardii perhaps the
most readily identifiable of the 22 known species of beaked whales
(Best 2007, see Jefferson et al. 2015). Furthermore, M. layardii appears
to be common within its circumpolar range in the cold/temperate waters
of the Southern Hemisphere—it has been cited as the most commonly
stranded mesoplodont worldwide (Mead 1989), including in South Africa
(Best 2007) and Australia (Bannister et al. 1996). However, despite being
distinctively patterned and relatively common, M. layardii has seldom
been identified alive in the wild. This may be due, in part, to the fact that
juveniles are mostly uniform gray and superficially similar to several other
mesoplodont species—their distinguishing field marks have not been ade-
quately described. In addition, for well over a century, the type specimen
of the recently resurrected spade-tooth beaked whale (M. traversii)was
misidentified as M. layardii due to similarities in tooth and skull morphol-
ogy. There are no descriptions of fresh specimens of M. traversii,aliveor
dead, and it could resemble M. layardii (van Helden et al.2002,
Best 2007).
Herein we describe ontogenetic stages in M. layardii color patterning
and tooth development based on a series of photographs from two
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****2
Figure 1. (a) The skull of an adult male strap-toothed beaked whale
(Mesoplodon layardii) showing how the single tooth from each lower jaw
wraps up and back, over the rostrum, restricting the ability of the animal to
open its mouth; the denticle on the top tooth has been worn off. (b) A small,
sharp, up-turned denticle at the top of the tooth (arrow) is the only part of the
tooth that makes contact with opponents. Photo courtesy Iziko South Africa
Museum/Jofred Opperman.
NOTES 3
recent observations at sea (Fig. 3–10). These descriptions should prove
useful for distinguishing M. layardii from other, similar-looking meso-
plodonts in the field, especially if no adult M. layardii are observed.
We also discuss group size and social structure in this species based on
these observations.
Figure 2. (a) A breaching M. layardii (sex undetermined) shows the
distinctive and unique adult color pattern of this species, including the
prominent cape, largely white beak, and white-tipped dorsal fin and flukes;
18 March 2005, 57240S, 39460W; photo by S. Howell; (b) An adult female
M. layardii stranded in Golden Bay, New Zealand, 21 January 2007; a pale
neckband extends from behind the black on the melon, to behind the
blowhole, and forms the base of a darker gray triangle on the back behind
the blowhole (see text). Photo © Department of Conservation, New Zealand.
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****4
Sightings
Sighting #1 was observed by MdB on 12 April 2017, just north of Tris-
tan da Cunha Island in the South Atlantic Ocean—the location was
33.39S, 11.07W; the water temperature was 22C. Three animals were
present, and they were traveling slowly, abreast, about 10 m apart. Two
of the animals showed different stages of subadult color pattern (Fig. 7,
9), and a third individual was not photographed well enough to docu-
ment its color patterning. It was not possible to tell if any had erupted
teeth, and the sex was not determined.
Sighting #2 occurred on 19 March 2018. The first four authors were
conducting killer whale (Orcinus orca) research off Western Australia
when a group of M. layardii surfaced several times in front of their tran-
siting vessel. The sighting location was 34.99S, 119.28E, approximately
64 km due south of Bremer Bay, Western Australia, over Cheyne Can-
yon. The water depth was 2,200–2,400 m, and the sea surface tempera-
ture was 20C–21C. Two individuals surfaced initially, and the rest of
the group appeared several seconds later. Individuals were spread out
over an estimated 30 ×30 m area and seemed to coalesce during the
Figure 3. (Sighting #2). (a) A pair of young M. layardii in juvenile coloration
surfaces off Bremer Bay, Western Australia; species identification was based on
the presence of adult and subadult males photographed in the same group (see
Fig. 8, 10). Notice the long beak, paler melon with a slightly darkened “neck-
stripe”just behind the blowhole, and (b) otherwise nearly uniformly gray body.
NOTES 5
course of the observation. After all of the whales had surfaced 3–4 times,
the entire group sounded almost simultaneously. At the surface, they all
seemed hurried and somewhat agitated perhaps due to the presence of
our vessel, which was heading in their direction and did not alter course
or change speed during the sighting. Less than 20 min prior to the sight-
ing, we had observed killer whales, which are known to prey upon
M. layardii in the waters offshore of Bremer Bay (Wellard et al. 2016),
and this also could have contributed to their apparent skittish behavior
at the surface.
When the whales first surfaced, three of the authors had DSLR cam-
eras in hand, each with motor-drives and 100–400 mm lenses. Although,
based on photograph metadata, the total observation period spanned
only 25 s (16:10:51–16:11:16 LMT), 199 still images were recorded as the
whales surfaced approximately 200–300 m ahead of the vessel. Although
at the time each of the authors independently estimated that there were
Figure 4. (Sighting #2). A roll sequence of the same juvenile M. layardii
shown in foreground of Figure 3 showing (a) a relatively straight gape with a
slight downturn toward the rear, (b) slightly paler melon and darker neck-
stripe, and (c) the dorsal fin is starting to show a white tip; the orange patches
on the head and below the dorsal fin are diatoms.
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****6
approximately 10 animals present, only seven separate individuals were
identified from the images, including four males and three unsexed juve-
niles. Males were identified by the presence of erupted teeth; individuals
without erupted teeth were classified as unsexed. Six of the identified
animals appeared to be swimming in pairs, which may have been ran-
dom given the brevity of the encounter. Pairs included two unsexed
Figure 5. (Sighting #2). A male M. layardii in juvenile coloration off Bremer
Bay, Western Australia, March 2018; (a, b) its tooth is just starting to erupt from
the lower jaw; (a–c) it has lost its pale melon and darker neck-stripe, but has
not yet begun to transition into the adult color pattern; (d) its dorsal fin does
not have a white tip yet. Its traveling companion in the lower right (b, c) is
shown in Figure 8.
NOTES 7
juveniles (Fig. 3, 4); a juvenile male (Fig. 5) with a subadult male (Fig. 8);
and another juvenile male (Fig. 6) with another, unsexed juvenile (not
shown). The remaining individual in the group was an adult male
(Fig. 10), which appeared to be swimming alone, approximately 15 m
apart from the subadult and juvenile male pair. We did not identify any
females, i.e., individuals either having an adult or subadult color pattern
but without erupted teeth (see below) or with dependent calves.
Below, we present photographs and descriptions of ontogenetic color
pattern changes and tooth development in M. layardii based on these
two sightings; the figures are presented in order of what we infer to be
the youngest to the oldest individuals.
Young juveniles (Fig. 3, 4), sighting #2—These were almost uniform
pale gray with long beaks and moderately prominent melons; they
were identified as “young”juveniles because they had a noticeably pale
melon (Fig. 3). At the trailing edge of the pale melon, immediately
behind the blowhole, was a slightly darkened area, which formed an
indistinct stripe on the side of the neck (“neck-stripe”) that traveled
down toward each eye (Fig. 3a; see also Jefferson et al. 2008, p. 141,
bottom right photo; Jefferson et al. 2015, p. 160, bottom right photo).
The visible body posterior to the neck-stripe was also pale gray but
lighter than the neck-stripe and slightly darker than the melon. The
dorsal fin was low, triangular, slightly falcate, and set far back on the
body (Fig. 3b). The near animal in Figure 3b has a slightly whitish tip
to the dorsal fin (see also this same animal in Fig. 4c) and the far ani-
mal does not, although they seem to be of similar age. The distant ani-
malinFigure3aappearstohavealongerbeakthantheanimalinthe
Figure 6. (Sighting #2). A young male M. layardii sighted off Bremer Bay,
Western Australia, in March 2018; the slightly longer teeth and whitening lips of
the lower jaw suggest that it is a bit older than the male in Figure 5, although it
still retains the juvenile coloration.
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foreground, which could signify an older animal, a female (females of
at least some species of beaked whales have longer beaks than males;
von Haast 1876, Besharse 1971, Thompson et al. 2014), or, possibly,
individual variation. There was no visible blow when the animals
surfaced.
Figure 4 shows details of the animal in the foreground of Figure 3. It
has a long, uniformly pale gray beak, and the gape is largely straight,
with a slight downward turn toward the rear. It has a moderately sloping
forehead and the beak protruded at a 30–45angle when it surfaced
(Fig. 4a). The paler melon and darker neck-stripe is evident in
Figure 4b; the orange area at side of the melon is a diatom patch.
Figure 4c shows the unmarked, pale gray back and flanks, a dorsal fin
that is starting to acquire a white tip, and another diatom patch at the
base of the fin.
Figure 7. (Sighting #1). Two views of the same M. layardii of unknown sex
from a group of three photographed in April 2017 north of Tristan da Cunha
Island, in the South Atlantic Ocean. This animal is transitioning into the adult
color pattern: (a) the distal portion of the beak is white; (a, b) the white
neckband has formed, and (b) the shoulder patch, eye-patch, and melon are
darkening, and the rear, lower portion of the cape is starting to develop. Photo
by S. Steadman.
NOTES 9
Juvenile males (Fig. 5, 6), sighting #2—Figure 5 is a roll sequence of
one individual animal; Figure 6 is a different individual. Both were iden-
tified as juvenile males by their all gray coloration and the presence of a
tooth just starting to emerge from the lower jaw. As in all age classes,
the long beak jutted high up out of the water at a 30–45angle when
Figure 8. (Sighting #2). A roll sequence of a subadult male M. layardii off
Bremer Bay, Western Australia, in March 2018. (a, b) Although the teeth are still
only partially grown, it has acquired much of the adult color pattern - the head
is black and the beak bicolored, although not as white as a full adult (see
Fig. 10); (c, d) the dorsal cape, although well developed, is still somewhat
muted; (c) there is a dark gray, triangular patch on the dorsal mid-line, forward
on the cape; (d) the dorsal fin has a white tip and orange diatom patches are
evident in several places.
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****10
the animals surfaced. The tooth is just beginning to erupt and the beak
is still entirely gray (Fig. 5a, b). The forehead is moderately sloped, and
the pale melon and dark neck-stripe appear to be absent at this stage
(Fig. 5a–c, 6). The body color is slightly darker gray compared to the
younger juveniles in Figures 3 and 4, but there is still no hint of black
on the head, no whitening of the beak, and the dorsal cape has not
begun to develop (Fig. 5b–d). Pale mottling on the back (Fig. 5c, d)
appears to be due to scarring (including healed bites from cookiecutter
sharks, Dilatiidae, Isistius spp.; see below) and, perhaps, skin molt. The
dorsal fin is uniformly dark with no evidence of white tip (Fig. 5d), and
comparing Figure 4c with Figure 5d, it is clear that some animals acquire
a white tip to the dorsal fin later than others do.
Figure 6 shows a different juvenile male, also with a single tooth
erupting from each side of the lower jaw. The head, back and rostrum
are still uniform gray, but the lower lip in front of the right tooth is just
starting to turn white, perhaps the first sign of transition into the sub-
adult color pattern. This male appears to be slightly older than the one
in Figure 5 due to this whitening of the lower lips and the somewhat
longer teeth.
Figure 9. (Sighting #1). Associated with the animal in Figure 7, this animal
was a bit older. (a, b) It has a white-tipped beak, blackish shoulder- and eye-
patches; the white of the cape has advanced dorsally and forward and has
become continuous with the white neckband. A dark gray triangle is still
evident behind the neckband.
NOTES 11
Figure 10. (Sighting #2). A roll sequence of an adult male M. layardii sighted
off Bremer Bay, Australia, in March 2018. (a) The distal half of the beak is
snowy white, and the cream-colored tooth curves up and back, over the top of
the rostrum; (b, c) the dorsal cape is fully developed; it has completely merged
with the neckband and the gray triangle behind the blowhole is no longer
visible; there is a distinctive, fluted border on the trailing edge; and (c) the
dorsal fin has a conspicuous white tip. (b, c) Two dark spots on the cape are
recent bite wounds from cookiecutter sharks (Isistius spp.); (d) the wound
near the water line, mid-flank, is enlarged in to show its rectangular shape
(see text).
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Unsexed subadult (Fig. 7), sighting #1—This individual is transition-
ing from the juvenile to the adult color pattern. It is mostly dark gray
(Fig. 7a), with a dark shoulder patch and eye-patch (Fig. 7b); the distal
portion of the beak is white (Fig. 7a), and the melon is becoming black
(Fig. 7a, b). It was not possible to determine from the available photo-
graphs if it had erupted teeth. The cape is just beginning to develop,
and it appears to be formed by two separate processes. At this stage, the
most prominent part of the cape is a broad, pale, transverse band imme-
diately behind the black on the melon. This white “neckband”extends
rearward only a short distance behind the blowhole and terminates in a
straight line, perpendicular to the back, which drops down between the
shoulder patch and eye-patch (Fig. 7a, b; see also Fig. 9a, b). Another
pale area is also developing, low, mid-body, on the side (Fig. 7b), which
will form the lower, trailing edge of the cape. Presumably, as the cape
develops, this second pale area will radiate dorsally and forward, even-
tually joining over the back and merging with the neckband. Another
photograph of M. layardii with a pale neckband and an incompletely
formed cape is shown in Jefferson et al. (2015, p. 160, bottom left
photograph).
Subadult male (Fig. 8), sighting #2—The beak juts up at a ca. 45
angle when surfacing (Fig. 8a). The tooth is longer than that of the juve-
nile males in Figures 5 and 6, but it still does not extend above the top
of the rostrum. (A suggested feature for distinguishing M. layardii from
M. traversii is that the leading and trailing edge of layardii tooth is
tapered toward the tip, while on the shorter, traversii tooth, the edges
are parallel [van Helden et al. 2002, Best 2007]; however, this may not
apply to M. layardii with partially grown teeth; Fig. 8b). The distal por-
tion of the beak is white on top and bottom, but the white does not
extend as far toward the base of the beak as in older adults (cf. Fig. 8b
with Fig. 2a, 10a). The black on the rostrum extends farthest forward on
the mid-dorsal line and mid-ventral line. The head and base of the beak
are black, extending almost as far back as the blowhole (Fig. 8b). The
cape on this animal is more developed compared to that in Figure 7; it
extends from the trailing edge of the black melon, just in front of the
blowhole, to two-thirds of the way back to the dorsal fin, and reaches
furthest back on the mid-dorsal line (Fig. 8c). The trailing edge of the
cape has joined over the back, and the lower leading edge has merged
with the neckband on the side of the head (Fig. 8c). Embedded in the
cape, behind the blowhole, on the mid-line of the back, is a large dark
patch, which is the last area of the cape to develop. It is roughly triangu-
lar in shape, with the apex pointed toward the rear, and the base is
formed by the neckband (Fig. 8c; see also Fig. 9, and Jefferson et al.
2008, p. 141, middle photograph, right side). The cape of this animal
appears muted, more grayish, compared to the lighter cape of the older
adult in Figure 10; it also lacks the contrasting, fluted border at the trail-
ing edge along the flanks (cf. Fig. 8c with Fig. 10c). The dorsal fin has a
pale gray tip, which is also not as conspicuous as in the older adult in
Figure 10c. There are several patches of orange diatoms visible on ligh-
ter areas of this animal (Fig. 8b, c).
NOTES 13
Unsexed adult (Fig. 9), sighting #1—This animal appears quite pale
because its cape is nearly completely developed, and only the upper,
forepart of the body is exposed. Again, due to water splashes partly
obscuring the base of the beak, it was not possible to positively deter-
mine from the available photographs if it had erupted teeth. The distal
portion of the beak is extensively white (Fig. 9a); the melon, shoulder
patch and eye-patch are all blackish (Fig. 9b); and there is only a
remnant of the dark triangle behind the blowhole (cf. Fig. 8c). The
noticeably paler neckband is still evident, although the expanding cape
has begun to merge with it. The dark spot near the waterline toward
the rear of the cape is probably a recent cookiecutter shark bite (see
below).
Adult male (Fig. 10), sighting #2—This adult male has a long beak
that is snowy white on the distal half; the base of beak and head are
black (Fig. 10a, b). The tooth erupts at the boundary between the white
and black on the side of the beak, and it angles back, over the top of the
rostrum; the blow is faint and angled forward (Fig. 10a). The teeth of
adult males are sometimes cream-colored and easy to discern (Fig. 10a),
but more often they are darkened by greenish-brown diatoms and can
be difficult to detect (Best 2007; RLP, personal observation). There is a
sharp boundary between the black head and the white or pale gray of
the cape (Fig. 10b). The cape is uniformly pale; it has completely
merged with the neckband, in this case obscuring it, and the gray trian-
gle behind the blowhole is no longer visible (Fig. 10b). Although the
neckband on this animal is no longer discernible, Shirihai (2007, p. 389,
top right) shows an M. layardii with a fully formed cape and a still-
evident neckband. The cape extends from the black on the melon, just
in front of the blowhole, to approximately two-thirds of the way back to
the dorsal fin (Fig. 10c); it extends farthest back on the mid-dorsal line
and has a distinctive, fluted border along the trailing edge on the flanks
(Fig. 10c). The remainder of the back is black, and the dorsal fin is black
but with a conspicuous white tip (Fig. 10c). Although not evident from
our photos, M. layardii also has white fluke tips, which, like the dorsal
fin tip, also appear to become lighter with age (Fig. S1a, b).
The adult male in Figure 10 shows none of the obvious linear scarring
often found on adult male mesoplodonts and attributable to tooth rake
marks from aggressive interactions with male conspecifics (Heyning
1984, MacLeod 1998). Although it could be that this animal has not yet
had contact with other adult males, we suggest that the lack of visible
scarring is due to two other factors. First, unlike some species of Meso-
plodon, superficial flesh wounds on M. layardii heal the same color as
the adjacent skin, making these scars nearly invisible (see MacLeod
1998). As evidence for this, cookiecutter sharks are small, oceanic sharks
(to 54 cm, Compagno 1984) that sometimes feed by excising a round or
oval plug out of the flesh of larger animals, including cetaceans (Jones
1971, Best and Photopoulou 2016). These bite wounds typically leave
small (5–7 cm across long axis, Shirai and Nakaya 1992), round or oval
scars, which on some species of Mesoplodon heal conspicuously white
(i.e., unpigmented). On M. layardii, however, cookiecutter shark bites
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****14
heal the same color as the surrounding skin, so that the scars are nearly
invisible (Fig. S2a, b), and this would presumably apply to any tooth
rake marks that this species might acquire from conspecifics, also. A sec-
ond factor contributing to a lack of conspicuous rake-mark scarring on
M. layardii is that, although the teeth of adult males are very large, as
mentioned previously, the actual cutting point is a tiny, up-turned denti-
cle on the top, leading edge of the tooth (Fig. 1b; MacLeod 2000). These
denticles leave relatively superficial scratches on conspecifics, which are
much less conspicuous than the deep furrows left by mesoplodonts with
larger “tusks”(see below).
Currently, there are two recognized species of cookiecutter sharks,
I. brasiliensis and I. plutodus (de Figueiredo Petean and R. de Carvalho
2018), and because Isistius bite wounds and scars tend to be either
oval or round, it has been suggested that these shapes could be indica-
tive of the biting species (e.g., Williams and Bunkley Williams 1996,
Pérez-Zayas et al. 2002). However, the adult M. layardii male in
Fig. 10c-d has a conspicuous rectangular bite wound low on the mid-
body. Although less common than the round or oval shapes, rectangular
wounds and scars have been observed on various species of oceanic
cetaceans in low latitudes (Baird 2016; RLP, personal observation) and
could be evidence for another species of shark of similar size and feed-
ing habits, but with, perhaps, a different jaw structure (e.g., Grace
et al. 2015).
Group Size
Mesoplodonts generally occur in small groups (Mead 1989, MacLeod
2014)—the largest that we are aware of was a mass stranding of a pur-
ported 28 Gray’s beaked whales (M. grayi, including holotype) from the
Chatham Islands, New Zealand, in 1875 (von Haast 1876), but confirm-
ing details about that event are lacking. Loughlin et al. (1982) reported
seven sightings of Stejneger’s beaked whales (M. stejnegeri) from the
Aleutian Islands, Alaska, and the two largest groups were an estimated
12 and 15 individuals, respectively. These are the largest reported
group-size estimates for free-swimming mesoplodonts in the modern era
that we are aware of (see also MacLeod and D’Amico 2006). However,
body length estimates for these two sightings were “about 6 m,”and
estimated body lengths for two other sightings were 5–8 and 6–8m,
respectively (Loughlin et al. 1982). These length estimates overlap the
size range of a newly identified, but yet undescribed, diminutive form of
Berardius sp. found in the North Pacific, including the Aleutian Islands
(Kitamura et al. 2013, Morin et al. 2017). The maximum reported body
length for M. stejnegeri is 5.7 m, and two adult males of the new dwarf
Berardius measured 6.6 and 7.3 m, respectively, compared to a maxi-
mum of 11.0 m for Baird’s beaked whale (Berardius bairdii; MacLeod
2006, Morin et al. 2017). MacLeod and D’Amico (2006) suggested that
beaked whales fall into two group-size categories: mesoplodonts were
in the small group category (mean 2.9 individuals, n= 354, range 1–15;
the “15”comes from the aforementioned Loughlin et al. 1982), and
Berardius spp. was in the large category (mean 7.9 individuals, range
NOTES 15
1–100, n= 335). Furthermore, Loughlin et al. (1982) reported that the
animals in at least some of their sightings “appeared to be traveling
abreast, almost touching each other. They surfaced and submerged in
unison.”This shoulder-to-shoulder swimming behavior is commonly
observed in Berardius spp. (including B. arnuxii in the Southern
Ocean) and rarely, if ever, seen in mesoplodont groups (RLP, personal
observation). Based on these comparisons of length, group size, and sur-
face behaviors, it seems possible that at least some of the Loughlin et al.
(1982) sightings could pertain to the recently identified Berardius sp.,
and that their group-size estimates may not be applicable to
Mesoplodon.
The largest credible group-size estimates for free-ranging mesoplo-
donts that we are aware of come from Claridge (2013) and Baird (2016)
who both reported maximums of 11 M. densirostris in the Bahamas and
off Hawaii, respectively. A group of 8–10 Sowerby’s beaked whales
(M. bidens) was reported from the eastern North Atlantic by ynes
(1974; the same sighting was reported on by Benjaminsen et al. 1976
and Christensen 1977). Hooker and Baird (1999) also reported a group
of 8-10 M. bidens, from the western North Atlantic, and Patel et al.
(2016) reported a mass-stranding of 10 M. grayi in the Chatham Islands,
New Zealand. For M. layardii, Shirihai and Jarrett (2006), Groom et al.
(2014), and MacLeod (2014) all reported group sizes of up to 5, while
Best (2007) cited 2–6 individuals from at-sea sightings in the
South African subregion. Our sighting of an estimated 10 individuals off
Bremer Bay appears to be the largest group of strap-toothed beaked
whales reported to date, and may be near the maximum for all
mesoplodonts.
Social Structure
The Bremer Bay sighting (#2) offers some insight into the social struc-
ture of M. layardii and perhaps other mesoplodonts as well. Among the
seven individuals identified in our photographs, at least four were males
of various ages, and the other three were unsexed juveniles. No adult
females or dependent calves were photographed or otherwise observed.
Lien et al. (1990) reported six M. bidens swimming near shore off New-
foundland in 1986; three that eventually stranded were all mature males,
and the authors speculated that M. bidens “may form all male social
groups.”Similarly, a group of 8–10 M. bidens observed by Hooker and
Baird (1999) in the western North Atlantic included at least four adult
males; they also reported another group of three that was all adult
males, and an aerial photograph in Jefferson et al. (2015, p. 151, top
left) shows what appears to be four adult male M. bidens swimming
together. Eight Stejneger’s beaked whales (M. stejnegeri) that stranded
at Adak Island, in the Aleutian Islands, Alaska, in August 2018, were
adult females,
2
and 10 Baird’s beaked whales that stranded on an island
in the Gulf of California, Mexico, in 2006 were all mature males (Urbán
2
Personal communication from Marc Webber, U.S. Fish and Wildlife Service, Homer,
AK, September 2018.
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****16
et al. 2007). The prevalence and adaptive significance of sexual segrega-
tion among beaked whales is unknown, but it could be fairly common
and widespread.
What little is known about mesoplodont social structure suggests
some interesting variability. Although more evidence is needed, the
observations above suggest that at least some species occur in sexually
segregated groups, or, at the very least, occur in groups with multiple
males present, although some mesoplodont species clearly do not. For
example, in M. densirostris, typically only one adult male associates with
one or more females in what appears to be a harem arrangement
(McSweeney et al. 2007, Claridge 2013, Dunn 2014), while M. grayi
(Patel et al. 2016), M. bidens (references above), and possibly
M. layardii (this study) occur in groups with multiple males, including adults.
Heyning (1984) was perhaps the first to suggest a possible link
between tooth development and social behavior among mesoplodonts,
and we further propose that the degree of rostrum and tooth develop-
ment among the different species might correlate not only with aggres-
siveness, but could also have implications for social structure and
mating systems. For example, adult male M. densirostris have two mas-
sive tusks, each raised on a boney arch above the level of the head
(think horns), and a stout, densely ossified rostrum that helps protect
the beak against trauma when forcibly engaging opponents (Heyning
1984, MacLeod 2002). When M. densirostris males rake each other in
combat, they leave “deep ruts”(MacLeod 2002; RLP, personal observa-
tion), which is presumably why there is almost never more than one
adult male seen in a social group (MacLeod and D’Amico 2006, McSwee-
ney et al. 2007, MacLeod 2014). On the other hand, adult males of spe-
cies with longer, more delicate beaks and/or smaller, more fragile teeth
(e.g.,M. bidens,M. grayi,M. layardii) tend to leave relatively light
scratches when they rake each other. The teeth of adult male ginkgo-
toothed beaked whales (M. ginkgodens), for example, barely erupt
above the gum line; as a result, adult males have almost no linear rake
marks on their bodies, which Heyning (1984) interpreted as a sign of
decreased aggressiveness. Reduced aggression among less well-armed
mesoplodont species could allow for multiple males within groups and
an altered social dynamic. Although M. layardii has the largest tooth of
any beaked whale and might seem to be an exception to the rule, as
mentioned above, its effective tooth size is actually very small because
only the tiny, upturned denticle at the tip of the tooth is used for raking
opponents (Fig. 1b; Best 2007).
ACKNOWLEDGMENTS
We thank Legend Charters and Captain Andrew “Dundee”Johnson and Kyle
Simms of the M/V Dhu Force for their cheerful and capable support during our
research off Bremer Bay. We also thank Captain A. Nazarov and crew of the
M/V Plancius for their efforts and support allowing us to study the whales in
the South Atlantic. Research in Australia was conducted under the auspices of
Department of Environment/Cetacean Permit: 2014-0007 and Flinders Univer-
sity/Animal Welfare Committee: Project E460/17. A. van Helden provided some
NOTES 17
thoughtful comments on an earlier draft of this paper; A. van Helden, S. N.
G. Howell, and S. Steadman provided photographs for this note.
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Received: 24 June 2018
Accepted: 27 November 2018
SUPPORTING INFORMATION
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Figure S1. (a) An aerial view of an adult M. layardii (an adult based
on uniformly pale cape; see text) photographed at 60280S, 155490W,
6 January 2014, showing prominent white tips to the flukes. Photograph
by E. Muirhead/Sea Shepherd; (b) top of the flukes of a female
M. layardii that stranded in Golden Bay, New Zealand, 21 January 2007
MARINE MAMMAL SCIENCE, VOL. **, NO. *, ****20
