Toothed whales, order Odontocetes, use a variety of call types to navigate,
detect prey, and communicate (Conner et al. 1994). For example the bottlenose
dolphin, Tursiops truncatus, use acoustics to identify kin relations, differences
in male and female roles (Sayigh et al. 1990), and identity of sender (Richardson
et al. 1995). Killer whales use acoustical signals for echolocation and repetitious
pulsed calls for social communication (Ford 1987).
Killer whales are second only to humans for being the most-widely distributed
mammal on earth (Ford et al. 2000). They are sighted from the tropics to temperate
coastlines and the ice packs of the poles (Bigg et al. 1987). Killer whales
in British Columbia have been studied for over three decades. A long-term field
study initiated by M. Bigg in 1973 (Bigg et al. 1987, Bigg et al. 1990) has
provided much information on the distribution, abundance and social organization
of killer whales.
There are distinct killer-whale ecotypes including northern and southern residents,
transients, and offshores that differ in travel pattern, pod size, behaviour,
and feeding habits. These ecotypes never travel together and do not mix when
in the same area (Ford 1987). Resident killer whales travel in stable social
associations of mixed ages and sexes called pods (Ford 1987). Pods are made
up of one or more maternal groups, each containing breeding female and her offspring.
Resident pods are typically seen in predictable coastal locations during the
summer, but their winter distribution is unknown (Ford 1987). Residents forage
on fish and usually have 6 or more members per pod (Bigg et al. 1990, Ford et
al. 2000). The resident pods are divided into northern and southern communities
with separate distributions. This unique social structure of resident killer
whales has functioned to produce group-specific dialects (Ford 1989, 1991).
Dialects may serve as a mechanism for maintaining cohesion and integrity of
killer whale pods (Ford and Fisher 1983, Ford et al. 2000). Each resident killer
whale pod produces a limited repertoire of 7 to 17 discrete call types (Ford
et al. 2000) which remain stable for at least 25 years (Ford 1987). These calls
vary per pod which give group specific dialects. Dialects are passed down generations
by vocal learning (Ford et al. 2000).
Transients have 6 or fewer members per pod with individuals coming and going
from groups (Bigg et al. 1987, Fore et al. 2000). They have sharper dorsal fins
more white (closed) saddle patches relative to resident killer whales (Ford
et al. 2000). Transients eat mammals and are found in coastal areas of BC throughout
the year but are encountered less often than resident killer whales (Bigg et
al. 1990). Underwater recordings of transients imply that they share the same
dialect of 4 to 6 discrete call types, none of which are used by the residents
(Ford 1987). (Ford 1987).
Within the past decade, a third group of killer whales, called offshores, have
been discovered as a distinct population off the coast of North America. They
are found in large groups of 30 to 60 individuals (Ford et al. 2000). They have
been seen as far south as southern California and as far north as southeast
Alaska (Ford et al. 2000). They are smaller than resident and transient killer
whales. Their dorsal fin shape and mottled saddle patches resemble resident
killer whales suggesting a closer common ancestor than with transients (Ford
et al. 2000). Offshores have been recorded emitting 97 discrete call types (Johnston
and Park 1994, Provencher 2001). This is a very large repertoire of calls compared
to that of residents and transient killer whales.
All members of the dolphin family rely on underwater sound for communication.
Sound is a practical and dependable means for whales to communicate with conspecifics
(Ford et al. 2000). They have good eyesight but water visibility is usually
less than 50 meters (Ford et al. 2000). Sound travels further and faster under
water than through the air and it is efficient during the night or daytime.
The underwater vocalizations of killer whales are comprised of clicks, whistles,
and discrete calls. Clicks are very brief bursts of sound, typically emitted
in a series, used for echolocation. Whistles are non-pulsed or continuous sound,
characterized by a waveform, which appear on a spectrogram as a single narrow-band
tone with little or no harmonic or side-band structure. Whistles are commonly
used when killer whales are socializing (Ford 1987). Acoustic exchanges among
killer whales are dominated by repeated calls that can be organized into discrete
categories (Ford and Fischer 1983). Discrete calls emit rapid pulses and contain
high-energy harmonics (Ford 1987). Some categories of call types will have more
variability (Ford 1987). Different categories of discrete calls can be distinguished
by ear (Ford 1987). Each call type has distinctive features and thus different
calls can be visually identified on spectrograms without ambiguity (Ford 1987).
Air is forced through the nasal cavity to produce loud complex calls that can
be heard for 10 km underwater. Killer whales have excellent sensitivity to sound
and its direction (Ford et al. 2000).
Calls and whistles are signals used for social communication within and between
pods. Discrete calls are distinct call categories. These calls are usually less
than 2 seconds long and are brief bursts of sound. The killer whales generate
complex signals by varying the timing and frequency structure of these calls.
Each resident pod has about dozen discrete call types. A resident killer whale
will emit the entire call repertoire of its pod. Calls generally serve as contact
signal, coordinating group behaviour, and to keep in touch with pod members
when out of sight with one another (Ford et al. 2000). The call repertoire allows
the group to identify each other acoustically. This may allow individual whales
to recognize their family pod from an unrelated pod and thus determine mating
patterns. When a resident killer whale emits a call others in the pod they respond
with the same call before moving on to another call (Ford et al. 2000). Some
calls are used more often for certain activities like resting of socializing
(Ford et al. 2000). Emotional state of resident killer whales seems to reflect
call type use and how it is emitted (Ford et al. 2000). When excited, pitch
is increased and call time is shortened. It is probable that each whale produces
calls in a consistently subtly different way than the other whales in its pod
relaying their identification in the call signal.
Little is known about offshore killer whales and similarity in their acoustic
repertoire to resident killer whales may reveal similar social communication
system. In this study I compared offshore killer whales calls recorded in 2001
and 2002 to the catalogue of previously documented calls (Provencher 2001).
I identified 7 new offshore discrete calls. Groups recorded in 2001 used newly
identified calls significantly more than groups recorded in 2002. Each group
could be identified by a different highly repeated call.
METHODS
Field recordings were made of offshore killer whales on 3 and 5 December 2001
in Broughton Strait and 26 and 27 April 2002 in Johnstone strait. Recordings
were made with a recorder fitted with a hydrophone and calls were digitized
at a later date.
Continual spectrograms were made of each recorded encounter using a 0-22 kHz
frequency range on CoolEdit (2000). Recordings were 25 minutes long on 3 December
2001, 50 minutes on 5 December 2001, 10 minutes on 20 April 2002, and 40 minutes
on 27 April 2002. Calls were transcribed by hand using a notation system representing
structure characters of calls. Every sound on these recordings was analyzed.
Each call was copied into its own spectrogram file. Pulsed calls were of two
types: discrete and miscellaneous. Variable calls, or miscellaneous discrete
sounds that were not repetitive (less than three times), and aberrant renditions
of pulsed calls were not included in this analysis. Structure character and
sound identified discrete calls. Spectrogram analysis served to clarify call
classification and permit quantitative comparisons between call types. Some
of the discrete calls were confirmed by measuring side-band intervals, duration,
and frequency of call segments. Avisoft (Specht 2002) was used to analyze and
print the calls.
Frequency distribution of call types was graphed for each encounter and all
four encounters. Contingency tables and chi-squared statistics were used to
test the hypothesis that the frequency of calls was independent of sample. By
using these tests it is assumed that the recordings were of random samples of
the offshore repertoire.
The existing offshore killer whale catalogue consists of 97 identified call
types. To be consistent with the existing catalogue, newly classified calls
were given the nest call types number in the catalogue. Starting with the oldest
recording (3 December 2001) and ending in the latest recording (27 April 2002),
discrete calls were assigned the next number in the catalogue as calls were
identified. FiledmakerPro was used to document newly classified calls. On each
sheet a spectrogram, waveform, and call sound were entered. The call identification
number, date, and location of the recording were also included.
RESULTS
A total of 488-pulsed calls were identified including 183 discrete calls and
305 miscellaneous calls (Table I). The discrete calls were highly repeated and
had distinct tones that could be recognized by the ear and visually on the spectrograms.
A total of six existing calls (Figure 1 a-f) and seven new offshore killer whales
calls were identified (Figure 1 g-m).
Table I. Offshore killer whale call analysis.
Figure 1. Waveforms and spectrograms of pre-identified (a-f) and newly identified
offshore
discrete calls (g-m).
The first recording was made in Johnston Strait, British Columbia, on 3 December
2001. The sample contained 173-pulsed calls and included 75 discrete calls (43%)
and 98 miscellaneous calls (57%). Four new discrete calls were identified. The
frequency distribution of each discrete call type ranged from 5.3% to 40 % (Figure
2). The main call used by this group was OS101.
Figure 2. Proportion of call use by offshore killer whales recorded on 3 December
2001.
The second recording was made in Broughton Strait, British Columbia, 5 December
2001. A total of 136-pulsed calls were distinguished. These included 46 discrete
calls (34%) and 90 miscellaneous calls (66%). There were two new discrete call
types identified. The frequency of the number of calls used by this group ranged
from 9% to 91% (Figure 3). The main call used by this group was OS103.
Figure 3. Offshore killer whale call frequency on 5 December 2001.
The third recording was made in Johnstone
Strait, British Columbia, on 26 April 2002. A total of 90-pulsed calls were
distinguished from analysis. Of these calls there were 29 discrete calls (32%)
and 61 miscellaneous calls (68%). There was one new discrete call identified.
The frequency distribution of each discrete call type ranged from 21% to 59%
(Figure 4). The main call used by this group was OS26.
Figure 4. Offshore killer whale call frequency on 26 April 2002
A total of 89-pulsed calls were distinguished from analyzing the 27 April 2002,
recording made in Johnstone Strait, British Columbia. Of these calls there were
26 discrete calls
(29%) and 63 miscellaneous calls (71%). The frequency distribution of each discrete
call type ranged from 8% to 39% (Figure 5). The main call used by this group
was OS26.
Figure 5. Proportion of call use by offshore killer whales recorded on 27 April 2002.
The total frequency distribution
of each discrete call type recorded ranged from 1.7% to 21% (Figure 6). There
are more new calls (66%) than old calls (34%) used by these recently recorded
offshore killer whales. The main calls used were OS26, OS101, and OS103.
Figure 6. Total proportion of offshore
killer whales recorded in 2001 and 2002.
There is significant difference (x^2 (0.01,3) = 11.4, p ² 0.01) between the
number of new and old calls used by the offshore killer whales per sample (Table
II). The 2001 groups of offshores utilized existing calls significantly more
than the 2002 groups of offshore killer whales (x^2 (0.01,1) = 25.3, p ² 0.01).
DISCUSSION
The wide movements of cetaceans, greater variability of marine environment over
large temporal scales relative to that on land, and stable social groups could
effect the evolution of cetacean behaviour. Vocal repertoires reflect cultural
and genetic distinctiveness of sympatric killer whale populations (Ford 1991,
Strager 1995). Past studies by Johnston and Park (1994) and Provnecher (2001)
have identified 97 discrete call types of British Columbia offshore killer whales.
Seven new offshore killer whale types were discovered making a total of 104.
The significance of these additional discrete calls may be that they represent
the existence of a unique and diverse call repertoire for the offshore killer
whales. Newly identified calls were used significantly more by the groups recorded
in 2001 than the offshore killer whales recorded in 2002. Thus recordings of
offshores in 2001 and 2002 may have been from different pods. The groups in
2001 could be identified by two discrete calls The OS101 call was emitted only
from the 3 December, 2001, group and the OS103 call was mainly heard from the
5 December, 2001, group. The groups recorded in 2002 both emitted the OS26 call
with high frequency and are probably the same group. Photo-identification and
observation support this finding and therefore are the same group.
Barrett-Lennard (2001) found that British Columbia resident and offshore killer
whale share some of the same mitochondria D-loop sequences and therefore share
a common ancestor. Residents and transients do not share common calls and likely
do not share a recent common ancestor. Offshores have four call that are similar
in structure and sound to calls of the resident community of British Columbia
(Provencher 2001). Thus, there is genetic and acoustic evidence that residents
are closer related than the other ecotypes of killer whales. The northern and
southern residents do not share any call types, which also suggest that northern
residents are more related to offshores than to southern residents (Provencher
2001). Since total range of offshores is unknown it is difficult to determine
where they may have originated. In Norway, resident-like killer whale pods have
a unique dialect suggesting a distant ancestry from British Columbia killer
whales suggesting they have shared an overlapping range (Strager 1995).
The presence of unique discrete call types used by the offshore killer whales
may be the key in determining that offshores are a separate form of killer whale.
Future acoustic studies are required to reveal insight into communication systems
and social interactions of these mammals. This study reveals that offshore killer
whales could have a social system similar to residents. Difference in discrete
call use per group may be a means of recognizing related individuals to prevent
inbreeding and be a way of maintaining cohesion and coordination. Further research
may broaden our knowledge to identify offshore killer whale ancestral and social
relations.
ACKNOWLEDGMENTS
I would like to thank Jane Watson, Alison Keple, and my colleagues for their
support, insights, and expertise throughout the course of this project. I would
like to thank John Ford for his instruction and analysis software. I would also
like to thank Paul Spong and Helena Symonds of Orca Lab, and Jim Borrowman of
Stubbs Island Charters who provided the acoustical recordings for this analysis.
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