half-sib controls demonstrated that call convergence did not result from
maturational processes, acoustic adaptation to the physical environment, or
heredity. Instead, half-sibs who transferred to new groups were more similar
to their new, unrelated group mates than to relatives in their natal group
(Boughman 1998). Call modification was not immediate, and the time
required for individuals to match a new group could provide protection
against outsiders who might feign identity to obtain access to food resources
controlled by groups. Thus, in this species, cheater-protection mechanisms
appear to have evolved in concert with group signatures.
3.2.2. Cetaceans—Killer Whale Pod Signatures
Killer whales (Orcinus orca) live in stable social groups called pods. Both
transient and resident whales are found in the coastal waters of western
North America. These types differ in foraging strategy, social structure, and
vocal patterns, and they are reproductively isolated (Barrett-Lennard et al.
unpublished). Here, we focus on the resident killer whales off the British
Columbia (BC) and Alaska coasts, for which the bulk of work on vocal
behavior has been done. Residents feed primarily on fish and live in
extremely stable matrilineal groups. Neither males nor females disperse
out of their natal pods (Bigg et al. 1990; Olesiuk et al. 1990; Hoelzel et al.
1998), so pods are composed of close relatives organized along matrilineal
lines. Mating occurs between members of different pods (Barrett-Lennard
unpublished). Pod members travel, feed, and socialize together. There is
some suggestion that pod members cooperate in finding and capturing prey
(Bigg et al. 1987; Hoelzel 1993), and foraging benefits may be one factor
that favors living in stable groups.
Like other odontocetes, killer whales use echolocation to find prey. They
produce several types of calls, but most work on vocalizations has focused
on social calls, which combine tonal and pulsed elements (Fig. 4.7A). Resident pods have a repertoire of 3–16 discrete calls, with an average of nine
calls (Ford and Fisher 1983; Ford 1989, 1991). Similar patterns were found
for whales in coastal waters off Norway, which appear to have social structure and call patterns similar to BC residents (Strager 1995). Pods differ in
their call repertoires. Although most calls are shared by several pods, some
are unique to a particular pod (Fig. 4.7). The degree of call sharing between
pods may depend on geographic distance, the amount of contact, or genetic
similarity. Pods that rarely encounter one another share almost no calls,
whereas those that interact frequently share some calls (Ford and Fisher
1983). The function of such call sharing has not been investigated. In addition to variation among pods in the type of calls given (termed repertoire
variation), more subtle structural variation is present. Pods differ in
frequency and temporal features of several calls, with those pods that associate least and least closely related showing the largest differences (Deecke
1998). An additional class of graded calls appears to be used at close range
4. Comparative Vocal Learning
177
maturational processes, acoustic adaptation to the physical environment, or
heredity. Instead, half-sibs who transferred to new groups were more similar
to their new, unrelated group mates than to relatives in their natal group
(Boughman 1998). Call modification was not immediate, and the time
required for individuals to match a new group could provide protection
against outsiders who might feign identity to obtain access to food resources
controlled by groups. Thus, in this species, cheater-protection mechanisms
appear to have evolved in concert with group signatures.
3.2.2. Cetaceans—Killer Whale Pod Signatures
Killer whales (Orcinus orca) live in stable social groups called pods. Both
transient and resident whales are found in the coastal waters of western
North America. These types differ in foraging strategy, social structure, and
vocal patterns, and they are reproductively isolated (Barrett-Lennard et al.
unpublished). Here, we focus on the resident killer whales off the British
Columbia (BC) and Alaska coasts, for which the bulk of work on vocal
behavior has been done. Residents feed primarily on fish and live in
extremely stable matrilineal groups. Neither males nor females disperse
out of their natal pods (Bigg et al. 1990; Olesiuk et al. 1990; Hoelzel et al.
1998), so pods are composed of close relatives organized along matrilineal
lines. Mating occurs between members of different pods (Barrett-Lennard
unpublished). Pod members travel, feed, and socialize together. There is
some suggestion that pod members cooperate in finding and capturing prey
(Bigg et al. 1987; Hoelzel 1993), and foraging benefits may be one factor
that favors living in stable groups.
Like other odontocetes, killer whales use echolocation to find prey. They
produce several types of calls, but most work on vocalizations has focused
on social calls, which combine tonal and pulsed elements (Fig. 4.7A). Resident pods have a repertoire of 3–16 discrete calls, with an average of nine
calls (Ford and Fisher 1983; Ford 1989, 1991). Similar patterns were found
for whales in coastal waters off Norway, which appear to have social structure and call patterns similar to BC residents (Strager 1995). Pods differ in
their call repertoires. Although most calls are shared by several pods, some
are unique to a particular pod (Fig. 4.7). The degree of call sharing between
pods may depend on geographic distance, the amount of contact, or genetic
similarity. Pods that rarely encounter one another share almost no calls,
whereas those that interact frequently share some calls (Ford and Fisher
1983). The function of such call sharing has not been investigated. In addition to variation among pods in the type of calls given (termed repertoire
variation), more subtle structural variation is present. Pods differ in
frequency and temporal features of several calls, with those pods that associate least and least closely related showing the largest differences (Deecke
1998). An additional class of graded calls appears to be used at close range
4. Comparative Vocal Learning
177