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The base of the killer whale's social structure is the matrilineal unit
including adult female and her offspring of different sexes. Some
matrilineal units, being in remote relationship and spending together
most part of the time, form a pod, consisting from 2 to 40 - 50 animals.
Each pod has unique vocal repertoire including either sounds used
only by pod members or sounds shared with animals of other pods. Some
pods can collaborate with each other for successful hunting or social
interactions. All pod members are relatives therefore we can suggest
that they mate only during different pod's associations. Canadian
researchers has described uncommon places of Canadian coasts, where
killer whales come for scratching themselves by gravel bottom in shallow
waters. There one can meet playing or resting whales from different
pods.
And now about the English name of these animals - "killer whales"!
Are they so bloodthirsty? Scotland researcher Erich Hoyt collected
all reports about killer whale's attacks on different vertebrate animals
and found, that the diet of orcas include 24 species of cetaceans,
14 species of pinnipeds, 31 species of fishes, 9 species of birds,
2 species of cephalopods, 1 turtle species and sea otter. Further
analysis of orca food preferences shows that Orcinus orcahas wide feeding spectrum but each separate population behaves as specialist,
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preferring to eat only few, common for an area, most plentiful
species of prey. Prey selection determines basic sociobiological
features of each population. Long-term investigations of Canadian
orca population showed how can differ animals of one species
hunting for different prey. Researchers distinguished two
sympatric (living at one area, but not mixing) populations:
"resident" and "transient". |
Resident orcas eat fish exclusively: salmon, herring, greenlings,
cod, etc. For chasing of school of fish "resident" orcas use different
methods, urging it to the beach or surrounding
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in the center of school one after another (carousel method,
described also for white whales and bottlenosed dolphins).
For such hunting they need large groups, therefore pods of
"resident" orcas include from 5 to 15 members (10 on average).
"Transient" form of Canadian orcas is a real "killer"
whale, whose bloodthirstiness breathes on reputation of these
sea predators. In stomach content of transients there have
been found remains of pinnipeds, small cetaceans and sea birds.
Pods of "transient" killer whales are smaller then "resident":
there are from 1 to 5 |
carousel fishing |
animals in the pod (on average 3). It can be caused by their hunting tactics: transient orcas make an ambush, using bottom relief near seal rookeries. Only one animal snap a seal, othershave to stay
on a distance and come to the hunter if seal has already been catched.
The most dramatic show is intentional stranding of killer whales for
catching pinnipeds on their rookeries at South America coasts. This
hunting tradition passes from generation to generation of killer whales,
mothers teach their youth to strand on uninhabited beaches on the
distance from seal rookeries. If calf can't return to the water by
himself, his mother always helps him, stranding near him and pushing
him to the water. Transient killer whale catch small dolphins alone
or surround their group, hunting together with another pods. When
attacking large whale orcas hunt like wolves pack. Often they try
to exhaust one whale from the group or separate a calf. Attacking
sperm whale orcas don't let him dive because they can't chase him
on the large depth. Sometimes killer whales don't eat the whale: they
can choose only tongue, lips and throat area. May be it is caused
by high energetic value of this parts of whale's body.
An important distinction of transients from residents is their vocal
activity level: transients keep silent, because marine mammals can
hear them. That's why for orientation and prey detection residents
use active echolocation (i.e. original production of clicks and listening
their sound reflection). Transients just listen to ocean noises passively
and are guided by them.
Analysis of nuclear and mitochondrial DNA of resident and transient
killer whales showed that there is no interbreeding between them.
It is possible that differences in social and sexual behaviour of
killer whales specialized on different prey can create strong barrier
preventing cross pairing.
Analogues of Canadian transient and resident orcas were found at Antarctic
waters by Russian scientists A.A. Berzin and V.L. Vladimirov (1982).
They have found "yellow" and "white" killer whale forms. These color
differences were caused by different amount of algae biofouling, which
can paint animals in yellow or fawn tint. These animals could be close
to each other, but scientists didn't observe any mixed groups. "Yellow"
animals prefer to keep near ice zone in groups of 150 - 200 animals,
and "whites" were at ice-free zone and their groups didn't exceed
of 15 animals. Studying of stomachs content showed that in stomachs
of "yellow" orcas 98.5% of food consisted of fish and in stomachs
of "white" animals 89.7% of food was marine mammals. Differences between
"white" and "yellow" forms were shown in square of pectoral fins,
configuration of cranium, etc. Basing on these considerable differences
and absence of visible insulating barriers, A.A. Berzin and V.L. Vladimirov
concluded that there was genetic isolation. They proposed to distinguish
a new species - Orca glacialis, but this initiative was not supported.
Currently, researches distinguish three forms of Antarctic killer whales.
In general, we know little about killer whales in Russian Far Eastern
seas. Most of the studies of these animals have taken place at 1950-1970
years and earlier. While reading these old reports you can find that
orca's groups consisted of 5 - 12 animals, and there were larger groups
(30-50 animals per group) in the places of fish accumulation. Y.I.
Betesheva and Y.I. Ivanova (1961) studied killer whale's diet, dissecting
stomachs of beached and killed orcas. They have founded that the half
of stomachs contained remains of fish and squids and others were empty.
Fish diet of Kamchatka killer whales was confirmed by observations
of M.M. Sleptsov, describing orca's hunting for school fish, deep-water
perch and squid.
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