Adaptive
evolutionary divergence in
sympatric killer whale (Orcinus
orca) ecotypes
Two
sympatric killer whale (Orcinus
orca) ecotypes of the northeast
Pacific exhibit dramatic resource
polymorphisms, with one (residents)
feeding exclusively on fish,
and the other (transients)
on marine mammals. Thus far,
behavioural studies of these
killer whale ecotypes have
focused on their vastly different
vocal repertoires, echolocation
use, foraging strategies,
and sociobiology. Recent genetic
studies have corroborated
behavioural evidence that
the resident and transient
forms are reproductively isolated
despite the absence of any
geographic or temporal barrier.
This segregation between the
sympatric ecotypes is apparently
maintained by cultural mechanisms
alone, which is extremely
unusual among non-human mammalian
species.
It remains to be seen whether
divergent morphologically
adaptive evolution has occurred
that reflects the dramatic
differences in the foraging
strategies, sociobiology,
and genetics of residents
and transients. Subtle morphological
differences in pigmentation
and dorsal fin shape have
been described and are useful
for distinguishing residents
and transients in the field.
However, these differences
have no known adaptive significance
and likely resulted from genetic
drift. Since members of the
two ecotypes use fundamentally
different foraging techniques
that almost certainly exert
different selective pressures,
I am interested in whether
residents and transients are
evolving along divergent morphological
trajectories, or whether the
present dietary dichotomy
is too recent or ephemeral
for adaptive evolution to
occur.
The goal of my research is
to test the hypothesis that
adaptive evolutionary divergence
has in fact occurred. I am
characterizing and comparing
the cranial skeleton of residents
and transients both qualitatively
and quantitatively by analyzing
linear, meristic, outline,
and landmark data from the
skulls and mandibles of northeast
Pacific killer whales in public
and private collections. The
ecotype of each specimen will
be determined by mitochondrial
DNA sequences. The results
will be interpreted in the
context of biomechanical advantages
of, or constraints on, cranial
structure in relation to ecotype-specific
differences in diet and foraging
behaviour. I predict that
transient killer whales that
bite and tear apart large
mammalian prey have more robust
skulls, mandibles, and dentition
than the piscivorous residents
that handle smaller prey items.
This work is being conducted
under the supervision of Dr.
Lance Barrett-Lennard (UBC
Zoology and Vancouver Aquarium
Marine Science Centre) and
Dr. Bill Milsom (UBC Zoology).
Links:
The
Cetacean Research Lab, Vancouver
Marine Science Centre
The
British Columbia Wild Killer
Whale Adoption Program
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