Inside the killer whale matriarchy - Darren Croft
Off the rugged coast
of the pacific northwest,
pods of killer whales
inhabit the frigid waters.
Each family is able to survive here
thanks mainly to one member,
its most knowledgeable hunter:
the grandmother.
These matriarchs can live eighty years
or more,
while most males die off
in their thirties.
Though killer whales inhabit
every major ocean,
until recently we knew
very little about them.
The details of their lives
eluded scientists
until an organization called
the Center for Whale Research
began studying a single population
near Washington State and
British Columbia in 1976.
Thanks to their ongoing work,
we’ve learned a great deal
about these whales,
known as the Southern Residents.
And the more we learn,
the more this population’s elders’
vital role comes into focus.
Each grandmother starts her life as a calf
born into her mother’s family group,
or matriline.
The family does everything together,
hunting and playing, even communicating
through their own unique set of calls.
Both sons and daughters spend their entire
lives with their mothers’ families.
That doesn’t mean a young whale
only interacts with her relatives.
Besides their own special calls,
her matriline shares a dialect
with nearby families,
and they socialize regularly.
Once a female reaches age fifteen or so,
these meetings become opportunities
to mate with males from other groups.
The relationships don’t go much
beyond mating—
she and her calves stay with her family,
while the male returns to his own mother.
Until approximately age forty,
she gives birth every 6 years on average.
Then, she goes through menopause—
which is almost unheard of
in the animal kingdom.
In fact, humans, killer whales and
a few other whales
are the only species whose females
continue to live for years
after they stop reproducing.
After menopause,
grandmothers take the lead
hunting for salmon,
the Southern Residents’ main food source.
Most of the winter they forage offshore,
supplementing salmon with other fish.
But when the salmon head
towards shore in droves to spawn,
the killer whales follow.
The matriarch shows the younger whales
where to find the most fertile
fishing grounds.
She also shares up to 90% of
the salmon she catches.
With each passing year,
her contributions become more vital:
overfishing and habitat destruction have
decimated salmon populations,
putting the whales at near-constant
risk of starvation.
These grandmothers’ expertise
can mean the difference between
life and death for their families–
but why do they stop having calves?
It’s almost always advantageous for a
female to continue reproducing,
even if she also cares for her existing
children and grandchildren.
A couple unique circumstances
change this equation for killer whales.
The fact that neither sons or daughters
leave their families of origin
is extremely rare—
in almost all animal species,
one or both sexes disperse.
This means that as a female
killer whale ages,
a greater percentage of her family
consists of her children
and grandchildren,
while more distant relatives die off.
Because older females are more closely
related to the group than younger females,
they do best to invest in the family
as a whole,
whereas younger females should
invest in reproducing.
In the killer whale’s environment,
every new calf is another mouth to feed
on limited, shared resources.
An older female can further her genes
without burdening her family
by supporting her adult sons,
who sire calves other families will raise.
This might be why the females have evolved
to stop reproducing entirely
in middle age.
Even with the grandmothers’ contributions,
the Southern Resident killer whales
are critically endangered,
largely due to a decline in salmon.
We urgently need to invest in restoring
salmon populations
to save them from extinction.
In the long term, we’ll need more studies
like the Center for Whale Research’s.
What we’ve learned about
the Southern Residents
may not hold true for other groups.
By studying other populations closely,
we might uncover more
startling adaptations,
and anticipate their vulnerabilities
to human interference
before their survival is at risk.
