OF
PREDATORS
& THEIR
PREY
To study the predatory
behavior of the GW, scientists must travel to a location where GW's
are known to hunt their prey. Luckily, about 30 miles off the coast of
San Francisco are the Farallon Islands, home to a large elephant seal
population - making this area the local burger shop for GW's in the
neighborhood and thus a hotspot of GW research.
A regular watch began in
1987 on Southeast Farallon Island and it has produced a great deal of
information about the GW's predatory habits. For example, it seems from
this data that most attacks occur during the day in late summer or early
winter. Furthermore, the attacks took place at around the same time each
day, most likely due to the tide schedule.
To give you an idea of what
a GW considers to be the ultimate meal - the Big Mac of the sea, if you
will - think of those cute seals and sea lions you often see performing
at a Sea World near you. They are collectively known as pinnipeds
and they are the preferred food of the GW - over
fish, other sharks, or your Aunt Sally.
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Although
the two animals look similar, they are quite different. The most
striking difference is the design of their flippers. The seal has highly
developed hind flippers and smaller, underdeveloped fore flippers. Sea
lions are just the opposite.
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A female northern
elephant seal and her pup resting on the shore |
A pair of sea lions, a
large male and a smaller female. Compare the female sea lion to the
female seal at right, noting the difference in the design of their
flippers. |
Going back to the
observational data, some interesting differences in attack strategies
were noted based on the species of the shark's prey. For example, in the
case of the seal, the animal is often attacked just beneath the surface
by a GW rising from below. A large elongating blood stain at the surface
indicates that the shark carries the seal underwater for a distance
before removing a bite and releasing the carcass which then floats to
the surface. When this initial attack took place near the head of the
seal, an area rich with networks of blood vessels, death by exsanguination
(loss of blood) or decapitation was the norm. On other
occasions, the GW would disable the seal by attacking from behind,
biting the strong hind flipper. Nature is pretty grisly stuff, huh?
With the sea lion, attacks
are usually observed with the sea lion at the surface of the water, the
GW striking brutally - even throwing itself out of the water with the
sea lion clamped in its jaws. The sea lion, lacking the same network of
blood vessels flounders at the surface until the shark returns for the
final kill and feeding.
Prior to the study I refer
to above, a prevalent theory relative to GW predatory behavior held that
a primary attack strategy of the GW is to capture and bite their prey,
release them wounded but alive, and then remain nearby until the prey
animal is still, indicating death or severe injury. The GW would then be
afforded an easy meal. This 'bite, spit, and wait' behavior was not
observed in the over 130 attacks recorded as part of this study. Does
this mean it is not a valid hypothesis? Not at all - 130 attacks in a
single location on a limited prey selection can hardly be called
universally representative of a GW's predatory behavior. However, these
same 130 observed events provide an excellent glimpse at how the GW goes
after its favorite meal.
SEAL
SAYS
"I
WAS
ATTACKED
FROM
BELOW!"
FILM
AT ELEVEN!
I bet you didn't know seals
could talk!
Of course, they can't. This
is common knowledge. So it seems would be an attack from below. After
all, this is because when a shark attacks an item at the surface, any
attack other than a 'Jaws'-like (and thus camera-friendly) attack at the
surface would be from below. But what path does the shark take when
attacking? Does the shark travel in a horizontal manner beneath its prey
before striking or does it prefer the vertical attack made famous by the
poster for Mr. Spielberg's most famous fish story? Turns out Hollywood
got it right. The GW likes to attack from a deep angle (45-90 degrees)
below the prey item. Is this because there are a limited number of other
attack vectors or because this is the best way to catch dinner?
In examining this question,
a couple of benefits become apparent, namely that an attacker from below
is harder to spot and also to elude. Think about it - for many reasons,
a shark deeper than its potential meal is less visible than a shark
swimming on the same plane. The greater the angle from and the further
below the shark is from a seal means the seal is going to have a tough
time seeing it before it becomes a statistic in a South Farallon Island
study.
At the same time, a seal's
best approach to evade the GW is to bust a move quickly in the opposite
direction of the shark's attack. Think of 'The Little Mermaid' when
Ariel and Flounder swim horizontally away from the shark who wants to
make them an appetizer. They've got lots of room and if they swim
quickly, they can (and did, of course!) evade their attacker. However, a
seal attacked from below can realistically only travel up . . . and
there's only so far it can go in that direction!
BIBLIOGRAPHY
Klimley, A. P., Anderson,
S. D., Henderson, R. P., and Pyle, P. (1996) A description of predatory
attacks by white sharks on pinnipeds. In "Great White
Sharks: the biology of Carcharodon carcharias" (A. P. Klimley and
D. G. Ainley, eds.), pp. 175-191
