80 Sharks: The Animal Answer Guide
advanced have larger brains. The relatively primitive squatiniform angel
sharks and lamniform Basking Sharks have relatively small brains, as do
squaliform dogfishes and rajid skates. White Sharks, also lamniforms, are
intermediate. Carcharhinid requiem sharks, hammerheads, and dasyatid
stingrays have relatively large brains, and odontaspid Sand Tigers have even
larger brains. Mobuline manta and devil rays, which are generally consid-
ered the most advanced evolutionarily of all cartilaginous fishes, have the
largest brains. Encephalization quotients move things around a bit, with
freshwater stingrays moving to near the top of the list (see the next ques-
tion, “Do sharks use tools?”) and White Sharks slipping down. Regardless,
by these measures, sharks have relatively large brains, at least when com-
pared with bony fishes and even with higher vertebrates. The relative brain
size of most sharks is larger than that of most bony fishes and even than
that of about one-third of bird species and some mammals. Much of the
increase is due to a relatively large and complex cerebellum.
What about learning ability? What we know is based on a limited sam-
ple size, involving lab tests on a few carcharhinids (particularly Lemon
Sharks and Bull Sharks), orectolobids (Nurse Sharks), and small stingrays
such as Round Stingrays and freshwater stingrays (Potamotrygonidae). We
know of no such studies on chimaeras. In standardized learning studies, a
shark receives a food reward if it learns a simple task, such as head-butting
a button when hearing an underwater bell. Sharks do as well as bony fishes
and mice in these tests and in tests of black versus white color discrimina-
tion. The sharks learn quickly and retain the knowledge, taking as much as
six weeks to forget the task. Lemon Sharks learn quicker than Bull Sharks.
No big deal.
How relevant are such tests to the field conditions and circumstances
under which sharks have evolved? Put another way, are these valid tests
of a shark’s “intelligence” given the sorts of abilities that would be useful
to a wild shark? The bell-head-butt-food-reward trials could be viewed as
parallel to real world situations, such as a shark’s learning that bony fishes
make distress calls and erratic swimming noises when injured, a clue to a
potential meal. In fact, later studies on newborn Lemon Sharks showed just
this—that newborns don’t react excitedly to prey distress sounds until they
are “rewarded” with prey, after which they get excited when the sounds
are played through underwater speakers. This study also showed that the
sharks do better in a group, suggesting they may watch other individuals
and learn from their behavior.
Other studies have shown that with experience, White-Spotted Bam-
boo Sharks become better foragers, in terms of being able to capture and
consume novel prey more quickly. They also remember what they have
learned, capturing prey efficiently even if they are allowed to forget for as
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