Shark Behavior 81
long as 18 days. Motoro Stingrays (Potamotrygon motoro) develop a mental
map of their surroundings, learning the location of food items based on vis-
ible landmarks in their holding pens. This would certainly be a useful skill
in the real world of a reef or a kelp bed, where various prey species tend to
hide in certain places. It would also help them learn where divers will con-
gregate with handouts during “interactive dives.”
Do sharks use tools?
This is another one of those human-centered questions. We associ-
ate tool use with intelligence because we use tools and therefore wonder
whether other animals are smart like us. Tool use implies taking an external
object in the environment and using it to perform a useful task that other-
wise would be difficult. There are a number of animal examples, such as a
thrush (or a wrasse) banging a snail against a rock or a rock against a snail,
or chimpanzees fishing termites out of a mound with a long blade of grass.
In this area, sharks pretty much fail, at least in terms of using external
objects in food processing. By strict definitions, thresher sharks whacking
prey with their tails, sawfish using their saws, stingrays displacing sand by
flapping their wings or blowing water out their mouths so that they can ex-
cavate clams, or hammerhead sharks knocking stingrays to the bottom by
delivering a downward blow with their “hammer” don’t qualify as tool use.
(See “How do sharks catch their prey?” in chapter 7.) One interesting vari-
ation involves Lesser Spotted Dogfish holding food items down with their
tails. They apparently take advantage of the rough dermal denticles on the
tail to hold the food in place while they tear pieces out of it with their teeth.
The example of rays excavating clams actually falls into a gray area be-
cause the ray isn’t just using a body part to accomplish the task, but is in-
stead pushing water with its pectoral fins or mouth. The water then be-
comes the tool, manipulated by the undulating fins or mouth muscles, to
achieve a goal of obtaining food. In this context, researchers have studied
Vermiculate River Stingrays (Potamotrygon castexi, also called the Otorongo
Ray). Stingrays are potentially good subjects for investigation because ba-
toids have the largest relative brains among elasmobranchs.
The study looked not only at tool use but also at learning ability in
a problem-solving situation. The researchers placed food rewards inside
an opaque plastic tube that had a mesh screen divider, which meant the
food could be obtained only if the ray tried to extract the food from the
“correct” end. (Such “tube-traps” are commonly used in studies of animal
learning.) All five test animals improved their performance by figuring out
progressively more quickly with each trial which end of the tube to work
from, correcting themselves if they first tried to extract the food from the