178 Sharks: The Animal Answer Guide
Can scientists estimate the size and species of a shark
from bite marks?News accounts of shark attacks almost always speculate on what kind of
shark was responsible and, of course, how big it was. Any attack—whether
on a surfboard, a person, or a submarine—happens quickly (remember:
most sharks rely on surprise to be successful). Consequently, victims or by-
standers have only a fleeting glimpse of the animal involved. Even trained
observers find making accurate assessments of size and species challenging.
This is where forensic analysis—a sort of CSI (crime shark investiga-
tion)—can contribute information. Research has shown that the size and
even species involved can be estimated from as little as a few scratches on
a surface produced by a shark’s teeth. An investigator looks at any tooth
fragments left behind, measures bite profiles, analyzes location and habitat,
and compares the information with species distribution guides and mu-
seum collections of teeth and jaws from known sharks. Or the investigator
calculates shark size from equations developed by careful measurements
that compare tooth and jaw characteristics of sharks of known sizes.
You might think that the size of a tooth left behind would indicate shark
size. But teeth vary between individuals and between locations in the jaw,
as well as among the rows of teeth in the jaw, all reducing the accuracy of a
size estimate. What about the width of the chunk taken out of a surfboard
or of a wound on a whale carcass? Aren’t these good indicators of shark
size? Yes and no. The shark might not have gotten its entire mouth around
an object, in which case we might underestimate the shark’s size. Or the
shark might have enlarged the wound by shaking its head or moving its
jaws from side to side; this larger wound would suggest a larger shark than
was actually the case. For this reason, analysis based on bite wound size will
state that the shark was at least or about a certain size. This kind of reason-
ing was used in determining that the shark that attacked 13-year-old surfer
Bethany Hamilton in Hawaii was a Tiger Shark with “a minimum length
estimate of 4493 mm,” given the circumference of the piece taken out of
her surfboard.
Recent data indicate that the best forensic analysis of size comes not
from the width of the bite but from measuring the distance between the
impressions or holes left by the tips of the teeth on either side of the middle
of the jaws, the “interdental distance,” or IDD. The IDD is fairly constant
for a particular size and species of shark, and even differs slightly between
upper and lower jaws. Because a bite is likely to leave several tooth tip im-
pressions above and below the bitten object, an investigator can make sev-
eral measurements, and the more measurements that are made, the greater
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