Explanation of Playback Videotapes
Michael Wilson
One of the most useful tools for studying the
communication and cognition of wild animals is the playback
experiment. In a playback experiment, researchers use a
hidden speaker to play back a recording of an animal's call
to an audience of other wild animals. How animals respond to
such playbacks can help researchers understand things such
as the meaning of different calls, how animals respond to
predators, and how animals defend their territories.
Playback experiments have been conducted with birds,
primates, lions, elephants and even whales. With my thesis
advisors Marc Hauser and Richard Wrangham and a team of
Ugandan research assistants I conducted the first successful
playback experiments with chimpanzees.
Chimpanzees have hostile relations with other chimpanzee
communities, and sometimes attack and kill members of
neighboring communities. Many researchers have compared such
attacks to human warfare. Such attacks occur rarely,
however, and there is still a great deal we don't know about
intercommunity relations. We conducted the playback
experiments to better understand what factors affect the
response of chimpanzees to foreign chimpanzees.
The video footage
is from two playback experiments conducted in Kibale
National Park, Uganda, in 1998. In each experiment, we
played a recording of the 'pant-hoot' call of a
single foreign male chimpanzee. The foreign chimpanzees,
recorded by John Mitani, live in Mahale Mountains, Tanzania,
over 500 km (300 miles) south of Kibale. We hid the speaker
about 300 m (330 yards) from the target chimpanzees.
Although chimpanzees live in communities of 50 or more
individuals, they spend most of their time in smaller groups
called 'parties'; that range in size from one to 20
chimpanzees. I played back calls of single male strangers to
parties of different size and composition to see if
chimpanzees responded differently depending on who they were
with. I also played back calls in different parts of their
territory to see if they responded more aggressively at the
center or edge of their range.
I found that the response to the playback depended mainly on
the number of adult males present. Parties with only females
stayed quiet and sometimes traveled away from the speaker.
Parties with 1-2 males stayed quiet and either stayed where
they were or (in about half the cases) slowly approached the
speaker. Parties with 3 or more males gave a loud vocal
response and traveled quickly towards the speaker. The
location of the experiment didn't seem to matter. These
results show that chimpanzees can 'count': they
approach faster and more noisily the more they outnumber the
intruder.
The video footage illustrates the different responses
given by parties with few and many males.
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