CAN BE IMPORTANT TO RATS
by Whit Gibbons
April 16, 2006
a colleague looking out my office window onto a patio where I spread sunflower
seeds noted that squirrels occasionally ventured into the open area but
that birds were nearly always around at the time. Birds are absent about
as often as they are present, whereas squirrels never seem to be present
unless birds are. Such a phenomenon could be from random chance of when
we happened to be watching. But based on a behavioral ecology study conducted
on birds and mammals eating together, a meaningful biological pattern
could be involved.
ecology is a field that attempts to identify what animals do and explain
why from a biological standpoint. Many behavioral patterns are obvious;
for example, mothers of many animal species actively protect their young.
Honeybees defend their hive and prairie dogs whistle to alert their close
kin that a predator is near. But in many instances the behavior of individuals
or groups can be protective of or otherwise beneficial to members of a
completely different species. Sometimes both species may benefit from
a relationship. The little birds that ride unmolested on the backs of
rhinoceroses rid the big animals of insect pests and warn them of approaching
danger, while picking up a meal in exchange. Some relationships are one-way
streets, with only one species profiting, although the other may be unaffected.
For example, Spanish moss provides no obvious benefit or harm to oak trees
whose limbs are used as a means of support for the uninvited guest.
that benefits one species but not necessarily the other was discovered
in a study conducted by David Young, a University of Georgia graduate
student. The study confirmed and helped explain an earlier observation
made by Ronald Pulliam of an unusual group behavior between sparrows and
rats. Anyone with a bird feeder is aware that sparrows often feed in groups.
Behavioral ecologists attribute this to a survival tactic in that an approaching
predator, such as a hawk by air or a fox by land, is more likely to be
seen if several pairs of eyes are on guard. Thus, when one bird flies
in fright, the entire flock is alerted and can scatter to avoid the danger.
are fat, brown balls of coarse fur that inhabit the southern tier of states
and are considered an ideal meal by many predators. Cotton rats know that
they are considered "good eating" and are highly cautious about
venturing into open areas where they might be caught. Nonetheless, if
a board covered with seeds is placed in a field, cotton rats will eventually
move onto the open area to get the free meal--usually on one condition.
A flock of feeding sparrows must already be on the board.
In the study,
video cameras were used to record the response of sparrows and cotton
rats to the presence of large plywood squares that had been sprinkled
with seeds. Shortly after the food was put out, sparrows began to come
and go. And so did the cotton rats. However, the rats would wait along
the margins until the sparrows arrived. The rats and sparrows ignored
one another while they were eating, but if the sparrows flew away, the
rats would scurry off the board.
were taking advantage of the sparrows as sentinels and were too wary to
venture into the open without benefit of the warning system. Confirmation
that the cotton rats were aware of the potential danger of feeding in
the open was made on the very few occasions when a rat was present with
no sparrows around. When feeding on the board with sparrows, a rat had
a food intake per minute (as measured on the videotape) ten times that
of one that dined alone. The reason? Solitary cotton rats spend most of
their time scanning the skies for hawks and have little time to enjoy
the free meal. Whether the sparrows derive any benefit from having cotton
rats at their dinner table is still to be determined.
the case with sparrows and cotton rats, the outdoor world has many intriguing
mysteries awaiting our discovery.
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