September 2012, Week 4


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Sun, 23 Sep 2012 22:04:15 -0400
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Flying Math: Bees Solve Traveling Salesman Problem
By Virginia Morell, ScienceNOW
Wired Science
September 21, 2012

Bumblebees foraging in flowers for nectar are like
salesmen traveling between towns: Both seek the optimal
route to minimize their travel costs. Mathematicians
call this the "traveling salesman problem," in which
scientists try to calculate the shortest possible route
given a theoretical arrangement of cities. Bumblebees,
however, take the brute-force approach: For them, it's
simply a matter of experience, plus trial and error,
scientists report in the current issue of PLoS Biology.

The study, the first to track the movements of
bumblebees in the field, also suggests that bumblebees
aren't using cognitive maps - mental recreations of
their environments - as some scientists have suggested,
but rather are learning and remembering the distances
and directions that need to be flown to find their way
from nest to field to home again.

A team of researchers from Queen Mary, University of
London outfitted seven bumblebees with tiny radar
transponders, which they stuck on the bees' backs with
double-sided tape. They trained the bees to forage
nectar from five blue artificial flowers (see video).
Each artificial flower had a yellow landing platform and
a single drop of sucrose, just enough to fill one-fifth
of a bumblebee's tank capacity, to ensure that the bees
would visit all five flowers on each foraging bout.

The scientists placed the flowers in a field at
Rothamsted Research, a biological research station north
of London, in October - a time of year when there are
few natural sources of nectar and pollen and the bees
are more likely to focus on the artificial flowers. They
arranged the flowers in a pentagon and spaced them 50
meters apart; that distance is more than three times as
far as bumblebees can see, so the bees must actively fly
around to locate their next target. A motion-triggered
webcam was attached to each flower to record the bees'
visits. Then, every day for a month, each bee was freed
to forage for 7 hours.

"We'd done a similar experiment in our lab," says
Mathieu Lihoreau, the lead author of the study and a
behavioral ecologist now at the University of Sydney in
Australia. "But that was in quite a small area for a bee
- only 7 by 7 meters." Seeing the bees forage in the
wild was entirely different. At first, Lihoreau says, he
tried to track the bees' movements by running alongside
them as they flew from flower to flower, "but they are
so fast, it wasn't possible." The transponders
eliminated the need for Lihoreau's sprints, and also
collected each bee's flight trajectory, travel distance,
and ground speed. From all those data, the scientists
recreated the bees' journeys and modeled them
mathematically - and discovered that they may be
employing a relatively simple method to find the most
efficient route between the flowers.

"Initially, the bees' routes were long and complex, and
they revisited empty flowers several times," Lihoreau
says. "But they gradually refined their routes through
trial and error."

At first, the bees visited the flower nearest to their
nest, and then the next closest flower. They kept track
- that is, they remembered - the total distance traveled
on each foraging trip. They tried new routes to increase
their efficiency, and if a route was shorter, they kept
it. If not, they abandoned it. As their experience
increased, they rarely altered the sequence of flowers
they visited.

After trying about "20 of the 120 possible routes, the
bees were able to select the most efficient path to
visit the flowers," Lihoreau says. "They did not need to
compute all the possibilities." A naïve bee traveled
almost 2,000 meters on its first foraging bout among the
pentagonal array; by her final trip, she'd reduced that
distance to a mere 458 meters.

Perhaps most surprising to the scientists was how
quickly the bumblebees learned from their trial-and-
error method. Before this study, such sophisticated
learning was "thought to be something only larger-
brained animals were capable of," says Lars Chittka, a
behavioral ecologist at Queen Mary, University London
and another member of the team.

Although the researchers did not set out to test whether
bumblebees use cognitive maps, the study's results
suggest that they do not. "The idea of a cognitive map
is very contentious," Lihoreau says. "But it's not a
very parsimonious hypothesis; it seems a lot to expect
from a small brain with less than one million neurons."
Using a simple rule, as the bumblebees did in this test,
may better explain what appears to us as complex
behavior, he says.

"It's a lovely study," says Mandyam Srinivasan, a
neuroethologist at the University of Queensland in
Brisbane, Australia. "It shows that bumblebees, with
their diminutive 1 milligram brains, are capable of
finding a nearly perfect solution to the traveling
salesman problem, with relatively few attempts and in a
relatively short time." This doesn't prove that
bumblebees do not possess a cognitive map, he adds, "but
it does demonstrate that they can get by without one."

This story provided by ScienceNOW, the daily online news
service of the journal Science.


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