Questions 31-40 are based on the following passage.
Only a few millimeters long, rock ants (Temnotho-
NATURAL SCIENCE: This passage is adapted from the article "Swarm Savvy" by Susan Milius (©2009 by Society for Science & the Public).
rax albipennis) prove difficult to track in the wild but
excellent for the tabletop world of the laboratory.
When something terrible happens to a rock ant
5 home, such as a researcher lifting off the roof, the
majority of ants cluster in the ruins. A quarter to a third
of the colony scurries out looking for new possibilities.
"I think of the ants as a sort of search engine," ant
biologist Nigel Franks says. In one set of tests, he and
10 his students disrupted a nest and watched to see what
the ants would make of a series of new possibilities that
improved with distance. The best nest was almost three
meters distant, nine times as far from the original home
as a nearby but less appealing choice. "It was just such
15 fun doing this experiment because the ants won,"
Franks says.
In spite of the epic distances, the ants typically
found and agreed to move into the best nest. "They're
fantastic at it," Franks says.
20 Franks and Elva Robinson, both of the University
of Bristol, monitored rock ants by fitting them with
radio-frequency identification tags. The data suggest
that each scout follows a simpler rule than previously
thought, Robinson, Franks and their colleagues
25 reported online in Proceedings of the Royal society B.
Instead of making direct comparisons between
sites, a scout follows a threshold rule. If she finds a
poor site, she keeps searching. When she finds a site
that exceeds her "good enough" threshold, she returns
30 to the original nest
Next, previous work shows, the scout recruits a
new scout to join her on a trek to the good site. She
dashes around tapping her antennae on other ants and
releasing a pheromone from her sting gland, explains
35 Stephen Pratt of Arizona State University in Tempe.
Usually she finds a volunteer within a minute or so, and
the two set off tandem running.
Scout A, who knows the way, runs back toward the
nest while her follower, B, jogs closely enough to tap
40 antennae against the leader. Should A sprint a little too
fast and dash beyond antennae range, she slows until
her partner catches up. Periodically the two ants stop,
and the newbie looks around as if learning landmarks.
It's a slow way to get to the site, and Franks argues that
45 it qualifies as animal teaching.
When the ants do reach the possible site, the
recruit explores it and, depending on her assessment,
returns to recruit yet another scout.
As with bees, it's the quorum of scouts at the sites
50 that matters. When enough of them gather at a particu-
lar place to encounter each other at a sufficiently high
rate, they've got a decision.
Once scouts reach that decision, their behavior
changes. Each scout dashes back to the nest, but instead
55 of coaxing a nest mate for a tour, she just grabs some-
body. She uses a mouthpart hook, an over-the-shoulder
throw, and off she goes with the passive nest mate
curled on her back in an ant version of the fetal posi-
tion. Carrying takes about a third as long as leading
60 would, and scouts can haul the rest of the colony to a
new home within hours. The ants shift from the inde-
pendent info gathering of scouts to group implementa-
tion of the quorum's decision.
Rock ants' willingness to thrive in the lab allows
65 experiments on finer points of collective decisions
making, Pratt says. For example, forcing a crisis among
the ants demonstrates that they will, in a pinch, trade
accuracy for speed. When researchers destroy an old
nest so that ants are completely exposed, the ants scope
70 and relocate within hours. Other experiments that just
offer the ants a better nest but don't ruin their current
one can result in days of deliberation. Speed has its
costs, and ants in a hurry now and then make mistakes,
such as splitting the colony between two nests. Slower
75 moves prove more accurate.
The quorum system could be widespread in group
behavior in nature, Pratt says. Overall it's a beautiful
tool, allowing for carefully balanced independence plus
some shortcut speed. Yet the system "has a dark side,"
80 he acknowledges. Once individuals have made their
independent assessments and then a quorum has
reached agreement, fellows copy the quorum behavior.
The chances are low that the whole quorum will reach
the same wrong decision. But flukes can happen. In
85 most uses of a quorum, "it's going to make a decision
more accurate," he says, "but it also slightly increases
the incidence of these rare events when you get it really
spectacularly wrong."