www.sciencemag.org SCIENCE VOL 343 28 FEBRUARY 2014 975
pete fire ants (8). A fire ant, when drenched
with formic acid, has no apparent defense and
falls over dead (9). In contrast, the crazy ant,
when dabbed with fire ant venom, will use its
own weapon as its ultimate defense: It rinses
itself clean with its own formic acid. LeBrun
et al. show that 98% of crazy ants survive
fire ant venom; when they are experimentally
denied access to their formic acid, survival
drops to 48%. Moreover, when crazy ants
competed against eight Texas ant species,
each of which uses some form of chemical
defense, the red imported fire ant triggered
seven times more application of formic acid.
The authors hypothesize that formic acid
rinses are an adaptive trait in crazy ants,
evolved in its native range and employed
when the two old foes are reunited. But for-
mic acid rinses don’t seal the crazy ant’s
advantage. Forty years ago, Bhatkar et al. (9)
found that the lowly lawn ant Lasius neoniger
(which also produces formic acid) can groom
away a dose of fire ant poison; it just loses a
chemical war of attrition to the more popu-
lous colonies of the fire ant. Crazy ants can
achieve worker densities that are 100 times as
high as those of species in the invaded habitat
(2). Its antidote gives it the edge.
Biological control efforts often build on
the premise that successful invasive species
have escaped the parasites and predators of
their native ecosystem (10). LeBrun et al.
make a strong case that the red imported fire
ant owes its long ride in the American South
to its escape from a competitor. The crazy ant
may be the fourth in a sequence of ant species
that have hit the American Gulf Coast in the
past century, each replacing the preceding as
common and pernicious (2).
Given their ubiquity and impact (10),
invasive ant species are model ecological sys-
tems for studying the many factors that reg-
ulate populations. As successive invasions
reconstruct the population interactions of
a South American ant community in South
Texas, a logical next step is to search for the
crazy ant’s Achilles heel. One fruitful avenue
may lie in evolutionary games of rock-paper-
scissors (4), where round robins of toxins and
antidotes make the competitor of your com-
petitor your friend. A more basic puzzle in
our homogenizing world is why some—or
perhaps all—disruptive invasions eventually
crash (7, 11).
1. E. G. LeBrun et al., Science 343, 1014 (2014).
2. E. G. LeBrun, J. Abbott, L. E. Gilbert, Biol. Invasions 15,
4. B. Kerr, M. A. Riley, M. W. Feldman, B. J. M. Bohannan,
Nature 418, 171 (2002).
5. L. Chao, B. R. Levin, Proc. Natl. Acad. Sci. U.S.A. 78,
6. G. H. Orians, D. H. Janzen, Am. Nat. 108, 581 (1974).
7. W. R. Tschinkel, The Fire Ants (Harvard Univ. Press, Cambridge, 2006).
8. D. H. Feener Jr. et al., Ecology 89, 1824 (2008).
9. A. Bhatkar, W. Whitcomb, W. Buren, P. Callahan,
T. Carlysle, Environ. Entomol. 1, 274 (1972).
10. D. Simberloff, Invasive Species: What Everyone Needs
to Know (Oxford Univ. Press, Oxford, 2013).
11. M. Cooling, S. Hartley, D. A. Sim, P. J. Lester, Biol. Lett. 8,
The Surface Mobility of Glasses
Fei Chen,1 Chi-Hang Lam,2 Ophelia K. C. Tsui1,3
Surface diffusion on frozen polymer glasses
is influenced by the surface dynamics of the
The diffusion of atoms and mole- cules on a crystal surface plays an important role in myriad applications including thin-film deposition, sintering, and heterogeneous catalysis (1, 2).
Surface diffusion is frequently observed at
temperatures appreciably below the crystal’s
melting point, implying a role for enhanced
surface mobility in the process. However,
understanding the dynamics of surface diffusion in glasses is a research area still in
its infancy. On page 994 of this issue, Chai
et al. (3) present an experimental technique
that enables detailed quantification of the
near-surface mobility of glasses.
Although enhanced surface mobility was
found by Chai et al. as well as by others in
small-molecule and polymer glasses (4–7),
there is a noteworthy distinction between
these and the analogous observations in
crystals. In crystals, the substrate surface is
frequently much less mobile than the surface
atoms or molecules (see the figure, panel A).
In glasses, however, the first or several sur-
face monolayers are molten even below the
glass transition temperature Tg (where the
glass freezes), and the change in dynamics
from the surface is gradual (see the figure,
panels B and C). The reason for such a dif-
ference may be that the temperatures com-
monly used in studies of glass surfaces are
close to Tg. This proximity in temperature is
attributable to a broad interest in connect-
ing enhanced surface mobility, if present,
with the anomalous Tg reduction observed
in polymer films (8) and, more recently, fast
organic crystal growth and the formation of
ultrastable glasses (7).
Computer simulations have consistently
revealed the presence of a surface mobile
layer in glasses (9). Experimental verifica-
tion has been made only recently. In one
method, the relaxation time for the flattening
of nano-dimples created on a polymer sur-
face was measured (4). In another, polymer
films were doped with fluorescent molecules
whose dynamics are tied to those of the poly-
mer ( 6); the relaxation time and relative pop-
ulation of the component exhibiting faster
dynamics were measured. However, it is
generally not straightforward to relate these
relaxation times to familiar transport mea-
sures such as mobility or diffusivity. Typi-
cally, the mobility is determined by monitor-
1Department of Physics, Boston University, Boston, MA
02215, USA. 2Department of Applied Physics, Hong Kong
Polytechnic University, Hung Hom, Hong Kong. 3Division of
Materials Science and Engineering, Boston University, Boston, MA 02215, USA. E-mail: email@example.com; c.h.lam@
Moving along. Mobile adatoms on a crystal sur-
face (A) and their counterparts in the surface mobile
layer of an organic glass (B) and a polymer glass (C).
The mobile species are shown in red; the less mobile
bulk-like species are in blue.