August 2020, ScientificAmerican.com 39
ADITYA VISTARAKULA
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did not choose to den alone, the scientists suspected:
they were being shunned. To confirm their hunch, the
investigators placed several lobsters in aquarium tanks,
allowing healthy crustaceans to choose an empty arti-
ficial den or one occupied by either a healthy or a dis-
eased compatriot. In a 2006 article in Nature, the sci-
entists reported that when disease was absent, healthy
lobsters preferred being social and chose dens with a
healthy lobster over empty ones. And lobsters strong-
ly avoided the dens containing virus-infected lobsters,
even though it meant they had to go it alone.
In a follow-up study published in 2013 in Marine
Ecology Progress Series,^ Behringer and his colleague
Joshua Anderson showed that healthy lobsters spot
afflicted ones by using a sniff test. It turns out that
infected lobsters have chemicals in their urine that
serve as a danger signal to healthy group mates. When
scientists used Krazy Glue to block the urine-releas-
ing organs of infected lobsters, healthy animals no
longer avoided the sick ones.
When lobsters detect an afflicted animal, they are
willing to take considerable risks to stay disease-free.
When Mark Butler of Old Dominion University and
his colleagues tethered a sick lobster to the home den
of healthy lobsters in the Florida Keys, they saw that
healthy animals often abandoned safe havens for open
waters, where they were at much higher risk of get-
ting eaten. When Butler’s team repeated the experi-
ment with a tethered healthy lobster, there was no
mass exodus. In their research, published in 2015 in
PLOS One, the scientists used mathematical models
to show that avoidance, while not without costs, pre-
vents viral outbreaks that would otherwise devastate
lobster populations.
PROTECT THE VALUABLE AND VULNERABLE
lobsTers are far from the only animals that have
found the benefits of social distancing sometimes out-
weigh the costs. Some other creatures, in fact, have
developed ways to boost the payoff by practicing social
distancing strategically, in ways that protect the most
valuable or vulnerable in their group. The most im -
pressive examples occur in social insects, where dif-
ferent members of a colony have distinct roles that
affect the colony’s survival.
In work led by Nathalie Stroeymeyt of the Univer-
sity of Bristol in England and published in 2018 in
the journal Science, researchers used tiny digital tags
to track the movements of common garden ant colo-
nies during an outbreak of a lethal fungus, Metarhi-
zium brunneum. The spores of this fungus are passed
from ant to ant through physical contact; it takes one
to two days for the spores to penetrate the ant’s body
and cause sickness, which is often fatal. The delay
between exposure and sickness allowed Stroeymeyt
and her colleagues to see whether ants changed their
social behaviors in the 24 hours after they first detect-
ed fungal spores in their colony but before fungus-
exposed ants showed signs of sickness.
To measure how ants respond when disease first
invades their colony, the researchers applied fungal
spores directly to a subset of the forager ants that reg-
ularly leave the colony. The foragers are most likely to
inadvertently encounter fungal spores while out
searching for food, so this approach mimicked the nat-
ural way this fungus would be introduced. The behav-
ioral responses of ants in 11 fungus-treated colonies
were then compared with the same number of control
colonies, where foragers were dabbed with a harmless
sterile solution. Ants in fungus-exposed colonies start-
ed rapid and strategic social distancing after treatment.
Within 24 hours those forager ants self-isolated by
spending more time away from the colony compared
with control-treated foragers.
Healthy ants in fungus-treated colonies also strong-
ly reduced their social interactions, but the way they
did so depended on their roles. Uninfected foragers,
which interact frequently with other foragers that
might carry disease, kept their distance from the col-
ony when disease was present. This prevents them
from inadvertently putting the reproductively valu-
STRATEGIC
DISTANCE:
Garden ants ( 1 )
stay away from
their colony when
exposed to a
fungus. House
finches ( 2 ) avoid
other birds that
appear ill.
1
2
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