Scientific American - USA (2020-08)

August 2020, ScientificAmerican.com 39

ADITYA VISTARAKULA

Getty Images

(^1 );

GETTY IMAGES

(^2 )

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

© 2020 Scientific American © 2020 Scientific American