40 Scientific American, August 2020
RALF GELFANDGetty Images(^1 ); MIKE HILLGetty Images(^2 )able colony members (the queen and “nurses” that
care for the brood) at risk. The nurses also took action,
moving the brood farther inside the nest and away
from the foragers once the fungus was detected in the
colony. The cues that the ants use to detect and rapid-
ly respond to fungus exposure are still unknown, but
this strategic social distancing was so effective that all
queens and most nurses from the study colonies were
still alive at the end of the experimental outbreaks.
Garden ants protect the most valuable members
of their colony, but some birds use a different strate-
gy, perhaps guided by the strength of their own
immune responses and resistance to infection. Max-
ine Zylberberg and her colleagues placed house finch-
es in three adjacent cages. Each central bird was
flanked on one side by a healthy finch and on the oth-
er side by a finch that appeared sick. (It got an injec-
tion that made it act lethargic.) By observing the
amount of time that the central bird spent on each
side of its cage, the researchers showed that finches
generally avoid birds that appear sick, but the degree
of avoidance varied with the power of their own
immune systems. Birds with higher bloodstream lev-
els of antibodies and of one other protein that may
signal broader immune activation showed less aver-
sion. But birds with weaker levels of immunity avoid-
ed sick birds most strongly, the investigators report-
ed in Biology Letters in 2013.
A similar pattern was detected in guppies affected
by a contagious and debilitating worm called Gyrodac-
tylus turnbulli. In work published in 2019 in Biology
Letters, Jessica Stephenson of the University of Pitts-
burgh placed individual guppies that did not yet have
worm infections in a central aquarium flanked by twotanks. One was empty, and one contained a group of
three guppies that represented potential contagion risk.
Many guppies preferred the side of the tank near oth-
er guppies, as expected for a social species. But some
male guppies strongly avoided the side of the tank near
the other fish, and these distancing guppies were later
shown to be highly susceptible to worm infections. It
makes sense that evolution would favor a strong ex -
pression of distancing behavior in those most at risk.THE TIES THAT BIND
sT raTegiC soCial disTanCing sometimes means main-
taining certain social ties even when they raise disease
risk. Mandrills, highly social primates with strikingly
colorful faces, illustrate this approach. This species can
be found in groups of tens to hundreds of individuals
in the tropical rain forests of equatorial Africa. Groups
typically have a mix of extended family members that
frequently groom one another; grooming improves
hygiene and cements social bonds. But they adjust
their grooming behaviors in particular ways to avoid
contagious group mates, Clémence Poirotte and his
colleagues noted in a report published in 2017 in Sci-
ence Advances. The scientists observed the daily
grooming interactions of free-ranging mandrills in a
park in Gabon and periodically collected fecal samples
to learn which animals were heavily infected with
intestinal parasites. Other mandrills actively avoided
grooming those individuals. The mandrills could de -
tect infection status based on smell alone: mandrills
presented with two bamboo stalks rubbed in feces
strongly avoided a stalk rubbed with droppings from
another mandrill that had lots of parasites.
And yet mandrills sometimes forgo social distanc-RELATIVE RISK:
Mandrills ( 1 ) groom
close relatives even
if they have para-
sites but avoid
other contagious
group mates. Band-
ed mongooses ( 2 ),
heavily dependent
on group coopera-
tion, groom both ill
and healthy animals
in their troop.
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