The Economist April 9th 2022 Science & technology 73W
henthePrussiannaturalistAlex
ander von Humboldt arrived in
South America in 1799, the colours aston
ished him. “Look at the blossoms, the
birds,” he wrote. “Even the crayfish are
blue and yellow.”
In the intervening centuries, Hum
boldt’s musings have morphed into an
informal, if controversial, hypothesis
about the world’s living things: that
organisms in equatorial climes are more
colourful than those nearer the poles.
In a new paper in Nature Ecology &
Evolution, Chris Cooney, an evolutionary
biologist at the University of Sheffield,
and colleagues at Britain’s Natural Histo
ry Museum collection in Tring, offer
what they call the most comprehensive
examination of this hypothesis to date,
demonstrating that it does indeed apply
for an order of birds known as passer
ines, a family of songbirds which in
cludes the blue tit and the robin.
Over several years, Dr Cooney’s team
photographed 4,527 birds from the mu
seum’s archives—representing roughly
half the total bird diversity on Earth. As
passerine birds are sensitive to ultravio
let (uv) light as well as visible, they see
more colours than humans. The re
searchers therefore took pictures under
uvlight, to get true bird’seye views.
A machinelearning algorithm was
then let loose on the images, to identify
the colours of the plumage (in values of
red, green, blue and ultraviolet) at hun
dreds of points on each specimen. The
total number of colours found on each
bird was then subsequently mapped onto
the geographic distribution of their
parent species.
The researchers used this data to
derive a measure for how colourful dif
ferent regions of the world were. The key
factor here was the diversity of colours
on individual birds. The paradise tanager
(Tangara chilensis pictured above), for
example, which struts through the Ama
zon rainforest in a green domino mask,
an azure bib fading into a skyblue apron
and a black cape splashed with all the
colours of a sunset, is a sign of a col
ourful region. The monochromatic
dunnock (Prunella modulari), meanwhile,
which flies across Europe with its brown
wings streaked with darker brown, mod
ulated by brown dots scattered across a
dark grey face, was surefire evidence of a
less colourful region.
Equatorial birds, the researchersconcluded,wereindeedmore colourful
than their cousins in temperate regions,
with colourfulness declining with lat
itude. The birds sampled at the equator
had, on average, between 90 and 100
different colours, while birds at 60 de
grees latitude had closer to 70 colours.
Such trends related to latitude are not
uncommon in ecology, with species
diversity also having been shown to peak
at the equator.
Over the decades, numerous explana
tions have been tendered for tropical
colourfulness. Some have speculated
that warmer climates make more re
sources available to their inhabitants,
allowing them to spend energy on devel
oping adornments. Others have suggest
ed that the bright colours may arise from
chemical compounds in their diet, or
else allow for breeding pairs to spot one
another in the darker environment of a
tropical forest floor.
The authors of the latest study do not
make any definitive conclusions on the
matter, but their analysis shows that
colourfulness is strongly associated with
the resources available to birds in an
environment, as well as its diversity.
Whether such results can be extrapolated
to other families of birds—or, indeed,
other classes of animal—remains to be
seen. After years spent photographing
avian specimens from a variety of angles,
Dr Cooney has high hopes that a similar
study can be performed on butterflies.
Crucially, they can be made to lie flat.EvolutionBird-swatching
Songbirds get more colourful the closer they live to the equatorWho’s a pretty bird then?had noticed from their own previous work
that people from different cultures de
scribed odours differently. They also knew
from past experiments by other research
ers that culture was important in deter
mining which sorts of faces people found
beautiful. Thus, they expected to see a sim
ilar phenomenon with smells.
To study how scent and culture relate,
Dr Arshamian and Dr Majid presented nine
different groups of people with ten odours.
These varied from pleasantsmelling va
nilla extract to isovaleric acid, the chemi
cal responsible for the revolting scent of
stinky socks. More intermediate odours,
which the team thought might split opin
ions, included octanoic acid with its mod
erately rancid smell; the sweetsmelling
eugenol, which comes from cloves; and
octenol, a musty and earthy scent found in
many mushrooms.
The cultures doing the smelling varied
widely too. They included huntergatherer
communities along the coast of Mexico,
subsistence farmers living in the high
lands of Ecuador, shoreline foragers, swid
den horticulturalists living in the tropical
rainforests of Malaysia, and city folk from
Thailand and Mexico City. All 235 partici
pants were asked to rank odours according
to pleasantness. The team compared their
results to earlier work on New Yorkers who
had been exposed to the same scents.
Writing in Current Biology this week,
the researchers noted that pleasantness
rankings of the odours were remarkably
consistent regardless of where people
came from. The smell of isovaleric acid
was reviled by the vast majority of the par
ticipants, only eight giving it a score of 1 to
3 on the pleasantness scale (where 1 was ve
ry pleasant and 10 was very unpleasant). On
the other hand, more than 190 people gave
vanilla extract a score of 1 to 3 and a tiny mi
nority, only 12 people, found it revolting
enough to rate 8 to 10. Overall, the chemical
composition of the odourants that the re
searchers presented explained 41% of the
reactions that participants had. In con
trast, cultural upbringing accounted for
just 6% of the results. Dr Arshamian and Dr
Majid point out that this is very different
from how visual perception of faces
works—in that case a person’s culture ac
counts for up to 50% of the explanation for
which faces they find beautiful.
Even so, while culture did not shape
perceptions of odours in the way that it is
known to shape perceptions of faces, the
researchers did find an “eye of the behold
er” effect. Randomness, which Dr Arsha
mian and Dr Majid suggest has to be com
ing from personal preferences learned
from outside individual culture, account
ed for 54% of the variance in which smells
people liked. “Olfactory bulb of the behold
er” does not slip off the tongue so easilybut
it too appears to be a real phenomenon.n