The EconomistNovember 30th 2019 Science & technology 692 challenge. Typically, evolutionary biolo-
gists assume that traits shared widely
across a related group are likely to have
evolved in an ancestral population, not re-
peatedly and separately in each lineage. Ms
Monk and her colleagues argue that cogni-
tive biases in the subject’s practitioners
have pushed them to look for fantastic ex-
planations for the evolution of same-sex
behaviours in a range of animals, rather
than considering the perhaps more reason-
able explanation for its persistence, that it
is a low-cost ancestral trait that has little
evolutionary reason to disappear.
Although the idea that same-sex behav-
iour has always been a norm is scientifical-
ly intriguing, the paper’s authors are also
making a broader point about human be-
ings’ pursuit of knowledge. Ms Monk says
that the paper’s authors met through a
Twitter account which promotes the work
of lgbtscientists. This was a serendipitous
encounter which gave them space to ex-
plore an idea that might have been dis-
missed at first sight in a more conventional
setting. The group includes people with a
range of sexual orientations, so naturally
they had an incentive to ask whether main-
stream evolutionary biology’s view of sex-
ual orientation is correct.
Their hypothesis still needs testing.
That will mean zoologists gathering more
observational data on sexual behaviour of
animals in the wild—and doing so with an
open mind. The authors themselves are
also mulling approaches involving com-
puter modelling, which might show that a
group of organisms behaving according to
their theory is capable of reaching the dis-
tribution of sexual behaviours seen in the
wild today. If their hypothesis is con-
firmed, it raises the question of which oth-
er facets of scientific knowledge might be
being obscured because the backgrounds
of practitioners in those fields do not lead
them to ask unconventional questions. Ms
Monk’s and her colleagues’ theory may yet
turn notions of the evolution of animal
sexual behaviour on their head. With a
broader array of minds focused on other
problems, other fields might follow, too. 7
D
espite theideas of Julia Monk and her
colleagues on the frequency and nor-
mality of same-sex mating behaviour
among animals (see previous article),
some species do work hard to attract the
opposite sex. That is why birdsong fills the
air during the spring and summer breeding
season. What has proved vexing to orni-
thologists is understanding why birds that
migrate to warmer climes in winter often
carry on singing even though there is no
breeding to be done and no need to defend
a territory. But a study just published in Be-
havioral Ecology and Sociobiologyby Abel
Souriau of Charles University in Prague
and Nicole Geberzahn of the University of
Paris, Nanterre, casts light on the matter.
Mr Souriau and Dr Geberzahn suggest that
winter is a period of practice for the sum-
mer performance to come.
Thrush nightingales are close kin to the
common nightingale familiar in western
Europe, but have a more easterly summer
range. Males are innovative songsters, fre-
quently plagiarising phrases from rivals
and integrating them into their own tunes.
They are also among the birds that carry on
singing after they have migrated to Africa
for the winter. So Mr Souriau and Dr Geber-
zahn decided to study the nature of their
African songs.
First, they themselves flew south, to a
part of Tanzania that previous work had
shown to be a place where populations
from two summer nesting grounds—one
in Poland, the other near Moscow—spent
their winters. They recorded thrush night-
ingales there and then, later, recorded
them in those summer grounds as well, to
provide a baseline for comparison. With
both sets of recordings in the bag, they ran
an analysis of what they had collected.
Bird song is divided by ornithologists
into a hierarchy of notes, syllables (com-
posites of notes), phrases (composites of
syllables) and songs (composites of
phrases). Mr Souriau and Dr Geberzahn
were interested, in particular, in the syllab-
ic and phraseological structure of songs.
In Europe the males sing according to a
particular template. The first part of a song
lets them show off their virtuosity. This ca-
denza varies from male to male and, for a
given male, from song to song. Neverthe-
less, males’ propensity to pinch snatches
from their neighbours means the songs
sung in one locality are distinguishable
from those in another. The second andthirdpartsaretwostereotypical phrases,
known as “castanet” and “rattling”, that
mark the end of a particular song.
As the researchers expected, the record-
ings they collected in Europe followed
these patterns. But those from Africa did
not. Around 90% of them were abnormal in
one way or another. Their structures were
extremely variable—chaotic, almost, with
castanet and rattling often absent. And
they often included syllables not found in
summer songs.
That the birds are singing at all in Africa
needs explanation, for singing consumes
energy which might be deployed for other
purposes. The explanation Mr Souriau and
Dr Geberzahn propose is that male thrush
nightingales employ the winter months as
a period of experimentation and rehearsal
for the summer. They can try out new sing-
ing techniques with little consequence,
and perhaps gain an edge early in the next
breeding season, before their neighbours
can plagiarise them.
Whether these winter rehearsals really
do result in songs that are better at repel-
ling rivals and luring mates remains to be
discovered. To find out would mean track-
ing a fair number of individuals over their
entire lives, to monitor both the evolution
of their songs and how their mating suc-
cess varies accordingly. But apparently ef-
fortless superiority is usually the result of a
lot of hidden hard work. In the case of
thrush nightingales, it seems quite plausi-
ble that the European former is a conse-
quence of the African latter. 7Male nightingales spend the winter practising their artBirdsongRehearsing for Berkeley Square
Practice makes perfect