27 February 2021 | New Scientist | 17THE rivers of the tropical Americas
hum and crackle with electric
fields generated by knifefish.
The fish use electric discharges to
search their murky surroundings
for food and to communicate with
mates. But new research suggests
these electric signals may also
be used to develop and maintain
a sophisticated social order.
In 2016, Till Raab at the
University of Tübingen in
Germany and his colleagues went
to study brown ghost knifefish
(Apteronotus leptorhynchus) in
their natural habitat in Colombia.
They were surprised to find 30 fish
communicating electrically in just
a 9-square-metre area.
The researchers knew “there
must be some kind of hierarchy
to avoid constant, repetitive
fighting”, says Raab, so they
brought some of the knifefish
back to the lab.
They paired 21 of the fish
in 37 different combinations in
tanks each containing a shelter
made of PVC pipe. The fish
competed for access to the
shelters, and the researchers
recorded behaviours using
infrared cameras and monitored
electric discharges using electrodes
in the tank.
The team found that fish denied
access to a shelter by a competitor
would commonly target the other
fish with electric “rises”: gradual
increases in discharge frequency
followed by a rapid fall to normal.
Initially, Raab thought these might
be submissive gestures, but what
he found “was way more complex”.
They seemed to be provocations,
triggering the dominant fish to
chase and bite the subordinate
agitator. While this didn’t allow
the loser to supplant the
dominant position of the other
fish, it appeared to provide a slight
increase in social status – one
that seemed to boost the chances
of success in future conflict
(bioRxiv, doi.org/fv58).
For instance, Raab recounts
one pairing where a subordinate
male repeatedly made rises
against a dominant female and
she eventually granted access
to her tube shelter.
In that way, the rises may
allow knifefish to improve their
standing while recognising andrespecting the social hierarchy,
which suggests the fish are capable
of surprisingly complex social
manoeuvring, says Raab.
Such a system might keep
violence to a minimum, allowing
competitors to come to a mutual
understanding. “They don’t
have to fight too much, but
everyone gets a little bit of
what they want,” says Raab.“The fact that rises occur
before attacks and not in
response to them is intriguing
to me,” says Rossana Perrone,
a neuroethologist at the Clemente
Estable Biological Research
Institute in Uruguay, adding that
other electric fish make submissive
signals following conflicts.
Perrone cautions that since
each knifefish took part in
multiple pairings during the
experiments, winner and loser
effects – where a win or loss
makes a repeat outcome more
likely in the next contest – might
influence some results.
Next, Raab wants to see
how these electrically charged
encounters alter relationships
across an entire group of knifefish.
It is possible that a knifefish can
estimate its chances of winning
a competition by watching other
fish, he thinks.
Raab says he and his colleagues
are only just beginning to get to
grips with the social convolutions
of knifefish. The fish also have
an entirely different set of
electric “chirps”, with which
they communicate. ❚“Two teeth looked like they
belonged to ancestors of
the woolly mammoth, but
the third was a surprise”Animal behaviour
Jake Buehler
PROF.DR.^ RÜ
DIGER
KRAHEPalaeontology
First million-year-
old DNA comes from
Siberian mammoths
FOR the first time, preserved DNA
has been recovered from animal
remains that are over a million years
old. The DNA belonged to two
mammoths that lived around
1.2 million years ago – and it paints
a complicated evolutionary picture.
Patrícia Pečnerová, now at
the University of Copenhagen
in Denmark, and her colleagues
extracted DNA from three
mammoth teeth discovered in
north-east Siberia. Two of the teeth,
from Krestovka and Adycha, are
respectively 1.1-1.2 and 1-1.
million years old. The third, from
Chukochya, is between 500,
and 800,000 years old.
The team found that the Adycha
and Chukochya teeth both looked
like they belonged to animals
ancestral to the famous woolly
mammoth (Mammuthus
primigenius), a species that
survived until 4000 years ago.
But the Krestovka tooth was
a surprise. Despite being about
the same age as the Adycha one,
its DNA was quite different. Themammoth it belonged to was a
member of a separate Eurasian
lineage that branched off from the
other Eurasian mammoths at least
1.78-2.66 million years ago.
Team member Love Dalén at
the Centre for Palaeogenetics in
Stockholm, Sweden, believes that
the Krestovka mammoths then
colonised North America, crossing
a land bridge to what is now Alaska
perhaps 1.5 million years ago.Later, sometime about 500,
years ago, “a small band of woolly
mammoths [also] crossed the
Bering land bridge and entered
North America”, says Dalén.
These woolly mammoths then
hybridised with the Krestovka-like
mammoths already living in North
America to give rise to a new
species, the Columbian mammoth
(Mammuthus columbi).
The DNA of this species had
previously been sequenced and
it appears to be a 50:50 mix of
“Krestovka” and woolly mammoths
(Nature, doi.org/fv67). ❚
Michael MarshallKnifefish use electric fields to
develop complex social hierarchy
A juvenile brown ghost
knifefish (Apteronotus
leptorhynchus)