SCIENCE science.org 10 JUNE 2022 • VOL 376 ISSUE 6598 1181
the transcription of these genes,
allowing them to be expressed.
Increased understanding of the
mechanism is important because
brown fat thermogenesis is
crucial not only for maintaining
core body temperature but also
for energy balance. Some studies
have explored the potential of
brown fat in treating obesity. —DJ
Life Metab. 10.1093/
lifemeta/loac003 (2022).
NEUROSCIENCE
Flexible learning in the
cerebellum
The cerebellum is known to
participate in the acquisi-
tion of motor skills and motor
learning, but it also processes
reward-related information.
The interaction between these
pathways and their contribution
to learning is not understood.
Sendhilnathan et al. compared
the activity of cerebellar Purkinje
cells while monkeys were actively
learning new visuomotor associa-
tions and while they performed
an already familiar visuomotor
association task. These cells pro-
cess two different signals when
monkeys learn a new arbitrary
stimulus–response association.
One is a previously described
reinforcement learning error
signal, and the second, described
here, is a signal that describes the
state of learning. —PRS
J. Neurosci. 42 , 3847 (2022).VOLCANOLOGY
Eruption forerunners
Volcanic eruptions often have
different sorts of precursors
before the main event that range
from a change in the number of
small earthquakes to the compo-
sition of gases from fumaroles.
Flóvenz et al. show that pre-
eruptive uplift cycles occurred in
a nearby geothermal field before
the March 2021 Fagradalsfjall
eruption in Iceland. The changes
were likely due to fluid migra-
tion from the magma body
and occurred up to a year and
a half before the eruption. The
observations help provide differ-
ent constraints on parts of the
complex process that can lead toa volcanic eruption. —BG
Nat. Geosci. 15 , 397 (2022).NONLINEAR OPTICS
Relaxing constraints for
phase matching
The interactions of light in non-
linear optical materials produce
a number of effects, including
lasing, frequency conversion,
high-harmonic generation, and
spontaneous downconversion.
Such effects find application
in microscopy, optical com-
munication networks, and
quantum optics. Underlying
these effects is phase matching
of the interacting light beams,
which requires strict experi-
mental conditions to be met,
often leading to cumbersome
setups. Gagnon et al. show
that metamaterials engineered
with a low refractive index can
relax the constraints for phase
matching. Their demonstration
of direction-independent four-
wave mixing in a nanophotonic
structure illustrates how such
low-index materials can be usedto miniaturize nonlinear optical
devices. — ISO
Phys. Rev. Lett. 128 , 203902 (2022).CHEMISTRY
Molecules work on
self-control
Oscillation of a chemical system
between two or more states
underlies many important pro-
cesses in biology and may be a
useful model for understanding
how early cells formed and func-
tioned. Howlett et al. designed a
model supramolecular system
that achieves a rudimentary
metabolism: oscillatory self-
replication of a metastable
micelle powered by a chemical
fuel. The micelle contains a disul-
fide that can cross-react with
other components, leading to
dispersal and eventual regenera-
tion. The addition of a dye that is
incorporated into the transient
micelle provides a visual readout
of the supramolecular oscilla-
tions. —MAF
Nat. Chem. 10.1038/
s41557-022-00949-6 (2022).EVOLUTIONCetacean skulls from
land to ocean
C
etaceans have populated the ocean to
become charismatic subjects of human
legend, but it has taken 50 million years for
whales and dolphins to evolve from terres-
trial mammals into an array of specialized
aquatic species. Their shift to water required
enormous physiological and physical adaptations.
Specializations such as alterations in the posterior
skull to accommodate breathing at the water’s
surface, baleen for filter feeding, and biosonar for
echolocation are hallmarks of cetacean adaptations.
Coombs et al. analyzed in three dimensions the
shapes of skulls from hundreds of living and extinct
cetacean species. Their results show that rapid
evolution during the Eocene reshaped the front of
the skull. Further diversification of skull morphology
continued as cetaceans evolved to accommodate
new diets and feeding methods, as well as echoloca-
tion. —PJH Curr. Biol. 32 , 2233 (2022).Since the Eocene, when their ancestors re-entered
the sea, skulls of cetaceans evolved rapidly to adapt to
a subaquatic lifestyle.