R350 Current Biology 25, R345–R361, May 4, 2015 ©2015 Elsevier Ltd All rights reserved
*, Janne Valkonen
and Ossi Nokelainen
What is aposematism? The word
comes from the Greek apo (away) and
sema (sign) and describes a strategy
whereby animals warn predators about
their unproﬁtability. It consists of two
elements: a primary defence, such as
distinctive colours, odours or sounds,
that operates before the predator attacks;
and a secondary defence, be it chemical,
morphological or behavioural that make
prey unproﬁtable for predators. For
example, the bright colours of many
animals, such as poison frogs and wood
tiger moths, warn predators about their
toxic or distasteful chemical defences.
When predators encounter and attack
them in the wild, the prey will provoke
a bad experience that the predator
will learn to associate with the prey’s
colouration. As a result, predators will
start to avoid defended prey. After several
being conspicuous might be lower in
environments where alternative prey is
abundant, given that most predators
prefer familiar over unfamiliar food
objects. Aposematism has presumably
evolved several independent times, as
suggested by its occurrence in many
groups of animals.
What are the theoretical assumptions
about aposematism, and why is
variation in warning signals puzzling?
In order to work for the aposematic
animal, signals have to be clear and easy
to learn and remember for predators.
Warning signals thus should evolve
to be conspicuous and distinct. The
more individuals bearing the warning
signal, the more effective, easier to
learn and memorable the signal will be
for predators. Essentially, successful
aposematism relies on strength
in numbers. In fact, aposematism
could have been initially favoured in
aggregations of defended prey. Predators
presumably also learn more easily to
avoid one signal rather than several, and
that their learning depends on the rate
of unpleasant encounters with defended
generations of coevolution, aposematic
animals are often conspicuous and
distinctive (Figure 1), but not all
conspicuous animals are aposematic.
Likewise, not all aposematic species are
overtly conspicuous (Figure 1) and, thus,
aposematism should be considered as
a continuum of conspicuousness and
secondary defence rather than as an
unconditional anti-predator strategy.
How does aposematism evolve?
Although it has been studied since the
times of Wallace and Darwin, the origin
and evolution of aposematism is not
yet fully understood. Despite being
clear evidence of natural selection,
aposematism is somehow a paradoxical
adaptation. It is unclear how the ﬁrst
conspicuous individuals were able
to survive and reproduce such that
predators would encounter them often
enough to be able to learn about their
unproﬁtability. Conspicuousness, as
well as chemical defence, may have
increased gradually. Alternatively,
both defences could have been
selected for other reasons (e.g. sexual
selection). Moreover the initial cost of
Figure 1. Aposematic animals.
Top left: dyeing poison frog (Dendrobates tinctorius); top center: female of the wood tiger moth (Parasemia plantaginis); top right: coral snake
(Micrurus surinamensis); Bottom left: Brazil’s lancehead (Bothrops brazili) is not overtly conspicuous to us, but both the patterns and head shape
of some vipers can function as warning signals to predators; bottom center: ﬁrebug (Pyrrhocoris apterus); bottom right: common wasp (Vespula
vulgaris). (Photos: Bibiana Rojas; wasp: Tom Houslay).
Current Biology 25, R345–R361, May 4, 2015 ©2015 Elsevier Ltd All rights reserved R351
prey. Thus, selection is expected to
favour uniform warning signals and
suppress variation. Nevertheless,
warning signal variation is evident across
the natural world. The mechanisms
maintaining this puzzling variation are
still poorly understood, but it is thought
that this may arise for various reasons.
Some warning signals may serve
other purposes, such as intra-speciﬁc
signalling, or be a response to different
selective pressures which would trade-off
with the pressure exerted by predators.
For example, in the colour polymorphic
wood tiger moth (Parasemia plantaginis),
yellow males are generally better
defended from predators. In contrast,
under some circumstances, white males
are more successful at mating and have
higher ﬂying activity, which might help
them ﬁnd emerging females quicker or
compensate behaviourally for a less
efﬁcient anti-predator colouration. In
cold environments, increased black wing
pattern elements bring thermoregulatory
beneﬁts to these moths, but at the cost
of reduced warning coloration (white
or yellow). Recently, local predator
communities have also been shown
to aid in the maintenance of warning
signal variation. Hence, it is likely
that different properties of warning
colouration become costly or beneﬁcial in
changing environments. Finally, it cannot
be discarded that the variation is not
adaptive, but the product of hybridisation
Are warning signals honest?
According to the ‘handicap principle’,
signals that provide reliable information
about an individual’s quality should
be selected for. Such signals must
be costly for the signaller and, thus,
unaffordable for low-quality individuals.
Warning signals can be honest, if they are
reliable indicators of prey unproﬁtability.
Therefore, secondary defences may
vary as well, and this variation may by
no means be less relevant. For example,
in the strawberry poison frog (Oophaga
pumilio) great variation in toxicity among
populations is positively correlated
with conspicuousness. Likewise, in
the seven-spot ladybird (Coccinella
septempunctata), the amount of coloured
pigments correlates positively with the
level of chemical defences. At least for
the ladybirds, this correlation seems
to depend on resource availability.
This means that there can be costs
associated with the production of primary
or secondary defences, or both, that may
affect the effectiveness of aposematism.
Are there cheaters? Yes. When
predators learn to avoid a warning
signal that is shared among aposematic
individuals, organisms of other species
may mimic that signal and get protection
beneﬁts without investing in secondary
defences or predator education. In
Batesian mimicry, a palatable organism
is protected by its resemblance to an
unpalatable one. Thus, Batesian mimics
should not be considered aposematic,
because they lack a secondary defence.
The increase of Batesian mimics in a
population decreases the efﬁcacy of the
signal, because predators start to ignore
it as it becomes less reliable. Maybe the
most well known Batesian mimics are
hoverﬂies, which resemble wasps and
bees. In Müllerian mimicry, on the other
hand, two or more aposematic animals
have evolved a similar appearance
that is avoided by predators. Textbook
examples include the famous Heliconius
butterﬂies and dart poison frogs in the
Ranitomeya imitator complex. In fact,
mimicry is one of the ﬁrst and strongest
pieces of evidence for Darwinian natural
Where can I ﬁnd out more?
Alatalo, R.V., and Mappes, J. (1996). Tracking
the evolution of warning signals. Nature 382,
Cott, H.B. (1940). Adaptive Colouration in Animals.
Endler, J.A. (1991). Interactions between
predators and prey. In Behavioural Ecology.
An Evolutionary Approach, J.R. Krebs and
N.B. Davies, eds. (Cambridge University Press:
Guilford, T., and Dawkins, M.S. (1993). Are warning
colors handicaps? Evolution 47, 400–416.
Härlin, C., and Härlin, M. (2003). Towards a
historization of aposematism. Evol. Ecol. 17,
Mappes, J., Marples, N., and Endler, J.A. (2005). The
complex business of survival by aposematism.
Trends Ecol. Evol. 20, 598–603.
Poulton, E.B. (1890). The Colours of Animals: Their
Meaning and Use. (Kegan Paul, Trench, Trubner:
London), pp. 558–612.
Ruxton, G.D., Sherratt, T.N., and Speed, M.P. (2004).
Avoiding Attack: The Evolutionary Ecology of
Crypsis, Warning Signals and Mimicry. (Oxford
University Press: Oxford).
Stevens, M., and Ruxton, G.D. (2012). Linking the
evolution and form of warning coloration in
nature. Proc. Roy. Soc. Biol. Sci. 279, 417–426.
Centre of Excellence in Biological
Interactions, Department of Biology and
Environmental Science, University of
Department of Zoology,
University of Cambridge, UK.
Deaf white cats
and Stephen G. Lomber
What are deaf white cats? The term
‘deaf white cat’ is used to describe
domestic cats with completely white
fur (short-hair or long-hair) that
have no functional hearing; they
typically have blue eyes (Figure 1A).
It is estimated that in the overall
cat population, 5% are white, and
a subpopulation of these are blue
eyed. As early as 1868, Charles
Darwin noted in his book The
Variation of Animals and Plants under
Domestication that “white cats, if they
have blue eyes, are almost always
deaf”. This observation has been
substantiated in many subsequent
studies. Deafness identiﬁed in white
cats can be bilateral (both ears), or,
less frequently, unilateral (one ear)
with residual hearing in the opposite
What makes deaf white cats so
interesting? Any mammal can fail to
develop functional hearing. In many
species, such as domestic cats and
dogs, there is a higher incidence of
deafness in animals with a white coat.
The association between white coat
and deafness is greatest in white cats
with blue eyes. Animals bred for this
trait are a natural model for human
congenital deafness. Consequently,
deaf white cats are ideal for
studying the effects of hearing loss
on development and function of
the auditory system. Furthermore,
studies examining this animal model
have demonstrated the beneﬁcial
effects of hearing restoration with
cochlear prosthetics (implants).
These experiments were essential for
evidence-based recommendations on
the treatment of congenital deafness
in children. Today, approximately
400,000 hearing impaired individuals
world-wide beneﬁt from cochlear
implants in their daily life. Given the
present rate of implantation, the
number of people using cochlear
implants is projected to reach one
million in 2020. Overall, the cochlear
implant is the most successful