Current Biology 25, R345–R361, May 4, 2015 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 or drift.
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 selection.
Where can I find out more?
Alatalo, R.V., and Mappes, J. (1996). Tracking the evolution of warning signals. Nature 382, 708–710.
Cott, H.B. (1940). Adaptive Colouration in Animals. (Methuen, London).
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, 197–212.
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. 1Centre of Excellence in Biological
Interactions, Department of Biology and
Environmental Science, University of
Jyväskylä, Finland. 2Department of Zoology,
University of Cambridge, UK. *E-mail: firstname.lastname@example.orgDeaf white cats
Andrej Kral1 and Stephen G. Lomber2
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 ear.
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).