Directory UMM :Data Elmu:jurnal:P:PlantScience:ecology:21-40:

(1)

I

t has been more than a century since

Wallace1 _{proposed that conspicuous}

animal colour patterns could evolve to advertise the toxic or unpalatable nature of prey to visually hunting predators. In this time, aposematism (the name given by Poulton2 _{to describe this common}

association between bright coloration and unpalatability) has become a key system in which to study how receiver psychol-ogy can influence the evolution of signals3_.

In the first workshop* of its kind, re-searchers from a wide range of back-grounds gathered to discuss recent ad-vances in our understanding of warning signals and its associated mimicry. This meeting provided an opportunity to ex-plore how well perceptual mechanisms of predators can explain the evolution and design of aposematic signals.

Perhaps the most frequently asked question concerning aposematism is how warning coloration evolved. This is a well known paradox4–6_{because there is an}

ini-tial cost to an aposematic morph: not only will it be more detectable to predators, but also its rarity means that predators are less likely to have learnt the association between the colour and the unpalatability. Therefore, although the pattern might have a selective advantage once it is com-mon, at a low frequency, it will suffer from increased predation, and remaining cryp-tic will be strongly favoured. However, a pattern could become locally abundant in a small prey population, perhaps because of predator relaxation, allowing it to drift beyond the critical frequency required to increase the fitness of its bearers.

Jim Mallet (University College, London, UK) and Matheiu Joron (University of Montpellier, France) think that the pat-terns of warning coloration and mimicry observed in South American butterflies can be explained by this idea of ‘shifting balance’4,7_{, and that predator psychology}

is unlikely to be an important factor in large changes in colour pattern. However, receiver biases could aid this process; in particular, through neophobic responses shown by birds against novel prey items. Nicola Marples and Dave Kelly (University College, Dublin, Ireland) have compared rates of acceptance of novel food into the diets of some laboratory birds (domestic chicks, Gallus domesticus, and zebra finches, Taenopygia guttata) and of some

wild birds (territorial blackbirds, Turdus merula, and robins, Erithacus rubecula). Their results suggest that dietary conser-vatism is stronger in nature than we had previously thought. Although this could in part be due to age differences be-tween birds in each species, their study highlights the importance of testing the strength of selection pressures ob-served in the laboratory in wild predator populations8_.

An alternative psychological mecha-nism that could allow the evolution of warning patterns is a phenomenon known to psychologists as ‘peak shift’9,10_.

Tim Guilford (University of Oxford, UK) presented data showing that when birds are given a colour discrimination test, where one colour predicts unpalatable food and the other palatable, the colour that they avoid the most is not the one they have been trained with, but the one that is shifted along the colour spectrum away from the original discrimination. Although previous work has shown that predator biases against warning colours could be attributable to peak shift5,6_,

these data convincingly demonstrate that peak shift could be the mechanism be-hind the evolution by gradual change of aposematic signals away from their cryptic origins.

The role of predator psychology in warning signal design has been predomin-antly concerned with how insect colour patterns have evolved in the face of avian predators. It is now widely accepted that warning signals are designed to be detectable and easily learned but, given that the need to be memorable is vital to the success of a signal, it is curious that there has been no rigorous test of how memorable warning signals are. Mike Speed (Liverpool Hope University College, UK) and Nicola Marples are uncovering how visual signal components influence the memorability of the signal. Their experiments show that the visual attrib-utes that make signals easier to learn do not necessarily make a signal more memor-able. For example, prey gregariousness has been shown to increase the speed of aversion learning11_{, but gregariousness}

only appears to enhance the memory for a signal if it is cryptic. This result leads to the question, do different components of a visual signal perform different functions?

Daniel Osorio (University of Sussex, UK) has been investigating the roles of different visual components in colour patterns. In these experiments, chicks are trained to look for food in coloured paper

cones, the colour and pattern of which can be controlled and manipulated (unlike pre-vious experiments, which have used dyed food or real prey). This provides a power-ful technique for studying features of avian psychophysics. Experiments have already shown that chicks accurately remember hue but not spatial pattern contrast12_.

However, the addition of contrast to a colour pattern can aid the learning of a specific hue, so perhaps the function of black stripes and other patterns is to direct the attention of avian predators to the warning colour itself. These experi-ments probe the ways in which birds visu-alize and learn complex stimuli, and can be used to investigate further how features of a visual warning signal interact to make it easier to learn and remember.

We should not forget that warning signals can be aimed at predators other than birds, and that the final design might depend upon its combined effectiveness against multiple predators. This was emphasized by Juha Kauppinen (University of Jyväskylä, Finland), who showed that hawking dragonflies (Aeshna grandis) refrain from attacking wasps. By painting flies with wasp-like patterns, he demonstrated that the colour pattern itself was an effective deterrent. However, when do we ever consider the insect visual system and psychology as a selective force in the evolution of aposematism? Raphael de Cock (University of Antwerp, Belgium) also presented data from nonavian predators. In this case, toads (Bufo bufo) were shown to treat the flashes of glow worm larvae (Lampyris noctiluca) as if they were warning signals in the dark. Preoccupation with exploring the effects of predator psychology on signal design from an avian perspective might have excluded us from thinking more about the influence of alternative selective forces.

The meeting highlighted our recent progress in understanding the perceptual mechanisms underlying warning signal design, and yet, despite this, there was a strong feeling that we still have far to go before we can integrate this usefully into models of population dynamics or larger scale evolutionary processes. There is much new still to be learned about this oldest of evolutionary paradigms.

Acknowledgements

We thank J. Mallet, J. Mappes and N. Marples for helpful comments on this article, and to J. Mappes, R. Alatalo, L. Lindström and A. Lyytinen for organizing such a successful meeting. The meeting was supported by the Evolutionary Ecology Research Unit, the Dept of Biological and Environmental Science and the University of Jyväskylä, Finland.

NEWS & COMMENT

Aposematism: to be red or dead

(2)

NEWS & COMMENT

Dept of Psychology, Ridley Building, University of Newcastle, Newcastle upon Tyne, UK NE1 7RU

(candy.rowe@ncl.ac.uk)

Tim Guilford

Dept of Zoology, University of Oxford, South Parks Road,

Oxford, UK OX1 3PS (tim.guilford@zoo.ox.ac.uk)

References

1 Wallace, A.R. (1867) Proceedings of the Entomological Society of London, 4 March 1867, Ixxx–Ixxxi

2 Poulton, E.B. (1890) The Colours of Animals: Their Meaning and Use Especially Considered

in the Case of Insects, Keegan Paul, Trench, Trübner and Co

3 Guilford, T. (1992) Predator psychology and the evolution of prey colouration. In Natural Enemies: The Population Biology of Predators, Parasites and Diseases (Crawley, M.J., ed.), pp. 377–394, Blackwell

4 Mallet, J. and Joron, M. (1999) Evolution of diversity in warning colour and mimicry: polymorphisms, shifting balance, and speciation. Annu. Rev. Ecol. Syst. 30, 201–233

5 Lindström, L. et al. (1999) Can aposematic signals evolve by gradual change? Nature 397, 249–251

6 Gambarale, G. and Tullberg, B. (1996) Evidence for a peak-shift of predator generalization among aposematic prey. Proc. R. Soc. London Ser. B 263, 1329–1334

7 Wright, S. (1982) The shifting balance theory and macroevolution. Annu. Rev. Genet. 16, 1–19

8 Marples, N.M. et al. (1998) Response of wild birds to novel prey: evidence of dietary conservatism. Oikos 83, 161–165

9 Spence, K.W. (1937) The differential response to stimuli varying within a single dimension. Psychol. Rev. 44, 430–444

10 Guilford, T. and Dawkins, M.S. (1993) Receiver psychology and the design of animal signals. Trends Neurosci. 16, 430–436

11 Gagliardo, A. and Guilford, T. (1993) Why do aposematic prey live gregariously? Proc. R. Soc. London Ser. B 251, 69–74

12 Osorio, D. et al. (1999) Accurate memory for colour but not pattern contrast in chicks. Curr. Biol. 9, 199–202

W

hat could Cambridge University ex-pect to achieve by organizing a con-ference on conservation biology* and, moreover, making it a student confer-ence? The ability to party until a few hours before show time then present pol-ished talks that never ran over the time limit, illustrated with superior, and some-times dazzling, dexterity with Power-Point presentations, not only reminded the few invited senior scientists of incipi-ent senility but also of studincipi-ents’ superior skills – for which we would like to take credit for teaching. OK, so arenas for dis-plays of academic prowess are essential to size up one’s competitors for entry-level jobs, but there must be other motivations. One, obviously, is that unlike ecology-as-we-know-it, conservation biology is a crisis discipline. Within the lifespans of students, humanity will have decided how much – if any – of the world’s tropical forests it will retain, whether to alter global climate at a geologically unprec-edented rate and whether to eliminate a fraction of life on Earth not seen since the event that eliminated the dinosaurs. Did the students address these challenges?

Generally, papers reflected the inter-ests of academic advisors. Only half included science that had more than tenu-ous connections to practical issues. Ecolo-gists should not flatter themselves that all they do makes a difference. The defining meeting on conservation biology at the University of Michigan in 1985, which produced Soulé’s book Conservation

Biology1 _{in 1986, had several panels of}

managers who wondered whether any ecologist makes a difference. (‘Only’, one said, ‘if we give him a spade and have him dig holes for fence posts.’) Although con-servation science has grown since then, tying science to practical issues is diffi-cult. If so many students spent no more than a sentence or two thinking about what its consequences for management or policy would be, it is surely because academics determine their research. How many students have met a manager or a policy maker? For those that answered ‘no’, one assumes that their academic advisors haven’t either.

The geographical spread – students came from nearly 30 countries – provided an interesting contrast in this regard. Generally, those from biodiversity-rich countries outside Europe and the USA did address immediate conservation needs. For them, working with managers is unavoidable. For example, Abi Tamim (Wildlife Institute of India, Dehradun, Uttar Pradesh, India) had the task of radio-tracking tigers in a park of dry forest, the habitat that constitutes nearly half of the species’ range in India. In contrast to wet forest, range sizes in dry forests are huge – on the order of hundreds of square kilo-meters and nearly the size of the national park in which he worked. Clearly, some reserves are too small for viable popu-lations and those who protect tigers must rethink their strategy. Tamim’s colleague, Tanushree Biswas (Wildlife Institute of India), showed that active habitat man-agement is converting natural grasslands to plantation areas to benefit the Indian rhino, but to the detriment of other

species. Barend Erasmus (University of Pretoria, South Africa) modelled the ranges of well known taxa in South Africa and pre-dicted their future ranges under a probable scenario of climate change. Most species’ ranges will be smaller under warmer condi-tions and they will also need to move to places where there are few parks.

Finally, Joel Musaasizi (Institute of Tropical Forest Conservation, Bwindi Im-penetrable Forest, Uganda) studied prob-lems in Bwindi Impenetrable Forest. It holds half the world’s mountain gorillas, is surrounded by densely populated areas and has crops planted to the edge of the forest. Baboons and bush pigs from the forest raid the crops; therefore, Musaasizi’s mission was to find ways to reduce their impact. These species only venture about 150 meters from the forest but bushes grow up outside the forest providing corridors for raids. The farmers ask why the animals feed, while the people starve? They view the animals as the prop-erty of foreigners and researchers, creat-ing a tension further exacerbated by farm-ers being prohibited to enter the forest to hunt. Farmers don’t cooperate on remov-ing the bushes because they don’t own the land and thus will not necessarily reap the benefit of such work. They could plant buffer crops, such as tea, that will not be raided. And, if it were organized well, they could benefit from the revenue sharing that comes from ecotourism. We cannot expect the world’s poorest to take the bur-den for our passion for preserving gorillas, thus Musaasizi’s problem is acute and also complex. By demonstrating the inte-gration of ecology with the realities of soci-etal constraints Musaasizi won the confer-ence prize for practical conservation.

A different contrast involves the spe-cies and systems studied. Globally threat-ened species are not distributed uniformly and there was evidence of students seeking

Directory UMM :Data Elmu:jurnal:P:PlantScience:ecology:21-40:

I

NEWS & COMMENT

Aposematism: to be red or dead

NEWS & COMMENT

W

Conservation connections