FSBio-Hannover

[Noiraime]

Being someone else – the strategy of the Danaid Eggfly (Hypolimnas misippus) optimising chances of survival



Henry W. Bates, a British scientist, was the first to recognise on his journeys between 1849 and 1860 the phenomenon that individuals of the Danaid Eggfly (Hypolimnas missipus) look similar to individuals of another species – the Plain Tiger (Danaus chrysippus). The Plain Tiger is a common butterfly living in Africa. It is inedible and shows a warning colour to discourage predators (Smith, 1973). The Danaid Eggfly is, in contrast, palatable but it shows the same warning colour and behaviour patterns as the Plain Tiger. After reflecting on some possibilities, Bates found an explanation for this phenomenon which is now called Batesian Mimicry. Having published his thoughts, there were many critical scientists who were entertaining some doubts that that was really the truth. Actually Charles Darwin thought that this phenomenon was an accidental similarity (Salvato, 1997).
Since then Batesian mimicry has been a subject of debate and countless papers, but various field and laboratory experiments as well as observations had been performed to demonstrate the efficacy of mimicry as a defensive coloration as part of evolutionary adaptions.
Batesian mimicry, as a special form of mimicry, consists of at least two different species which have similar colour and behaviour patterns. It involves an unpalatable or protected species which often shows warning colours (the model) and a palatatble, unprotected species imitating the model in colour and behaviour patterns (the mimic) (Salvato, 1997). The model becomes undesirable to predators such as birds, mammals and amphibians by eating poisonous plants in their larval stadium or by synthesising noxious compounds themselves. The models´ warning colours, a part of defensive coloration, are usually bright and conspicuous. Some experiments have shown that bright and eye-catching warning colours (such as red, yellow and orange) are easier to learn by predators than unobtrusive patterns with green and brown – bright colours are adaptive colours (Mallet & Fowler/ Wallace in Mallet 2001). Predators that try to eat an unpalatable, bright coloured individual will learn to avoid other individuals with the same colour pattern, no matter if they are unpalatable or not. Mimics profit by the inedibility of their models. Regarding this point, Batesian mimicry is a kind of parasitism. The mimic hurts the model by using its technique to warn predators when the frequency of mimics becomes high because that will enhance the attacks on models and mimics. Batesian mimicry has a frequency-dependent advantage (Mallet & Fowler 2006). This probably explains the polymorphisms which exist in most Batesian mimicries. The Plain Tiger (Danaus chrysippus) shows four different forms and all these forms are mimicked by females of the Danaid Eggfly (Hypolimnas misippus) (Smith 1973). If one form increases it is better to have a different, bright and warning colour to decrease chances of being untroubled by predator attacks.
Indices for the efficacy of mimicry have been demonstrated by different laboratory and field experiments. In 1950 Jane van Zandt Brower and Lincoln Brower performed a laboratory experiment with blue jays and monarch butterflies to circumstantiate the mimicry theory. They fed the jays with unpalatable monarchs. The jays became sick and they learnt to avoid monarchs. Having learnt that, the jays also avoided mimics of the monarchs which were palatable.
A second experiment performed by Jeffords, Waldbauer and Sternberg in the field demonstrates the success of Batesian mimicry. They painted some palatable moths to resemble unpalatable species and some moths to resemble palatable species and let them fly. After a time they were recaptured by using pheromones. The recaptured rate of mimics of the palatable species was a mere 40% of that of mimics of unpalatable species. This experiment showed that Batesian mimicry is a very strong selection pressure (Mallet & Fowler 2006).
In the 1960s and 1970s when Hamiltons´ discovery of kin selection was generally accepted it was taken for granted that this selection pressure caused the evolution of mimicry. Warning colour as well as unpalatability were seen as a kind of altruism which benefits the group and not the individual. The bright and conspicuous warning coloured individual with noxious compounds gives a capacious service to its population but causes a high cost to itself. It teaches the predator to avoid its special colour pattern. The population has a large advantage but the individual pays with its life (Mallet & Fowler 2006).
But if altruism involves only benefit for the group and not for the individual, it could not evolve. At that point, Fisher suggested in 1930 that unpalatability and warning colour could evolve because of the advantages to the individuals´ siblings which was the first formulation of Hamiltons´ kin selection. All caterpillars in a group, descended from one female, have a degree of relationship of 50% . If one of these individuals gets eaten by a predator and teaches the predator to avoid its colour pattern this way its siblings have a larger chance of survival. In fact the share of the individuals´ genes will increase in the population gene pool although it dies because protecting its siblings (which have 50% identical genes) will save the genes in the pool.
The theory that unpalatability and warning colour evolution is part of kin selection was accepted since the 1980s. But now there are many indices that warning colour and unpalatability have evolved in rather different ways. Regarding unpalatability there are two components of costs: The metabolic energy to synthesise and sequester the poisonous compounds and the danger of being eaten as a result of being conspicuous. On the other hand unpalatability could be an enormous benefit to the individual if the noxious compounds are sequestered in tissues which are prone to attack and not damage (Mallet & Fowler 2006). Perhaps it is cheaper to sequester these compounds than detoxificating them. If the benefit of the individual outweighs the costs, unpalatability is not true altruism and it could evolve via individual selection. The question why caterpillars and adults nevertheless live in groups could be explained by selfish advantage: If a group of individuals could satiate predators in their territories it pays them to live together in groups. The probability of being eaten decreases for every individual the larger the group is.
Warning coloration is seen as a kind of frequency-dependent selection. When the warning colour pattern arises first, it has a large cost: It is conspicuous and predators do not know it. An individual with that pattern has an enormous probability of being eaten by a predator – the new pattern could be seen as a kind of altruism. But if the new pattern becomes commoner and predators have learnt it, it pays to have it. The predators remember the warning colour and avoid individuals with that new pattern. In this case it is not an altruistic component but a selfish one. Novel colour patterns could evolve when a mutation in a small and local group arises, provides local succes and is spread out to other groups.
Having now learnt how unpalatability and warning colour could evolve, it is just a small step to understand the evolution of mimicry. In fact, there are the same mechanisms of selection: The mutation of a palatable individual, which now looks similar to a protected unpalatable species, offers the individual a large advantage. Its probability of being eaten by predators is minimised and the mutated genes which cause the novel colour pattern will be transmitted to the individuals´ offspring – which also will be protected.
All these facts and arguments show that Batesian mimicry, as a special form of mimicry, is a good example of evolution by natural selection and offers a large possibility to change and optimise chances of survival. Although Bates only explained this phenomenon by regarding butterflies, there are many other examples of mimicry in various versions. It is just one possibility of species to ensure its own survival, based on Darwins´ theory about the survival of the fittest. It has arisen independently in so many different families and species that it seems to be one of the most important developments to assure the survival of species. There are still many questions which need to be answered, such as the genetic evolution and mechanism of mimicry, female polymorphism in mimic species, race formation and speciation and so on. After knowing these facts about mimicry, can we now be certain that everything we see is actually the truth? Be observant and look a second time before being sure!

Lisa Endres, December 2006

References:

- Mallet, James; Fowler,Kevin: “Warning colour and mimicry” 2006
- Salvato, Mark: “Most Spectacular Batesian Mimicry” April 15, 1997, University of Florida, Book of Insect Records, Chapter 28
-Iziko museum of Cape Town: “Insect mimicry” , 2004
- Smith, D.A.S.: “Batesian mimicry between Danaus chrysippus and Hypolimnas misippus (Lepidoptera) in Tanzania” , March 3, 1973, Nature 242
-Mallet, James: “Mimicry:An interface between psychology and evolution” , Juli 31, 2001, PNAS, vol. 98, no. 16
-http://en.wikipedia.org/w/index.php?title=Hypolimnas_misippus
-http://en.wikipedia.org/w/index.php?title=Mimic