By Paul Donovan
The animal world around us is full of impersonators who would have you believe they are something they are not. Harmless species do this, by playing on the strength and reputation of dangerous species, for their own safety or gain with a behaviour called Batesian mimicry. Mimicry comes from a Greek word for mime or copy.
The British naturalist, Alfred Russel Wallace (1823-1933) was fascinated by mimicry, and was convinced that mimicry and camouflage helped with natural selection, as an insect which blends in with its background is less likely to be eaten by a predator. As Wallace put it…. “Nature selects for successful traits’ ‘. The genes responsible for a particular trait which has become successful, will be passed onto future off-spring. The same reasoning can also be said of snakes which mimic one another.
In evolutionary biology, mimicry is the similarity of one organism to that of another that evolved to a selective behaviour which favoured that animal, such as appearance, behaviour, sound, or scent. This can be played out to serve concealment, or protection against predators. But the two organisms which are playing out this ruse, may be quite different from genus, species, class, family or even phylum classification. One of the most astonishing of which, is where the larva of a moth found in South America and Central America, called Hemeroplanes Triptolemus mimics a snake.
I am sure just about everyone is familiar with the mnemonic rhyme; ‘Red to yellow kill a fellow, Red to black venom lack’ (or one of the many variances of it). This is a commonly used rhyme when trying to distinguish between the venomous North American Coral snakes Micrurus, and Micruroides, and the harmless Milk snakes and Kingsnakes Lampropeltis. It is, however, known that this rhyme only applies to North American Coral snakes, and not the South America variants, as these show different patterns, with the red bands touching the black bands. Even with the North American species, the rhyme should not be taken as cast in stone, as a number of Coral snakes can display variations in colouration and patterning from one another, called polymorphism.
Mimicry is found throughout the world in all species of organisms. Out here in Africa I see it in a lot of insects, and some snakes, one of which is the Rhombic Egg Eating Snake, Dasypeltis scabra. This harmless African snake eats only eggs, so has no teeth to use for self defence. This leaves it in a vulnerable position when it comes to defending itself. But all is not lost for this harmless snake, for it mimics the venomous Rhombic Night Adder, Causus rhombeatus, and to some degree the Saw-Scaled Viper, Echis carinatus. It not only mimics appearance, but puts up a very convincing display when threatened; enough to make even the most persistent predator have second thoughts about approaching it, and the eager herpetologist to double check before grabbing it.
Although it is commonly said that the egg eater can be distinguished from the Night Adder by the slight variations in the V-shaped markings on the head, and the shape of the pupils (vertical in the egg eater, and round in the Night Adder), in some regions of Africa, the egg eater may have round pupils, so the latter does not always apply.
To the extreme
In 1977, an extreme case of mimicry was discovered in the Southern African Bushveld Lizard, Heliobolus lugubris (formerly Eremias lugubris). While adults are brown with yellow stripes, juveniles are jet black with creamy broken stripes. While it is not unusual for juveniles and adults to be patterned differently, what makes the juvenile patterning of this lizard so special, is why it is patterned like that.
Observations of juveniles showed that they were similar in size and colour to Ground Beetles of the genus Anthia, in particular Anthia thoracica. But the similarity does not stop there, for the beetles also mimic the way in which the beetles move. But why should a lizard mimic a beetle? Well, these beetles have robust mandibles and squirt acids from the abdomen in defence. Many predators give these beetles a wide berth for that reason. This is the most highly specialised form of mimicry in the reptile world, and is the only known case of a terrestrial vertebrate mimicking an invertebrate.
Mimicry in amphibians
As so many amphibians have poison glands, or poisonous skin secretions, one would have assumed that mimicry amongst this group would be widespread. In fact, mimicry is not well practiced amongst amphibians. One of the reasons for this, may be the type of predators which prey on them or, the fact that they are so well camouflaged and mainly nocturnal, they don’t need to practice mimicry.
If you look at the range of predators who feed on snakes, they are predominantly birds. If you look for the same for amphibians, they are predominantly (although not exclusively) snakes. Compare the colour perception of birds against those of snakes, and you will see birds are far superior. Birds also have the ability to learn and remember. An experience of a bird interacting with something which tastes bad and advertises to leave it alone with bright red colouration is quickly learnt, and remembered for next time. The same does not apply to snakes, they see the world in a different aspect, and lack the ability to learn from experience. That being said, I have seen some amphibian-eating reptiles avoiding certain frogs. It may be, and this is just my thought, the poison in the skin of these frogs gives off a different type of smell which the snake may be able to relate to. I will expand on this in a moment.
So, where is mimicry seen in the amphibian world? Possibly the greatest degree are those species mimicking the Poison Arrow Frogs Dendrobates such as Lithodytes lineatus and Eleutherodactylus gagei.
One of the most interesting cases of mimicry amongst this group, was discovered in 1999 during an expedition to Peru. During the expedition, a new species of Poison Arrow frog Ranitomeya imitator (formerly Dendrobates imitator) was discovered. It was eventually given the name ‘imitator’ because it was found to mimic another species of Poison Arrow frog, Ranitomeya variabilis (formerly Dendrobates variabilis). These two species are sympatric, that is they share the same geographical areas. This suggests that different populations of R. imitator have evolved to look like the members of the populations of other species that they are sympatric with. But why this should be, is still not known.
Not without its faults
Outwardly, Batesian mimicry would seem a positive evolutionary behaviour. If you can mimic something which is recognised by predators as being dangerous/poisonous, they will leave you well alone. But predators are smart animals, and some research has found that if batesian mimics are too common, the advantage they have over non-mimic species is lost, as the predators begin to ignore the warning signs. Moreover, this belief that mimicry is based on the principle of ‘once bitten, twice shy’, is a bit of a fallacy. No predator would learn anything from receiving a fatal bite from a Coral Snake. If it was distasteful, as many insects are, and advertised this through bright colouration/warning patterns, then a predator would learn to leave well alone because it would associate the two. As for the mimic, it has nothing to gain, either. What would be the point in a milk snake mimicking a coral snake, if it were still going to be attacked because its warning colours/patterning have no significance?
There are a lot of theories as to why warning colours have come about. An animal which could see colour may have a natural aversion to bright colours. Red being an obvious one. It is widely used in our daily lives. The brake lights of a car are red; the stop light of a traffic light is red, and of course we have learnt that most red berries are poisonous.
This is all well and good if the animal can see colours, but we know many animals are colour blind. How are they perceived by these? Well, these animals would recognise these bright colours in their basic form, as shades of dark and light banding. So, irrespective of how these bands were arranged, the animal would recognise it as being ‘dangerous’. This is why we can interpret the ‘red to yellow kill a fellow, red to black venom lack’ rule, but a colour blind animal couldn’t. And of course neither can one see colour, because it associates to the bright colours, and not to their arrangement.
A lot of speculation also reasons, that to be suddenly confronted by a brightly coloured snake shocks a predator for a few seconds, just long enough for the snake to make its escape. But this is only selective towards those which can see colour, and would be ineffective towards a colour blind animal. The reasons why mimicry evolved are interesting ones indeed.
Mimicry is still evolving?
Mimicry has been shown to be an evolution of harmless animals copying venomous animals when the two share the same sympatric geographic region. This poses an interesting question, “What happens if the species you are mimicking vanishes from the region you are sharing with it? Does the mimic colouration remain or regress? Considering that the colouration/patterning was evolved to avoid predation, would predators, over time, come to learn that the mimicker is just wearing a cloak of falsehood?
Well, studies in the Sandhills forest of North Carolina, can give us a clue as to what may happen. The region is home to the Scarlet Kingsnake, Lampropeltis elapsoides. It was also the home to the Coral Snake, Micrurus fulvius, but the coral snake vanished from the region during the 1960s. One would imagine that the absence of the coral snake would have an influence on the kingsnakes colouration. And it has, but not in the way we would envision. Researchers expected to find the patterning to begin regressing, but were surprised to find the kingsnake was actually evolving to resemble more closely the coral snake, where the red and black bands were becoming more equal in size.
Evolutionary biologists are still in a quandary as to why this is happening. Some think that the resemblance will become even stronger, while others speculate that eventually it will begin to regress, because they believe that predators will sooner or later learn that the snakes are not venomous. Evolution is a fascinating subject, but unfortunately it can take an awful long time to get the answer to the question.
Tail-end
Mimicry is characterised by two factors; an emitting organism (referred to as the model), and a receiving organism (the copier). It has evolved since the earliest forms of life inhabited the earth, and came about through a number of criteria, notably; exterior characteristics, behaviour, sympatric geographic, and the mimic’s inability to defend itself. Something which I have not touched on, is mimicry also requires little or no energy on the part of the mimicker, and it is perhaps another good reason to explain why it evolved.
On a final note. If we look at how red, or variations of it, and patterning are played out in the reptile world, even the humble Corn Snake, Pantherophis guttatus, plays on mimicry to a certain degree. The length of its body is covered in a series of saddle-markings which could be interpreted by a predator as meaning the snake is venomous. Is the Corn Snake showing mimicry evolution in progress?
Further readingHuey, R.B; Pianka, E.R (January 1977). Science 195: 201-203. Natural selection for juvenile lizards mimicking noxious beetles.
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