We all know that appearances can be deceiving, and when we venture out into the forest not everything is quite as it seems. In the quest to eat, and not be eaten, the many plants, animals and other living things in the woods and beyond have evolved different strategies to secure their place in the web of life. Some of these strategies include being obscure and concealed, fierce and aggressive, prickly or poisonous amongst much else. Some have even evolved methods of trickery and deception in order to survive. These look-alikes use a strategy known as mimicry: disguising themselves as something else and reaping rewards, in survival terms, for doing so. Some might appear to be more threatening or more benign than they really are, and some might appear to be just - well - something other than what they really are! Looking more closely at a few of the many forms of mimicry can give us a clear insight into the fascinating process of evolution.
Of the different kinds of mimicry, one of the most widespread and well-studied was identified by Henry Walter Bates, an English naturalist who spent over a decade surveying the Amazon in the mid-19th Century. It was through his intensive studies of butterflies that he observed and described what later became known as Batesian mimicry. This is where a harmless species evolves the appearance of something much more dangerous. A prime illustration Bates gave was the non-toxic butterflies that have the same warning coloration as species that are poisonous for predators to eat.
Closer to home, we have many Batesian mimics, a lot of which impersonate wasps and bees. The familiar black and yellow stripes of many wasp and bee species are warning colours, which effectively proclaim 'back off or I'll sting you!' - and mean it. A naive young bird may eat a wasp and get stung, but if it has any sense it will avoid wasps in future. The individual half-chewed wasp doesn't benefit directly, but the species as a whole is better off in that there is one less predator to worry about.
From then on the bird will avoid anything that it thinks is a wasp, regardless of whether or not it is the 'real McCoy'. Among the best known wasp and bee mimics are the hoverflies, which do an impressive impersonation of their more dangerous distant cousins. Many hoverflies have evolved these black and yellow 'don't mess with me' markings, but are just bluffing - they are completely harmless. The ruse is very effective however, and even humans who have never encountered hoverflies before are sometimes very nervous in their presence!
The bee beetle (Trichius fasciatus) looks and sounds a lot like a bumblebee (Bombus spp.), and there are even wasp-like moths: many of the clearwing moths (Family: Sesiidae) have black and yellow markings, transparent wings and also tend to buzz in flight. Again they rely on the disguise working, as they have no other form of defence. The wasp beetle (Clytus arietus), an insect that breeds in dead deciduous wood, takes wasp mimicry a step further. As well as having the markings we know so well, it even behaves like a wasp, walking jerkily and tapping the surface it is on with its antennae in a waspish way!
Of course not every strategy is infallible. The drone fly (Eristalis tenax) is a hoverfly that looks very much like a honeybee (Apis mellifera). While many animals are fooled, the spotted flycatcher (Muscicapa striata), a woodland-dwelling bird, is undeterred and eats them readily.
Wasps and bees are common 'models' for mimics to imitate but there are others. Ants are feared by many animals, and worldwide there are a lot of creatures that mimic these fierce insects. In the Scottish Highlands there are two species of spider that mimic ants: Micaria pulicaria and its close relative M. alpina which lives in high elevation habitats such as montane scrub.
The 19th century German naturalist, Fritz Müller, described a different kind of mimicry. Müllerian mimicry is when two species or groups of animals copy one another, but neither of them is bluffing. Many species of wasps and bees for instance have co-evolved the same coloration and markings, using a common 'language' to ward off would-be predators, although it is often difficult to tell which species copied which. When a group of different species evolves this common code, it is called a 'mimicry ring' and is highly effective. In our example about the young bird, as well as avoiding wasps and their harmless impostors in future, it would also steer clear of bees, without any bees having to be sacrificed to educate the bird. And of course the reverse is true: if a bird learned its lesson from a bee, it would be more likely to avoid yellow and black wasps. Thus the relationship is a mutually beneficial one.
Mimicry provides us with one of the clearest illustrations of natural selection. Let us imagine the process in action: a species of fly produces a new generation of offspring. They are all fairly similar, although there is inevitably some natural variation. One may have a mutant gene that perhaps gives it a bit of yellow on its otherwise black coloration, enough to make it look a little more wasp-like than the others. Thus it may just be spared from being eaten, while many of its siblings get devoured. It then passes that coloration gene on to its descendants, and again those that look the most waspish, even if vaguely and inadvertently so, have a slightly better chance of surviving than those that don't. This process of natural selection continues down the generations until the fly, although not closely related to wasps, looks remarkably similar to one.
Sometimes mimics look virtually identical to their 'model' (the organism that is being mimicked), but sometimes there is only a passing resemblance. Scientists suggest that in some instances the mimic is still in the process of evolving towards looking more like its model, so that in many generations down the line it will be nearly identical. In other cases the model is trying to 'escape' from the similarity. After all a mimic 'dilutes' the message that a dangerous animal is sending out. To use the example of our naive young bird, if it ate several hoverflies before eating a wasp, the wasp's 'black and yellow equals pain' message would be weakened. Thus it is in the model's interests to look different from its impostors.
The peacock butterfly (Inachis io) uses another, bolder kind of mimicry. If spotted and directly threatened, it flashes its conspicuous eye-spots to frighten the predator away. These false eyes give the impression that it is something much bigger than it actually is, and are a very effective defence. Similarly the caterpillar of the elephant hawkmoth (Deilephila elpenor) has a dramatic response to being attacked. When it is threatened it withdraws its 'trunk' and inflates its head, complete with eyespots, creating the illusion of being larger and scarier. It adds to the effect by waving about menacingly. Even though it may not be mimicking a specific species, it is still a powerful deception.
It has been suggested that the Scottish wildcat (Felis sylvestris) also uses a form of mimicry. When cornered, these cats are well known for their ferocious spitting and hissing, and some zoologists suggest that this strategy actually mimics a snake. The flattened ears, lashing tail and gaping, fanged mouth all add to the effect. Many animals have an instinctive fear of snakes and so are more likely to be daunted and back down.
Mimicry is not always about protection from predators. The cuckoo (Cuculus canorus) uses some astonishing forms of trickery to aid its parasitic lifestyle. In flight, cuckoos look a lot like hawks, having a similar body shape, flight pattern and even the same kind of barred markings on their breast. When a cuckoo flies into an area, other birds often mistake it for a hawk, and make a hasty exit to avoid being eaten. This gives the cuckoo the chance to carry out its next manoeuvre, and here the plot thickens. While it is famous for laying its eggs in the nests of other birds, what is less well known is the incredible subtlety of this deception. Cuckoos target a range of hosts but since, say, meadow pipits (Anthus pratensis) and dunnocks (Prunella modularis) (two of the main cuckoo hosts) have different coloured eggs, how does this work? Surely the host would spot an oddly coloured egg? The answer is that individual cuckoos specialise in certain host species. So some will target meadow pipits and lay the same spotted eggs as the host, while others will lay turquoise eggs in a dunnock nest.
This bird has an insect namesake with some similar habits. There are various species of cuckoo bee (Psithyrus spp.), each of which looks like a particular bumblebee (Bombus spp.). The cuckoo bee lays its eggs in the nest of a bumblebee host, which are tricked into raising the young.
It is not only birds and animals that use mimicry. Fungi and plants, being less mobile, sometimes use imitation to help the population to disperse. Walking through the woods you can sometimes smell the aptly named stinkhorn (Phallus impudicus) well before you see it. The stinkhorn is a fungus that mimics the smell of decaying flesh. This attracts flies which, having landed on the fungus, fly elsewhere carrying its spores on their feet.
Most mimicry has an obvious purpose, but some is more mysterious. Many species of birds do excellent copies of other sounds, particularly the calls of other birds. Among the many mimics are the starling (Sturnus vulgaris), song thrush (Turdus philomelos) and blackbird (Turdus merula). These calls can be astoundingly accurate, yet no one is really sure why they do it! There is evidence to suggest that using a wide and rich repertoire helps them to impress the opposite sex. It is also possible that they simply enjoy it (a difficult one to prove or disprove!), or that there is a combination of reasons.
Camouflage is effectively another form of mimicry, and the definitions can be somewhat blurred. After all, camouflage involves imitating the organism's background, and commonly uses what is known as cryptic coloration to help the creature to blend with its surroundings. However, there is a particularly strong overlap between mimicry and camouflage when animals disguise themselves as specific objects that their predators are likely to ignore. The buff-tip moth (Phalera bucephala) looks astonishingly like a broken twig and the bird dropping spider (Cyclosa conica) looks remarkably similar to (you guessed it!) a bit of bird faeces. In seeking to differentiate between camouflage and mimicry, camouflage can be consider as a more 'passive' attribute, in which an organism has evolved to blend in against a background, whereas mimicry has a more 'active' component to it, whereby an organism copies a specific species or behaviour. Camouflage is covered in more detail here.
The world of mimicry demonstrates the power of natural selection, as well as highlighting the fact that it is often worth taking a second look at what we think we have seen!
Sources and further reading