The sixth in James Ashworth‘s series on alien biology. The Zygons are ready to take over!

First seen in Terror of the Zygons in 1975, Zygon abilities include shapeshifting and mimicry of the voice of the being they are copying. In their first appearance, the Zygons seemed only to be able to copy humans, and the human whose ‘body prints’ they borrowed had to be kept alive. However, since their reappearance in 2013’s The Day of the Doctor, the Zygons have been seen to transform into animals, such as horses. They can kill the being they are copying as long as they can remember its body print, as well as keeping their victim’s memories. In nature, there are creatures who can change shape, change colour and change voice – but can they tell us anything about the talents of the Zygons?

We have the power to turn ourselves into replicas of your unpleasant form whenever it is necessary
– Zygon-Caber, Terror of the Zygons part two

The mimic octopus can imitate the forms of various species, including the sea snake, lionfish and flatfish, among others. It has been suggested that the ability to mimic various organisms, rather than focusing on one, is so that it may mimic an organism that preys on the organism that it sees, so the organism in question does not attack. It is suggested it is at least partly a Batesian mimic, where a non-toxic organism pretends to be a toxic one. The main reason the mimic octopus is thought to be a Batesian mimic is because the sea snake and lionfish, among the other species it mimics, are toxic, so that this mimicry makes predators avoid the octopus. The octopus changes its behaviour and shape, aided by the fact it has no rigid skeleton. This enables it to change its hydrostatic skeleton (a skeleton with no bones, just muscles and fluid) into a variety of shapes. It can hide the majority of its legs in holes or burrows in the sea floor to pretend to be a sea snake, where it stretches the other two legs into a long line to look like the snake. Another example is its flattening itself and collecting its legs together to mimic a flat fish.

Batesian mimicry doesn’t completely cover the abilities of the mimic octopus. It’s been described as the only species to practice dynamic mimicry, able to impersonate several different other animals. In the example of the flatfish, the octopus turns brown, while for the sea snake it mimics its distinctive black and white stripes. Colour change is another key aspect of dynamic mimicry. In many organisms, such as fish and lizards, colour change is controlled through hormones. One of the most important hormones is intermedin, which allows pigments to disperse in the cell, as well as adrenalin and melatonin, which cause the pigment to aggregate.

Big red rubbery thing covered in suckers. Surprisingly good kisser
– Tenth Doctor, The Day of the Doctor

Chromatophores are a cell or plastid that contains pigment. There are many types of chromatophores, that build themselves into dermal chromatophore units, or DCUs. On the bottom of the dermal chromatophore unit is a melanophore, which contains eumelanin, a black pigment. It has finger-like extensions that cover the iridiophores, to obscure them when the animal wishes to disguise itself in a dark background. The iridiophores themselves are the next layer up of the unit, and contain guanine plates that adjust themselves to give different blue-green colours through scattering or reflecting light at slightly different angles to change the hue. There are actually two types of iridiophores, and the other is the leucophore, which appear white. We are currently unsure how the iridiophores work, if it is through changing the structure of the guanine plates, or if they can reorient them.

The top layer of the dermal chromatophore units are the xanthophores, that contain pteridines and carotenes (which unsurprisingly are found in carrots) which give a yellow colour. In the absence of intermedin and the presence of melatonin, eumelanin aggregates into small pockets.The DCUs give a green hue in many species, due to the blue light of the iridiophores filtering through the yellow xanthophores. The exact hue can be altered as necessary by varying hormone levels.

I underestimated his intelligence, but he underestimated the power of organic crystallography
– Broton, Terror of the Zygons Part Four

In other species, where intermedin causes eumelanin to spread out, it appears white. With intermedin, the melanophores obscure the iridiophores, as the eumelanin disperses throughout the cell, covering the iridiophores as eumelanin spreads into the extensions above them. In the iridiophores themselves, the plates spread out, so their colour becomes dull enough to be obscured by the eumelanin, and the xanthophores expand to give a darker skin tone. The distribution of pigment in the melanophore is achieved by microtubules, specialised protein structures that spread throughout cells. These allow specialised motor proteins to move pigment vacuoles a large distance in the cell, via tubulin, and they are then dispersed in a small area along actin fibres, which are protein filaments which are distributed throughout the cytoplasm. This allows for pigment to be quickly moved across a cell to the rough area, whereupon it is then dispersed across that area to cause the colour change.

The other way of changing colour, which is used by cephalopods like cuttlefish and octopuses, is similar, but is controlled by muscles rather than hormones. A sacculus is a group of pigment-containing cells that form an organ, along with muscles attached around its circumference. When these muscles contract and relax, it varies the amount of pigment that can be seen by an observer. Contraction of the sacculus means that pigment is concentrated in a small area, which makes it hard to see, while relaxed muscles allow the pigment to spread throughout the sacculus and be seen, giving a colour change. Cephalopods have a mix of pigmented xanthophores, in red, yellow, brown and black. This, in combination with the blue-green iridiophores, allows for a much greater control of colour change, such as in the cuttlefish. Another reason for the much greater range of colour changes that are available, and their speed, is that muscles are controlled by neurons. Neurons act much faster and more specifically than hormones, which act generally across the body, and much more slowly.

The one thing you and I can never do is lie to each other
– Bonnie, The Zygon Inversion

The final aspect of the Zygons’ mimicry is vocal. A good example of a species that can mimic the calls and sounds of other species is the fork-tailed drongo. It uses its ability to steal food from other species, making it a kleptoparasite. It splits its time between foraging for food, and giving false alarm calls. It sits on a perch, where it can see a large area of the surrounding savannah. While it does give genuine alarm calls when it spots a predator, it also gives false alarm calls, mostly mimicking other bird species. In this environment, various species listen to the alarm calls of other species, to avoid predators. However, the fork-tailed drongo exploits this by giving a mix of alarm calls, of its own species and of others, which causes species such as the meerkat to flee to safety, while it then picks up their dropped food as there is no predator. The mix of species mimicked means that the target species cannot single out which species is making the false alarm calls.

The Zygons’ extraordinary abilities are strangely believable. The ability of the Zygons to change body shape and colour is very possible, the colour change particularly, if they use the sacculus chromatophores of the cuttlefish. The ability to change body shape is also quite realistic, as the use of muscles would allow the precise formation of a certain texture and shape for the body of the mimic. Vocal mimicry is also possible, as the fork-tailed drongo and indeed, any impressionist, can mimic the voice of another. However, it is the speed at which the Zygon can pick up the vocal habits of another that is unrealistic. The drongo is trained from birth, whereas an impressionist requires a lot of practice for a decent impression.

Those were the old rules, before Zygons could pluck loved ones from your memory and wear their faces
– Osgood, The Zygon Invasion

The Zygons’ other powers can be questioned too. The hydrostatic skeleton required to change body shape has its limits. While in the ocean, a hydrostatic skeleton is very malleable, due to the support given by the surrounding water, but on land the hydrostatic skeleton has no external support, and rings of muscles would be required solely to stop the organism from collapsing. While this would require a large energy expenditure on muscle contraction, it could still be possible, and would offer much greater control of the body shape. If the Zygons had such dense musculature that they could mimic the appearance and shape of a being with incredibly high accuracy, then this is quite realistic. Mimicking the voice of a target would take time, however, and a lot of studying of the speech patterns of the individual. A more recent development is being able to recall memories of the targets as seen in The Zygon Invasion/Inversion, is more difficult to rationalize, especially with the speed it was deployed in the episodes. So watch out for the silent people with unusually muscled bodies. They may be Zygons in disguise…

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