The highest level at which rapid colour-change occurs is among lizards,
and the finest exhibition of it is among the chameleons. These quaint
creatures are characteristic of Africa; but they occur also in
Andalusia, Arabia, Ceylon, and Southern India. They are adapted for life
on trees, where they hunt insects with great deliberateness and success.
The protrusible tongue, ending in a sticky club, can be shot out for
about seven inches in the common chameleon. Their hands and feet are
split so that they grip the branches firmly, and the prehensile tail
rivals a monkey's. When they wish they can make themselves very slim,
contracting the body from side to side, so that they are not very
readily seen. In other circumstances, however, they do not practise
self-effacement, but the very reverse. They inflate their bodies, having
not only large lungs, but air-sacs in connection with them. The throat
bulges; the body sways from side to side; and the creature expresses its
sentiments in a hiss. The power of colour-change is very remarkable, and
depends partly on the contraction and expansion of the colour-cells
(chromatophores) in the under-skin (or dermis) and partly on
close-packed refractive granules and crystals of a waste-product called
guanin. The repertory of possible colours in the common chameleon is
greater than in any other animal except the Æsop prawn. There is a
legend of a chameleon which was brown in a brown box, green in a green
box, and blue in a blue box, and died when put into one lined with
tartan; and there is no doubt that one and the same animal has a wide
range of colours. The so-called "chameleon" (_Anolis_) of North America
is so sensitive that a passing cloud makes it change its emerald hue.
There is no doubt that a chameleon may make itself more inconspicuous by
changing its colour, being affected by the play of light on its eyes. A
bright-green hue is often seen on those that are sitting among strongly
illumined green leaves. But the colour also changes with the time of day
and with the animal's moods. A sudden irritation may bring about a rapid
change; in other cases the transformation comes about very gradually.
When the colour-change expresses the chameleon's feelings it might be
compared to blushing, but that is due to an expansion of the arteries of
the face, allowing more blood to get into the capillaries of the
under-skin. The case of the chameleon is peculiarly interesting because
the animal has two kinds of tactics--self-effacement on the one hand and
bluffing on the other. There can be little doubt that the power of
colour-change sometimes justifies itself by driving off intruders. Dr.
Cyril Crossland observed that a chameleon attacked by a fox-terrier
"turned round and opened its great pink mouth in the face of the
advancing dog, at the same time rapidly changing colour, becoming almost
black. This ruse succeeded every time, the dog turning off at once." In
natural leafy surroundings the startling effect would be much greater--a
sudden throwing off of the mantle of invisibility and the exposure of a
conspicuous black body with a large red mouth.
§ 4