ANSC 420 Lecture Notes - Lecture 13: Abbott Handerson Thayer, Stenopus Hispidus, Hugh B. Cott
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Aggressive resemblance is used by predators or parasites. In special aggressive resemblance, the
animal looks like something else, luring the prey or host to approach, for example when a flower
mantis resembles a particular kind of flower, such as an orchid. In general aggressive
resemblance, the predator or parasite blends in with the background, for example when
a leopard is hard to see in long grass.
For adventitious protection, an animal uses materials such as twigs, sand, or pieces of shell to
conceal its outline, for example when a caddis fly larva builds a decorated case, or when
a decorator crab decorates its back with seaweed, sponges and stones.
In variable protective resemblance,an animal such as a chameleon, flatfish, squid
oroctopus changes its skin pattern and colour using special chromatophore cells to resemble
whatever background it is currently resting on (as well as for signalling).
The main mechanisms to create the resemblances described by Poulton – whether in nature or in
military applications – are crypsis, blending into the background so as to become hard to see
(this covers both special and general resemblance);disruptive patterning, using colour and pattern
to break up the animal's outline, which relates mainly to general resemblance; mimesis,
resembling other objects of no special interest to the observer, which relates to special
resemblance;countershading, using graded colour to create the illusion of flatness, which relates
mainly to general resemblance; and counterillumination, producing light to match the
background, notably in some species of squid.
Countershading was first described by the American artist Abbott Handerson Thayer, a pioneer
in the theory of animal coloration. Thayer observed that whereas a painter takes a flat canvas and
uses coloured paint to create the illusion of solidity by painting in shadows, animals such as deer
are often darkest on their backs, becoming lighter towards the belly, creating (as zoologist Hugh
Cott observed) the illusion of flatness, and against a matching background, of invisibility.
Thayer's observation "Animals are painted by Nature, darkest on those parts which tend to be
most lighted by the sky's light, and vice versa" is called Thayer's Law.
Colour is widely used for signalling in animals as diverse as birds and shrimps. Signalling
encompasses at least three purposes:
advertising, to signal a capability or service to other animals, whether within a species or not
sexual selection, where members of one sex choose to mate with suitably coloured members
of the other sex, thus driving the development of such colours
warning, to signal that an animal is harmful, for example can sting, is poisonous or is bitter-
tasting. Warning signals may be mimicked truthfully or untruthfully.
Advertising coloration signals an animal's capability to other animals. These may be of the same
species, as in sexual selection, or of different species, as in cleaning symbiosis. Signals, which
often combine colour and movement, may be understood by many different species; for example,
the cleaning stations of the banded coral shrimp Stenopus hispidus are visited by different
species of fish, and even by reptiles such as hawksbill sea turtles.
Darwin observed that the males of some species, such as birds of paradise, were very different
from the females.