Turning on the sexually fastidious female
HUMANS are not the only animal species to possess a sense of beauty. Recent research bears out the controversial view of 19th century British naturalist Charles Darwin that animals have an eye for beauty. He offered this as an explanation for the preference of females for the gaudiest males.
The males of many species have unusual and elaborate features, such as a stag's antlers or the bright and colourful plumage of tropical birds. The development of these traits is difficult to explain on the basis of the Darwinian theory of natural selection -- the survival and propagation of organisms and features that are best adapted to the environment. That's because these traits do not contribute directly to success in the struggle for existence.
In 1871, Darwin proposed a theory of sexual selection, which asserts beautification traits play an important role in the competition between males for access to females. Darwin assumed the female of the species discriminates between males and responds favourably to those with extreme traits. The female, he conjectured, shows evidence in exercising a preference of an aesthetic sense akin to that found in humans.
Darwin's theory was criticised as being anthropomorphic (attributing human form or personality to animals) by many of his contemporaries, who noted its major flaw is its inability to explain the origin and persistence of female choosiness, factors that are needed to account for the evolution of the male beautification traits.
This limitation of Darwin's theory has now been overcome by Swedish zoologists Magnus Enquist and Anthony Arak. By modelling the process of perception in female birds, they have come up with a plausible explanation for the evolution of the female choosiness that Darwin had interpreted as an aesthetic sense (Nature, Vol 361, No 6411).
Response simulation Using what are called artificial neural networks, Enquist and Arak were able to simulate how female birds recognise differences in a certain trait, such as the length of the tail. These neural networks are models of mental processes associated with perception, based on how animal brains supposedly function. They serve as convenient tools for uncovering general principles of recognition, without the complexity to be found in the nervous systems of real organisms.
The network was given an input, representing a male bird with a certain tail length. The intensity of the output signal signifies the female response. Enquist and Arak used a procedure that mimics natural selection to train a simple network to distinguish between males with long and short tails and give a strong response to the former and a weak response to the latter.
The two zoologists also found a system concerned with signal recognition is inevitably biased in its response to certain forms of the input signal. The female preference for males with long tails is a manifestation of such bias. The model lends credence to the theory that female choosiness could have evolved originally in response to stimuli quite unrelated to reproduction, such as warning signals from predators that were important for survival.
Tail elongation occurs even when increasing tail length is a threat to male survival. The results obtained by Enquist and Arak confirm Darwin's theory that traits that give an advantage in mating can evolve to such extremes, they decrease male survival.