Evolution as a process / edited by Julian Huxley, A.C. Hardy, E.B. Ford.

Date:
1954
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Evolution as a process / edited by Julian Huxley, A.C. Hardy, E.B. Ford. Source: Wellcome Collection.

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    stick-like and inconspicuous, translates itself, under provocation, into the likeness of a snake (Moss, 1920). Alternatively, the problem of colour conflict may be met by the exhibition of two rival sets of coloration in different groups of individuals within the species, rather than on different parts of the body in the same individual. In such cases the advertising dress is generally worn by the less vulnerable or less valuable members of the species, i.e. males or adults; and the cryptic dress by the more valuable or defenceless members, i.e. females or young. Among sexually dimorphic game-birds, ducks and others which nest in the open, it is typically the male that has utilized the biological advantages of conspicuousness, whether for purposes of display or distraction; while the female, upon whom devolves the duty of incubation, has been forced to take the evolutionary road leading to concealment. Parallel conditions obtain in the parent-child relationship: for instance, the young of many ground-nesting species such as Oyster- catcher, Avocet, and of gulls and terns, in which adults of both sexes are conspicuous, are themselves highly cryptic. Such temporary or inter-individual segregation of effacing and reveal ing functions is essentially related to the selective pressure imposed upon species by predators; and we may here note the interesting point made by Mottram (1915)—that marked sexual dimorphism is only found among birds specially liable to attack of enemies; and that among rela tively non-vulnerable species, such as large birds possessed of fighting strength, or formidable powers of offence, and colonial species with corporate defence in the flock, secondary sexual dimorphism rarely occurs. Here the opposing claims no longer operate, and the way lies open for the untrammelled use of advertisement by both sexes—as seen in the coloration of albatrosses, penguins, gannets, frigate birds, swans, cockatoos, ravens and many others. On the other hand, with small or otherwise defenceless species, the conflict between cryptic and sematic needs is acute. Such birds may take one or other of two evolutionary roads; many, like larks, pipits, partridges, quails, coursers, stone-curlews, and nightjars, forgo the advantages of conspicuousness, both sexes being typically cryptic, as is generally true of ground-nesting species in which incubation is shared by both parents; sematic characters are then often auditory. Another line of adaptation has led in the opposite direction, and it would appear that where advertisement has been developed in otherwise defenceless species, such as the black-and-white desert chats Oenanthe leucopyga and 0. lugens , it is often associated with distasteful- ness of the flesh (Cott, 1946). In this respect, birds fall into line with many other groups of animals, for which a broad correlation has been established between cryptic