REPRODUCTIVE BEHAVIOUR

any activity directed toward perpetuation of a species. The enormous range of animal reproductive modes is matched by the variety of reproductive behaviour. Reproductive behaviour in animals includes all the events and actions that are directly involved in the process by which an organism generates at least one replacement of itself. In an evolutionary sense, the goal of an individual in reproduction is not to perpetuate the population or the species; rather, relative to the other members of its population, it is to maximize the representation of its own genetic characteristics in the next generation. The dominant form of reproductive behaviour for achieving this purpose is sexual rather than asexual, although it is easier mechanically for an organism simply to divide into two or more individuals. Even many of the organisms that do exactly thisand they are not all the so-called primitive formsevery so often intersperse their normal asexual pattern with sexual reproduction. any activity directed toward perpetuation of a species. The enormous range of animal reproductive modes is matched by the variety of reproductive behaviour. Sexual reproduction is by far the most common mode of reproductive behaviour. The mixing of genes, which occurs when an egg unites with one sperm, produces a greater variation of characteristics in the next generation. This genetic variety enables the offspring to adapt to a wider scope of fluctuating environmental conditions than the offspring of asexual reproduction, which are primarily duplicates of the solitary parent. Millions of eggs must often be fertilized to produce just a few adults, even though a variety of behaviour patterns have been developed to protect the eggs, larvae, and young in their process of growth. In some cases, nature provides a specially large yolk in the egg; in others, parents or groups of adults care directly for the young. In social parasitism, the young are left in the care of other species (e.g., the cuckoo), and they will push the adopted species' offspring out of the nest in order to monopolize the food supply. Most one-celled organisms reproduce by splitting in two (fission), which is the basic form of asexual reproduction. In multicellular animals, reproduction may be asexual, sexual, or a combination of different forms at different times. In some cases (e.g., male bees), an egg will develop an embryo without fertilization (parthenogenesis). Another form of asexual reproduction is by budding, in which the larvae remain inside the adult until a wall is developed to separate the new animal from the original parent. Some species are hermaphrodites and can either split in two or copulate; each member usually produces sperms or eggs, but not both simultaneously, thus preventing self-fertilization. The stages of approach, identification, and copulation in sexual reproduction have been developed to avoid predators or even, as with spiders, to avoid being eaten by their own kind, as well as to avoid wasting eggs and sperm. These stages may or may not include courtship with its attendant displays, dances, and attractants, which vary from olfactory chemicals called pheromones to sounds that are specific for a species. Elaborations such as displays, crests, and pheromones are all forms of behaviour that maximize the survival of the species. The evolution of antlers and horns in some male mammals, for example, is the result of their usefulness in territorial dominance and the consequent possession of females. Additional reading Modes of behaviour related to reproduction in animals other than humans are dealt with in Margaret Bastock, Courtship: An Ethological Study (1967), an excellent survey; S.A. Asdell, Patterns of Mammalian Reproduction, 2nd ed. (1964), a comprehensive survey stressing anatomic and physiological aspects; Desmond Morris, Patterns of Reproductive Behaviour (1970), a compilation of some of his classic papers; Ari Van Tienhoven, Reproductive Physiology of Vertebrates, 2nd ed. (1983), a good survey with an emphasis on the hormonal and neurophysiological aspects of reproductive behaviour, especially chapters 9, 11, 13, and 14; and Martin Daly and Margo Wilson, Sex, Evolution, and Behavior, 2nd ed. (1983), a useful introductory textbook for undergraduates on the sociobiology of reproduction. Texts dealing with bird reproductive behaviour include John Sparks, Bird Behaviour (1970), an excellent introduction, well-illustrated; Joanna Burger and Bori L. Olla (eds.), Shorebirds: Breeding Behavior and Populations (1984), a collection of review essays for professionals and other advanced readers; and Nicholas E. Collias and Elsie C. Collias, Nest Building and Bird Behavior (1984), a fascinating look into nest building and other behaviour patterns and devices used by birds to protect their eggs and young. An important survey of reproductive behaviour in fishes is Charles M. Breder, Jr., and Donn Eric Rosen, Modes of Reproduction in Fishes (1966). Edward O. Wilson, The Insect Societies (1971), is one of the best general treatments of this group.