Sociobiology: The New Synthesis [1975, 2000] by E. O. Wilson

Author:E. O. Wilson [Wilson, E. O.]
Language: eng
Format: epub
Tags: LCC QL775 .W54, DCC 591.56
Publisher: Belknap Press of Harvard University Press
Published: 1975-01-01T02:00:00+00:00

The Origins of Polygamy

Because of the Bateman effect, animals are fundamentally polygamous. At the start, virtually all are anisogamous. In those species in which parental care is also lacking, variance in reproductive success is most likely to be greater in males than in females. Under many circumstances the addition of parental care will reinforce this inequality, because parental investment in postnatal care is seldom quantitatively the same in both sexes. Monogamy is generally an evolutionarily derived condition. It occurs when exceptional selection pressures operate to equalize total parental investment and literally force pairs to establish sexual bonds. This principle is not compromised by the fact that the great majority of bird species are monogamous. Although polygamy in birds is in most cases a phylogenetically derived condition, the condition represents a tertiary shift back to the primitive vertebrate state. Monogamy in modern birds was almost certainly derived from polygamy in some distant avian or reptilian ancestor.

Before examining the evidence behind these generalizations, let us define the essential terminology pertaining to mating systems. Monogamy is the condition in which one male and one female join to rear at least a single brood. It lasts for a season and sometimes, in a small minority of species, extends for a lifetime. Polygamy in the broad sense covers any form of multiple mating. The special case in which a single male mates with more than one female is called polygyny, while the mating of one female with more than one male is called polyandry. Polygamy can be simultaneous, in which case the matings take place more or less at the same time, or it can be serial. Simultaneous polygyny is sometimes referred to as harem polygyny. In the narrower sense preferred by zoologists, polygamy also implies the formation of at least a temporary pair bond. Otherwise, multiple matings are commonly defined as promiscuous. But the word promiscuity, as Selander (1972) has pointed out, carries the incorrect connotation that the matings are random, even though in fact they are usually highly selective in a way that leads to the evolution of secondary sexual characteristics. Selander has proposed the alternative expression polybrachygamy. Although the term is technically and etymologically correct, it may prove too cumbersome to gain wide usage.

Entirely by itself, anisogamy favors polygamy as broadly defined. There also exist five general conditions that promote polygamy still further. They are (1) local or seasonal superabundance of food; (2) risk of heavy predation; (3) precocial young; (4) sexual bimaturism and extended longevity; and (5) nested territories due to niche division between the sexes. All but the last were discovered in birds, where polygamous and monogamous species commonly coexist and provide the opportunity for evolutionary comparisons; but the same biases probably operate with equal force on other, less well studied groups.

Local or Seasonal Superabundance of Food

Armstrong (1955), on the basis of his study of the common wren, Troglodytes troglodytes, of Europe, hypothesized that monogamy in birds evolves wnen food is limiting, the population is at or near its maximum,

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