Bruce Bagemihl - Biological Exuberance

Once animals establish a family, an enormous number of different parenting arrangements can be employed—only a small fraction of which involve a “nuclear family” configuration with a mother and a father both caring for their offspring. In the majority of animals, single parenting (or no parental investment at all) is the rule. In most mammal species, for example, no long-lasting bonds are formed between the sexes, and females raise their young on their own. Even in birds, where “nuclear” two-parent heterosexual families are typical, single parenting also sometimes occurs. In a number of species, male-female pairs routinely separate and one bird takes over parental duties—often the female, but occasionally the male, as in Common Murres and whimbrels. Sometimes this occurs only a few days after hatching (e.g., whimbrels), or even before hatching (e.g., ducks). In other cases the brood is actually split between the two parents, as in some woodpeckers, Hooded Warblers, and many other perching birds. 130The opposite of single parenting is also found: many birds raise their young in communal breeding groups with multiple parents and caretakers of both sexes, and parenting trios occasionally form in species that otherwise have “nuclear families.” 131A phenomenon that could perhaps be called double-parenting also occurs in Golden Plovers, in which two heterosexual couples join forces and raise their youngsters as a quartet of parents. In general, then, a male-female parenting couple is neither a necessary nor a common arrangement among animals.

In addition to the wide range of family constellations in which animals raise their young, nearly 300 species of mammals and birds have developed adoption, parenting-assistance, and “day-care” systems, in which offspring are raised or cared for by animals other than their biological parents. Sometimes a sort of “baby-sitting” arrangement is formed (also known as alloparenting), in which a male or female assists another individual or couple in the care of their young (including “wet nurses” who suckle another female’s young). This helper may be a relative or may not be related at all. In other species, groups of youngsters are pooled together into groups—known variously as crèches (e.g., Botos, Flamingos, Cliff Swallows, and many other bird species), nursery groups or calf pools (e.g., Giraffes, Wapiti), and pods (e.g., Northern Fur Seals). These groups are usually looked after by one or two adult “guardians” while the parents are out foraging or socializing. Such systems can be viewed both as examples of adults being freed from their parenting duties by a natural “day-care” system and instances of animals forgoing a portion of their reproductive “responsibilities” in order to pursue other activities. Outright adoption and various forms of foster-parenting and stepparenting also occur across a wide spectrum of animals. Many Gull chicks actually desert or “run away” from their families as a result of neglect or violence (and are adopted by other families), and White Stork and lesser kestrel chicks also sometimes abandon their nests and switch to neighboring “foster families.” In birds, many other types of “adoption” result from eggs being abandoned by other birds, laid in other families’ nests, or even transferred to other nests by being carried or swallowed and regurgitated whole. 132In a few species, “kidnapping” of youngsters or stealing of eggs (with subsequent foster-parenting) may also take place. 133

Heterosexual mating systems also exhibit a dizzying variety of forms. Pair-bonding between males and females is found in some mammals and most birds, but the majority of animals have polygamous or promiscuous systems, in which animals mate or bond with several different partners. This can take the form of either one male with several females (polygyny, the most common form), one female with several males (polyandry), a combination of both (each sex mating and bonding with several partners, or polygynandry), or mating with multiple partners with no bonding between them ( promiscuity ). 134Even in species that form male-female pairs, however, there are many different arrangements. Heterosexual pair-bonding was long thought to be a simple and straightforward type of mating system, but biologists now recognize that—as in most other aspects of sexuality and social organization—animals exhibit considerable flexibility and diversity in their pairing arrangements. 135Many species such as willow warblers, Eleanora’s falcons, and sea horses are strictly monogamous. In numerous others, however, “infidelity” or nonmonogamous matings occur, among at least a subset of both males and females. 136Often, such copulations take place at times when females cannot be fertilized, so they are not entirely related to reproduction. In spotted sandpipers and Shags, for instance, almost all “unfaithful” copulations occur outside the females’ fertilizable periods, while female Razorbills specifically avoid full genital contact during nonmonogamous matings until after fertilization is no longer possible. 137Other, more complex arrangements are also found: the mating system of some species, such as Tasmanian Native Hens, is described as “social polygamy with genetic monogamy.” These birds live in polygamous groups, often several males mating with one female, but only one male fathers offspring with the female. This is the opposite of birds that form heterosexual pairs (“social monogamy”) but mate and produce offspring with other partners (“genetic polygamy”). In many species where individuals typically form pair-bonds, there is also usually a subset of individuals that form heterosexual trios. 138

Many pairing systems could probably be characterized as “serial monogamy.” Even in birds that tend to form lifelong pair-bonds, divorce occasionally happens, and in many species pairs break up much more frequently, individuals usually then remating with other partners. 139In Oystercatchers, for instance, divorce and remating is quite common (especially among females), and some individuals have as many as six or seven consecutive mates over their lives. Overall divorce rates vary widely between different individuals and species, from 0 percent in Australian ravens and wandering albatrosses to about a quarter of Kittiwake, a third of natal robin, and two-thirds of Lapland longspur pairs, to nearly 100 percent of all pairs in house martins and Flamingos. Divorce may result from a failure to produce offspring, but in many cases a complex interaction of multiple factors is involved, including general partner incompatibility. Other types of heterosexual family breakup also occur: extended families in Ocellated Antbirds may disintegrate when male-female pairs leave or grandparents isolate themselves; Warthog family units composed of a male and a female with young are generally less stable than female-only families; yearling White-tailed Deer are usually driven away by their own mothers; and Snow Geese family units may break up prematurely when juveniles leave. 140

Heterosexual mating and parenting arrangements come in a staggering variety of forms—it is simply not the case that one type of “family” configuration is utilized by all species, or even by all individuals within the same species, or by the same individual for all of his or her life. Animal heterosexuality (like homosexuality) is a truly multidimensional, polymorphous phenomenon.