Animal Family Values: Birth Control, Day Care, Divorce, and Infidelity

Although two young are born [in Pronghorn antelope], four to six embryos are implanted in the uterus, where they fight to the death, so to speak, for the limited space …. Long projections grow out of the embryonic sheath which puncture other embryos, causing their death. All but two embryos are reabsorbed by the mother’s body.

—VALERIUS GEIST, “Pronghorns”¹¹³

When most people think about animal families, they imagine a mother deer lovingly tending her fawns, or a father bear diligently protecting mother and babies. The realities of animal heterosexuality are a far cry from this romanticized view. More often than not, a mother deer viciously drives her yearlings away from the family group (when she hasn’t aborted her fetuses, that is) while father bear rarely has anything to do with his family—and when he does, it is often to kill and eat them. This section examines some of the stark realities of animal “family life,” beginning with a look at the many mechanisms that animals use to avoid having families altogether, by limiting reproduction and eliminating offspring.

In addition to infrequent copulation or mating during times when fertilization cannot occur, several other forms of “birth control”—i.e., ways of preventing pregnancy—occur in animals.¹¹⁴ In fact, more than 20 different strategies have been identified whereby females are able to limit, control, or prevent insemination. The widespread occurrence of these phenomena throughout the animal world has led one scientist to conclude that “copulation … seldom leads directly and inevitably to fertilization.”¹¹⁵ Some female birds such as brown boobies defecate during mating rather than performing the customary genital contractions, thereby preventing insemination, while females of a number of insect, bird, and mammal species actively eject semen after copulating.¹¹⁶ Among mammals, vaginal or copulatory plugs (sometimes also known as chastity plugs) are found in a number of species. Gelatinous barriers form (or are deposited) in the female’s reproductive tract in several different kinds of rodents, bats, insectivores, and wild pigs, as well as in some primates and dolphins. Although their function is not fully understood, it appears that plugs often serve to prevent insemination. In many species a male leaves a copulatory plug in the female following mating (or the semen simply coagulates to form the plug) so that other males will not be able to fertilize her. In Squirrel Monkeys and some bats, hedgehogs, and opossums, however, the female herself produces the plug (often from sloughed vaginal cells), probably to control or prevent inseminations by males. In addition, female squirrels sometimes entirely remove copulatory plugs—which contain all the semen deposited by a male—thereby effectively preventing insemination from their most recent mating.¹¹⁷ Finally, scientists recently discovered that female Common Chimpanzees employ an extraordinary form of birth control: nipple stimulation. As in a number of other mammals, the regular reproductive cycles of female Chimps are inhibited or interrupted while they are suckling infants (known as lactational amenorrhea). Some females without infants have learned that by stimulating their own nipples they can effectively mimic this physiological effect, thereby preventing themselves from conceiving even though they are not actually lactating. In some cases, Chimps have avoided pregnancy for as long as a decade by employing this ingenious “contraceptive” technique.¹¹⁸

Following conception, pregnancy can be blocked (the fertilized egg does not implant, a phenomenon found in many rodents, where it is known as the Bruce effect). ¹¹⁹ Embryos can also kill each other by puncturing or strangulation (Pronghorns) or by actively devouring one another inside the uterus (sand sharks, some salamanders).¹²⁰ Embryos may also be “eliminated” because the mother has too few nipples to accommodate all of them (some marsupials such as Northern Quolls), while embryos of many hoofed mammals are simply reabsorbed.¹²¹ Actual abortion occurs in many species, including primates (e.g., Hanuman Langurs, Pig-tailed Macaques, Savanna Baboons), marine mammals (Australian Sea Lions, other seals), hoofed mammals (feral Horses, White-tailed Deer), carnivores (Red Foxes), and numerous rodents and insectivores (wood rats, voles, coypus). Abortions may occur either spontaneously, or as a result of stress and harassment from males, or (in primates) possibly also from deliberate ingestion of abortion-inducing plants. Though usually a sporadic or isolated occurrence, abortions may be more commonplace in some species or populations. In California sea lions, for example, large numbers of females routinely abort their fetuses: hundreds of abortions take place each year on some breeding grounds, often as long as four months in advance of the usual birthing period.¹²² Many birds and other species that lay eggs practice the equivalent of “abortion”—that is, the termination of an embryo’s development—in the form of egg destruction (also known as ovicide), ejection of eggs from the nest, and/or clutch abandonment.¹²³

Following birth or hatching, many animals employ strategies of direct or indirect killing to “eliminate” offspring. Infanticide or direct killing of young is a widespread phenomenon in the animal kingdom, reported in all major animal groups.¹²⁴ It is also a decidedly heterosexual behavior, often revolving around the creation of new breeding “opportunities.” In one common form of infanticide, for instance, a male kills youngsters so that he can mate with their mother and sire his own young.¹²⁵ In another type of infanticide, females kills their own or related offspring—in black-tailed prairie dogs, for example, almost 40 percent of all litters suffer partial or complete loss to infanticide by females.¹²⁶ Cannibalistic infanticide also occurs, in which young animals are not only killed but also eaten.¹²⁷ Offspring are sometimes subjected to neglect and abuse as well, ranging from “absentee” parents who fail to incubate their eggs or properly attend to their youngsters (e.g., storm petrels, Oystercatchers, King Penguins) all the way to abandonment, physical brutality, and/or sexual violence inflicted on youngsters (e.g., Hanuman Langurs, Northern Elephant Seals, Ring-billed Gulls), sometimes resulting in death.¹²⁸ In many bird species, the size of families is regulated through a combination of factors. Parents frequently control the sequence of egg laying and hatching so that some offspring routinely perish (usually the last egg to be laid or hatched). In other cases “surplus” or extra young are produced as a “backup” strategy and usually end up fighting and killing each other (a phenomenon sometimes known as siblicide or cainism).¹²⁹

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.¹³⁰ The 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.”¹³¹ A 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.¹³² In 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).¹³⁴ Even 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.¹³⁵ Many 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.¹³⁶ Often, 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.¹³⁷ Other, 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.¹³⁹ In 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.