Identification: The largest living bird (over 6 feet tall), with striking black-and-white plumage in the male and powerful legs and claws.

EMU

Identification: The second-largest living bird (5—6 feet tall), with shaggy, brown plumage and bare patches of blue skin on the face and neck.

GREATER RHEA

Identification: Similar to Ostrich but smaller (up to 41/2 feet tall) and with overall grayish brown plumage in both sexes.

Social Organization

Ostriches associate in flocks and frequently form sex-segregated groups. All-male flocks may contain up to 40 individuals, many of them juveniles, who travel with each other for long periods. Emus generally associate in pairs or groups of 3—10 birds, while Greater Rheas gather in flocks of 15—40 birds outside of the mating season. All three species have a wide variety of mating systems (discussed below). These are notable for their various forms of POLYANDRY (females mating with several males) and the fact that—in Emus and Greater Rheas—all incubation and chick-rearing is performed by males without any help from females.

Description

Behavioral Expression: Male Ostriches perform a homosexual courtship dance to each other that is distinct from heterosexual interactions. Same-sex courtships consist of a sequence of three activities, performed by sexually mature adult males in full nuptial plumage (black-and-white feathers, with a red flush on the face and legs). First, there is a dramatic APPROACH in which one male runs rapidly toward his chosen partner—often achieving speeds of 25—30 mph—and stops abruptly just short of the other male. Then he launches into frenzied PIROUETTE DANCING, a high-speed, energetic circling in place beside his partner. This whirling may occur in a series of bursts, each lasting for several minutes. Finally, in KANTLING, the male drops to the ground next to his partner and rocks steadily from side to side, fluffing out his tail and sweeping the ground with his wings in an exaggerated fashion. All the while, he twists his head and neck in a continuous corkscrew action and repeatedly inflates and deflates his throat. The male being displayed to may acknowledge the dance with his posture, or he may simply maintain a calm stance devoid of alarm or aggression. Homosexual courtships are distinct from heterosexual ones in a number of respects: neither the running approach nor pirouette dancing occur in male-female interactions. Kantling is performed in heterosexual contexts but differs because it is usually accompanied by singing (when males display to females, they frequently produce a booming call), and it is significantly shorter. Same-sex displays last for 10—20 minutes, while opposite-sex ones rarely exceed three minutes. Also, symbolic feeding and nest-site displays are components of heterosexual but not homosexual courtships.

A male Ostrich (right, on the ground) courting another male with the “kantling” display

Although no copulation takes place between courting male Ostriches, homosexual mating has been observed in pairs of male Emus. A sexual interaction begins with one male approaching the other, stretching his neck upward and erecting his neck feathers so that they stand out horizontally, while grunting deeply. The two birds begin following and chasing each other; if the male who initiated the activity is behind the other, he may make treading movements with his feet, indicating his intention to mount the other. Often, however, it is the initiating male who lies flat on the ground as an invitation for the other male to mount. The males may also take turns mounting each other. The mounting male lies down behind his partner, resting his breast on the other’s rump, and uses his heels to slide forward until he covers most of the other male. While copulation is taking place, the mountee makes soft grunting noises (not usually heard during heterosexual matings), and the mounter gently toys with the feathers on his partner’s upper back. After mating, his erect penis is often visible: the male Emu, along with other ratites, is one of the few birds in the world that has a penis (most male birds simply have a cloacal, or genital, opening).

Joint parenting in male Greater Rheas: two males in Argentina (above) sitting on their double nest, which contains two sets of eggs (below) that are frequently rolled between the coparents

Male Emus also sometimes coparent with each other: two (and occasionally three) males may attend one nest at the same time, incubating all the eggs together. Such nests often contain SUPERNORMAL CLUTCHES of 14—16, and sometimes more than 20, eggs. This is over twice the number found in nests attended by single males, probably because more than one female has laid in them. Unlike single fathers, male coparents are able to take a break from incubating while their partner sits on the nest; they also sometimes roll the eggs between them while on the nest together. Although they are probably not sexually involved with one another, the two fathers cooperate in raising their chicks together, calling to them with “purr-growls” and jointly defending them from predators. A similar phenomenon is found in Greater Rheas: pairs of males occasionally sit on “double nests” that are close to or touching one another; they incubate the eggs together and jointly parent the chicks when they hatch. Most such nests begin as standard nests with only one male incubating, after which another male joins him and begins transferring eggs to his half of the nest; later, eggs may be transferred back and forth between the twin nests. Unlike Emu nests belonging to male coparents, Rhea double nests usually have a combined number of eggs that is the same as for single nests. Male coparents are different from male nest helpers, which are also found in Rheas. About a quarter of breeding males are assisted by an adolescent male, who incubates and raises (on his own) a clutch of eggs fathered by the adult while the latter goes off to start a new family. This differs from male coparenting in that the two nests are widely separated from one another, each contains the full clutch size of a single nest, the two males never share parenting duties, and the helper is always an adolescent male.

Female Ostriches are occasionally transvestite, having full black-and-white male plumage (along with underdeveloped ovaries).

Frequency: Homosexual courtship in Ostriches is quite common in some populations, occurring two to four times a day (usually in the morning). Sexual behavior between male Emus has so far only been observed in captivity, but it does occur repeatedly between partners. Among Greater Rheas, joint parenting between males occurs in about 3 percent of all nests; Emu coparenting probably occurs at a similar rate

Orientation: In Ostriches, 1—2 percent of all adult males engage in homosexual courtship in some populations. Male Ostriches who court other males typically ignore any females that may be present; they are probably solitary birds that participate in little, if any, heterosexual interactions. Most Emus and Rheas that participate in male coparenting have probably mated and/or paired with females earlier in the season prior to parenting with another male. Male Emus may also have a latent capacity for bisexuality, as evidenced by the occurrence of sexual behavior between captive males (at least one of whom had previously mated heterosexually). However, individual life histories and the full patterns of sexual orientation have not yet been systematically studied in this species.

Nonreproductive and Alternative Heterosexualities

Heterosexual mating in Ratites occurs in the context of an extraordinary variety of complex social arrangements that deviate significantly from the nuclear-family model. Ostriches have a mating system that has been described as SEMIPROMISCUOUS MONOGAMY. Male and female Ostriches form a type of pair-bond with each other that one biologist describes as a sort of “open marriage,” since both partners also copulate with a number of other birds besides their primary partner (birds often mate with a different primary partner each year as well). In addition, females often lay eggs in nests other than their own, especially if they are not the primary partner of a male. As a result, many of the eggs that a pair incubates (and the young they raise) are not necessarily their own. Adoption also occurs when broods are combined to form nursery groups or CRÈCHES—sometimes containing hundreds of chicks—that are looked after by one or more adults. Emus utilize SERIAL POLYANDRY in their mating system: a male pairs with one female who remains with him until incubation begins, at which point the female leaves her partner and pairs with a new male to begin a second clutch. Many females also seek nonmonogamous matings, copulating with males other than their pair-mate. One study found that the majority of copulations—nearly three-quarters—are promiscuous. In addition, copulation between pair members may be nonprocreative, occurring several months before egg laying. Greater Rheas have a variable mating system that can be characterized as SERIAL POLYGYNANDRY: a male associates with a “harem” of three to ten females, all of whom he mates with. The females lay their eggs communally in one nest; after the male begins to incubate, the females then move on to another male, repeating the process with up to seven different males. As noted above, most Emu and Rhea males are single parents, which can be an arduous task. While tending the eggs, male Rheas rarely leave the nest for more than a few minutes during the six-week incubation period. Male Emus often become severely emaciated and weakened from not eating, drinking, defecating, or leaving the nest during their entire eight-week incubation period. Nonbreeding and failed breeding attempts also occur at high rates among Greater Rheas: less than 20 percent of males even try to reproduce each year, and overall only 5—6 percent of males are successful at breeding each year.

As discussed above, sex-segregated flocks are common among Ostriches, many of whom are not involved in heterosexual pursuits. Also, heterosexual courtship is often not synchronized: females typically begin approaching males several weeks before the latter become sexually interested, and during this time the males often appear to ignore or be indifferent to the females’ advances. The onset of the males’ sexual cycle is marked by a red flush on the legs and face, as well as enlargement and erection of the penis, which is often displayed in a special “penis-swinging” ceremony. However, once males begin courting, nearly a third of their advances are, in turn, refused by females. Among Emus and Rheas, outright hostility often develops between the sexes once the male starts incubating. Fathers typically threaten, chase, or attack females that try to approach them, while female Emus have been seen responding with vicious double-footed kicks that can tumble males head over heels. Infanticide also sometimes occurs: females that are able to get close to a male tending his chicks may end up killing the youngsters. Egg abandonment or destruction takes place among Ostriches, often of eggs laid by another female. Abandonment also occurs in Greater Rheas, where nearly two-thirds of nests are deserted by males during incubation. In addition, female Rheas who are unable to find a nest and male caretaker for their eggs often lay them in the open and then abandon them; these are known as ORPHAN EGGS. Once (nonorphan) eggs hatch, fathers often adopt youngsters from other broods, raising them alongside their own. Nearly a quarter of male Rheas are adoptive parents, and up to 37 percent of each of their broods may be composed of foster chicks. Researchers have found that adopted young actually have a better chance of surviving than do their stepsiblings.

HUMBOLDT PENGUIN