Biological Exuberance: Animal Homosexuality and Natural Diversity

Social Organization

Western Gulls and Kittiwakes form pair-bonds and nest in colonies, some of which contain upwards of 10,000 pairs; Kittiwakes often nest on cliffs. Outside of the breeding season they are less sociable, occasionally gathering in loose aggregations when not solitary.

Description

Behavioral Expression: Female Western Gulls sometimes form homosexual pairs, as do Kittiwakes. In Western Gulls, the females participate in courtship, sexual, and parenting behaviors similar to those of heterosexual pairs in their basic patterns, yet different in many details. Two females court one another by performing HEAD-TOSSING (a stylized bobbing of the head with the bill pointed skyward) and COURTSHIP-FEEDING (in which a small amount of food is regurgitated and offered as a “gift” to the partner). In heterosexual pairs, males usually perform more courtship-feeding and females more head-tossing. In homosexual pairs, both birds perform these behaviors—often with equal frequency—although the overall rate of courtship behaviors for each female is similar to females in heterosexual pairs. Homosexual courtship-feeding differs from the heterosexual pattern in that a female does not offer as large an amount of food to her female partner and may even swallow the “offering” herself rather than give it to her mate. In some female pairs, one partner regularly mounts the other and may even utter the copulation call characteristic of heterosexual matings. Some females adopt unique mounting positions such as sideways or head-to-tail (not seen in heterosexual matings), and genital contact does not usually occur. Like heterosexual pairs, female couples establish territories that they defend against intruders. Both females spend a great deal of time on their territories (typical only of females in heterosexual pairs), while both also exhibit aggressive reactions to intruders (more typical of males in heterosexual pairs). Once a homosexual pair-bond is established, it usually persists for many years, and the two females return to the same territory each season: one study that tracked eight homosexual pairs found that seven of them remained together for more than one breeding season.

Homosexual pairs usually build a nest in which both females lay eggs; the resulting SUPERNORMAL CLUTCH contains 4—6 eggs in Western Gulls, up to twice the number found in nests of heterosexual pairs. Some of these eggs are fertile because females in homosexual pairs occasionally copulate with males (without breaking their same-sex bond). Although eggs laid by female pairs may be smaller than those of heterosexual females, homosexual parents successfully hatch and raise chicks, sharing all parental duties.

A homosexual pair of female Western Gulls in California

Frequency: As many as 10—15 percent of Western Gull pairs in some populations are homosexual; the percentage is much lower in Kittiwakes, about 2 percent of all pairs.

Orientation: Most female pairs in Kittiwakes are exclusively homosexual, never mating with males and laying only infertile eggs; the same is true for many pairs in Western Gulls. However, up to 15 percent of eggs laid by Western Gull same-sex pairs are fertilized, so at least some females are simultaneously bisexual—copulating with males while retaining their homosexual pair-bond.

Nonreproductive and Alternative Heterosexualities

Not all heterosexual birds in these species form lifelong, monogamous pair-bonds within which they raise their own young. About 30 percent of Kittiwake male-female pairs divorce. Some birds form polygamous trios consisting of one male bonded with two females, each with her own nest (about 3 percent of all bonds). Western Gull pairs (and rarely, single males) sometimes adopt and raise chicks that are not their own, and “stepmothering” occurs when females pair with a male that has lost his mate; foster-parenting also occurs in Kittiwakes (where about 8 percent of all chicks are adopted). In addition, many birds do not reproduce or do so only rarely: 30—40 percent of adult Western Gulls breed only once or twice in their lifetimes, and do not successfully raise any offspring, while 5 percent of all Kittiwakes that attempt to nest do not raise any young during their lives (and nearly two-thirds of all Kittiwakes never produce any offspring, usually because they die before breeding). Female Western Gulls that breed less often (or defer breeding until later in life) actually have higher survival rates than birds that reproduce more frequently. In Kittiwakes, nonbreeding birds form their own flocks or CLUBS on the outskirts of the breeding colonies. Western Gulls in heterosexual pairs also sometimes engage in nonprocreative sexual behaviors, such as REVERSE mounting (where the female mounts the male, typically without genital contact).

Some male Western Gulls are promiscuous, attempting to copulate with females other than their mates (usually birds on neighboring territories), although they are frequently unsuccessful. Occasionally a male will behave aggressively toward a female he has just mated with (nonmonogamously) and may even attack and kill her. Overall, more than 40 percent of aggressive incidents occur between members of the opposite sex. These include females defending themselves against promiscuous males, males attacking neighboring females that are courting them, and territorial disputes. Females may also refuse to copulate with their own mates, either by not allowing them to mount or by walking out from under them during mating. Some pairs, however, begin copulating even before the female’s fertile period; this also occurs in Kittiwakes, and many copulations in this species do not involve genital contact (more than 30 percent). In Kittiwakes, 15—27 percent of all heterosexual copulations are harassed and interrupted by other males. Occasionally, adult Western Gulls are violent toward chicks, who may be attacked and even killed if left alone by their parents. Kittiwake parents (especially inexperienced ones) sometimes neglect their chicks (e.g., starving them), and birds may also attack or toss their own or other parents’ chicks off cliffs. In fact, many adoptions and chick deaths in both species result from youngsters deserting or “running away” from their biological families as a result of neglect or direct attack. As many as a third of all Kittiwake chicks in some colonies abandon or are driven from their own nests. In addition, adults of both species may eat unattended eggs belonging to other parents.

Sources

*asterisked references discuss homosexuality/transgender

*Baird, P. H. (1994) “Black-legged Kittiwake (Rissa tridactyla).” In A. Poole, P. Stettenheim, and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, no. 92. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

Carter, L. R., and L. B. Spear (1986) “Costs of Adoption in Western Gulls.” Condor 88:253—56.

Chardine, J. W. (1987) “The Influence of Pair-Status on the Breeding Behavior of the Kittiwake Rissa tridactyla Before Egg-Laying.” Ibis 129:515—26.

———((1986) “Interference of Copulation in a Colony of Marked Black-legged Kittiwakes.” Canadian Journal of Zoology 64:1416—21.

*Conover, M. R. (1984) “Occurrence of Supernormal Clutches in the Laridae.” Wilson Bulletin 96:249—67.

*Coulson, J. C., and C. S. Thomas (1985) “Changes in the Biology of the Kittiwake Rissa tridactyla: A 31 Year Study of a Breeding Colony.” Journal of Animal Ecology 54:9—26.

———(1983) “Mate Choice in the Kittiwake Gull.” In P. Bateson, ed., Mate Choice, pp. 361—76. Cambridge: Cambridge University Press.

Coulson, J. C., and E. White (1958) “The Effect of Age on the Breeding Biology of the Kittiwake Rissa tridactyla.” Ibis 100:40-51.

Cullen, E. (1957) “Adaptations in the Kittiwake to Cliff-Nesting.” Ibis 99:275— 302.

*Fry, D. M., C. K. Toone, S. M. Speich, and R. J. Peard (1987) “Sex Ratio Skew and Breeding Patterns of Gulls: Demographic Toxicological Considerations.” Studies in Avian Biology 10:26— 43.

Hand, J. L. (1986) “Territory Defense and Associated Vocalizations of Western Gulls.” Journal of