The evolution of sex created a fundamental problem for nearly all plants and animals; namely, the need to fertilize eggs. Numerous types of mating systems have evolved as solutions to this problem, each molded by particular environmental circumstances and particular species attributes. Various hypotheses have been advanced to explain each type of mating system, but an integrated theory is only now beginning to emerge. Since mating behavior is affected by nearly all other aspects of an organism’s behavioral adjustments to its environment, such a theory must fit within a composite view of animal social behavior and hence must include explicit points of contact with related bodies of theory. In particular, mating system theory must mesh with theoretical advances concerning the evolution of territoriality, parental behavior, and animal sociality. By including the appropriate theoretical work from these other areas, an integrated theory of vertebrate mating systems can be developed.

The Evolution of Mating Systems in Birds and Mammals

Recent theoretical insights are helping to clarify how animal mating sys­ tems evolve. An integrated theory is beginning to emerge from the realiza­ tion that males and females have overlapping but nonidentical reproductive interests and that the type of mating system shown by a species results from interactions between the individual interests of each sex. The optimal mat­ ing system for promoting individual reproductive interests often differs for each sex, and when it does, the interests of one sex constrain the reproduc­ tive options open to the other. Thus, mating system theory revolves around two major issues: the factors determining which sex predominates in shap­ ing each mating system, and the factors determining which mating system is optimal for members of the "controlling" sex. The greatest progress has been made in explaining how polygynous mat­ ing systems evolved, particularly among territorial vertebrates, and more recent work is clarifying the specific environmental conditions involved (4, 46, 145, 146, 161, 227, 244, 246, 247). Promiscuous and polyandrous mating systems are less well understood, but several promising avenues toward a general theory have begun to develop (7 1, 9 1 , 126, 240, 245, 246). In contrast, theoretical work on the evolution of monogamy lags far behind, despite an enormous literature on monogamous animals.

The Evolution of Monogamy: Hypotheses and Evidence

Brood reduction in birds: Selection for fratricide, infanticide and suicide?

Obligate siblicide, the unconditional killing of an individual by its sibling, occurs in a small number of bird species. Although clutches of two eggs are frequently laid, obligate siblicide eliminates the younger hatchling before it reaches the age of independence. The phenomenon presents a challenge to adaptationist evolutionary theory because parents produce chicks that are virtually certain to be killed and because surviving offspring actively sacrifice the inclusive-fitness increment represented by the victim. Dorward's (1962) insurance-egg hypothesis explains the production of a second egg as insurance against the first egg's failure. The insurance-egg hypothesis predicts that, if a second egg sometimes produces a fledgling after a first egg fails, then the evolution of insurance-egg production in single-chick species is governed by the ratio of the benefit of the increased probability of producing a hatchling to the cost of egg production. A field study of the obligately siblicidal masked booby (Su...

Evolution of Obligate Siblicide in Boobies. 1. A Test of the Insurance-Egg Hypothesis

Behavior is one component of adaptation to the environment; thus examinations of particular kinds of behavior provide insights into the evolution of species. Comparing behaviors among animals provides a framework for generating ideas about how animals adapt. In birds, parental care involves considerable time and energy, and thus provides a good opportunity to examine the diversity, mechanisms, and adaptative significance of behavior. Parental care refers to all behaviors that contribute to the maintenance and survival of their young. Most references to parental care in the literature are limited to the early phase of care of the nestlings, whereas a few studies examine care up until “fledging.” In its broadest sense, parental care includes any behavior that contributes to the survival of the young once they have departed from the nest and are partially independent of their parents.

The Transition to Independence and Postfledging Parental Care in Seabirds

Summary

This paper summarises observations on the breeding behaviour of the Pink-backed Pelican Pelecanus rufescens at Rakewa, Nyanza Province, Kenya, where the species has bred for at least 200 years. Observations covered most of one breeding season, November 1962 to April 1963.



Of at least 250 nests, 35 were closely observed. The community consisted of about 815 pelicans of which about 540 were adults. The death rate is estimated at 13% per annum and the mean life-span at about seven and a half years.



The breeding site, in trees above a small swamp, is 15 miles from the favoured feeding ground. The colony is protected by local Luo people.



The pelicans feed and roost mainly at the Miriu Delta, 15 miles away, travelling between the two places so high up as to be unseen. They fish in the early morning, visiting the colony to feed young mainly between 09.00 and 13.00 hrs. Once the young are large both parents roost away from the colony at the Delta.



The breeding season takes place from August, towards the end of the rains, to March, at the end of the dry season. The birds breed in synchronised groups, the breeding cycle for any group occupying five months.



Nuptial display is performed on the nest trees, by single pairs or small groups. Two main displays are described, “pointing” and “bill-clapping”. Mating occurs on the nest, with little preliminary display.



Nests are slight stick structures, repaired from year to year, and used by other pelicans if abandoned. The clutch is normally two eggs, occasionally three. Both sexes incubate, with infrequent change-overs, for 33–35 days.



The chick is first brick-red, becoming covered with white down. Feathers break through at about 12 days and have covered much of the body by 30 days. At 40 days chicks can recognise their own parent. They fly at 70–75 days.



Parents feed chicks by regurgitation, sometimes into the nest. They brood them closely at first, but after 10–12 days leave them much alone. Large chicks thrust the head far into the parental gullet, and injuries result from such feeding struggles. Feeding usually occurs before mid-day, each parent normally delivering two feeds with a rest between. Curious convulsive movements of the young are probably begging displays.



Forty-two young hatched in 35 nests, an average of 0.6 chicks/egg laid. The heaviest mortality among young occurred between 10–30 days when 31% of all chicks died. Young which flew were produced at the rate of 0.47/egg hatched, 0.28/egg laid, and 0.57/pair.

L. H. Brown

Papers

Observations on the breeding of the pink‐backed pelican pelecanus rufescens