Causes and consequences of sex ratio bias in Nazca boobies

Nazca boobies are socially and genetically monogamous, long-lived pelagic seabirds with bi-parental care and similar parental roles. A large colony on the island of Española, Galápagos has been the focus of long-term reproductive and demographic studies. The adult sex ratio at this colony has been significantly male biased for more than twenty years. Knowledge of the ontogeny of this bias provides critical context that allows proper framing of hypotheses regarding parental sex allocation, the relative reproductive value of sons and daughters, mating systems, individual fitness and endangered species management. Nazca booby parents might adaptively alter the sex of their offspring according to current environmental or demographic conditions, or sex-specific mortality may occur in poor food years. Genetic sex determination of nestlings and fledglings showed that these processes could not account for the male-biased adult sex ratio in this population. Instead, the sex ratio bias arose after the period of parental care ended, during the juvenile/subadult stage between fledging and return to the colony. The deficit of female recruits is apparently a consequence of sex-specific post-fledging mortality. This finding has important implications for mate competition. The proportional representation of males among the adults ready to mate in a population at a given moment is a central concept in explaining variation in sex roles, the intensity of mating competition, and mate choice. Given the extensive period (approximately six months) of parental care in Nazca boobies, both males and females should benefit from choosiness in mate selection. The sex ratio bias in this population should allow females to choose the best available partner from a pool of potential mates. Behavioral observations and analysis of long-term mating patterns revealed that divorce was more common in this population than is typically expected for monogamous seabirds. Costs of reproduction provided a basis for females to adaptively switch mates, replacing a temporarily depleted male for a current non-breeder in better condition. Behavioral observations revealed that mate choice and divorce were driven primarily by female choice. Females appeared to use several long-term indicators of health and nutritional status to distinguish among potential mates. Males selected as mates were in better body condition, had lower circulating immunoglobulin G levels, and, as a group, had lower variance in serum albumin concentration than did unselected males. This body of work included the most complete documentation of the ontogeny of the sex ratio across the lifespan for any wild bird species, provided evidence for a previously undescribed mating system, uniquely investigated the behavior of mated pairs and non-breeding adults prior to divorce, and was the first study to examine the morphological and hematological parameters of individuals during the process of mate selection in a wild population.

Grantor dc:publisher

Wake Forest University

Year dc:date.issued

2008

Author dc:creator

• Maness, Terri

• sex ratio

Language dc:language.iso

en_US

Handle dc:identifier.uri

http://hdl.handle.net/10339/14724

OAI identifier oai:identifier

oai:wakespace.lib.wfu.edu:10339/14724