Date of Award

1-1-1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Intrasexual competition among males is a dominant feature of the mating system of most animals. Far fewer species exhibit female-female competition for mates. However, species showing reversal of the typical sex roles afford special opportunities to evaluate evolutionary generalities concerning sexual selection and mating systems.I studied Wilson's phalarope (Phalaropus tricolor), a sex-role reversed, nonterritorial shorebird, from 1982-1986 at Last Mountain Lake Wildlife Management Unit in southcentral Saskatchewan, Canada. Phalaropes were individually marked and data on breeding biology were recorded during daily observation periods. I focal sampled individual male phalaropes to assess weekly changes in the intensity of female-female competition for mates. Seasonal changes in four estimates of operational sex ratio (OSR) were examined using data from censuses, individual nest histories, focal sampling, and courtship chases.The mating system of Wilson's phalarope is characterized by extreme sex-role reversal. Females are more brightly plumaged and significantly larger than males, and compete for mates directly, rather than via resources. Males provide virtually all parental care for eggs and chicks. Nesting success differed among years owing to changes in clutch loss to predators. Clutches that were initiated early in the breeding season experienced greater risk of predation. On average, 16% of males renested following clutch loss; some renested twice. The weight of males declined during incubation.During the breeding season, females routinely outnumbered males. Weekly estimates of OSR varied widely owing to the nonterritorial nature of the social system. However, within each year, the four measures of OSR were positively correlated. Intrasexual competition differed significantly among weeks. In general, intrasexual competition for mates was positively correlated with changes in OSR: as sex ratios became more female-biased, female-female competition for mates increased.Natal philopatry and adult return rates were significantly male-biased, offering support for Greenwood's (1980) hypothesis, which relates sex differences in dispersal and philopatry to a species' mating system.These results offer support for several hypotheses regarding the interaction between sex differences in behavior and a species' mating system. Further studies examining fitness consequences of behavioral patterns would be illuminating.

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