- Colorado State University
Summary
Organisms which reproduce sexually may often provide differing relative investments in offspring of the two sexes,and factors influencing the proportions of these investments are poorly understood.Theoretical discussions (Trivers,1972; Charnov,1979) have rarely been supported by adequate field evidence (but see Cowan,1978; Alcock, 1979). Fisher(1958) initially described how, in random mating populations of sexual species, a 1:1 investment ratio should evolve and be maintained (evolutionarily stable strategy sensu Maynard Smith and Price, 1973).This theory has been elaborated upon, incorporating such factors as parental condition, offspring mortality, and structure of mating populations (Hamilton,1967;Trivers and Willard,1973;Werren and Charnov, 1978).The results, as Werren and Charnov(1978) express it, is that for a population to be in evolutionary equilibrium, it should be devoting half of the parental reproductive resources to each sex.However,a 1:1 investment ratio may not always be the optimum. Under circumstances of local mate competition, differential benefits between sexes from a resource, greater mortality of offspring of one sex during periods of parental care, or unusual sex determining mechanisms (see Maynard Smith,1978), biased investment ratios may evolve.
Methodology
Pine blocks, each drilled with a single hole 95 mm in depth,of one of five diameters (3 mm, 5 mm, 7mm, 9 mm and 11 mm) were placed in the field at Lake Opinicon,Ontario,Canada during the summer of 1977. These traps were bundled in groups of five,one of each hole diameter per bundle,with all openings facing in the samedirection.Filledtrapswereremovedoncompletionandreplacedwithanotheremptytrapof the same-diameter boring.Developing larvae were reared in the lab,and the sex of cell occupants recorded. Since off spring of each sextend to be laid in linear series, sex of cell occupants can usually be reliably determined even where larvae die prematurely. If a cell in which a larva prematurely died occurred closer to the blind end of a nest than a reared female-containing cell, it could be assumed to be female.Conversely,if such a cell occurred closer to the opening than a reared male-containing cell,it could be assumed to be male. (See also Cowan,1978.)