SEX RATIO VARIATIONS IN XYLOPHILOUS ACULEATE HYMENOPTERA

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. Studies often assume a 1:1 sex ratio, basing such assumptions on whole population studies for a single year, or for several years. Such broad, long term data may obscure important variations in sex ratios and investment ratios in local populations over a single generation, as discussed below. Xylophilous aculeate Hymenoptera normally construct nests consisting of a linear series of provisioned cells in abandoned beetle burrows or other holes in wood. Many such wasps and bees will accept artificial nesting sites (trap-nests) and are well suited to an examination of investment ratios, particularly since sex determination based on haplodiploidy and facultative insemination of eggs allow female manipulation of brood sex ratios. Krombein (1967) discusses general biological data on many species. Eggs of a given sex are laid in series within a single nest, rather than at random; female-containing cells are larger than male-containing cells, and usually occur toward the blind end of a nest. Adult females searching for nest sites will accept holes of various diameters, within a certain range, each species apparently preferentially selecting holes of given diameters (Longair, unpubl.). Previous studies have used similar methods, offering bundled traps of different diameters. However, these studies have suffered from two major failings, if data are to be used in determination of population sex ratios. Traps of different diameters were usually offered in different relative numbers (e.g., twice as many 5 mm traps as 7 mm traps). As various authors (Medler and Fye, 1956; Koerber and Medler, 1958; Fye, 1965; Krombein, 1967) have pointed out, and as shown here, sex ratio tends to vary with nest diameter, such that whereas cells containing females are larger than cells containing males, a greater proportion of male eggs are laid in traps with a smaller diameter boring. Therefore, greater numbers of one hole diameter could have affected overall estimates of sex ratios. Similarly, hole sizes offered in previous studies have often been only of a limited range, failing to fully encompass those diameters accepted by searching females. This again could bias sex ratio estimates. The mode of presentation of traps in this study was specifically intended to provide an adequate range of hole diameters, with equal numbers of each trap diameter offered; resulting distribution of hole diameter use is thus unbiased with respect to those factors discussed above, and subsequent sex ratio estimates are based on natural nest size use distribution for the species studied.