Experimentally Induced Density-Dependent Mating Patterns in Phyllobius maculicornis Germ. (Coleoptera: Curculionidae)

Abstract

The mating pattern of the weevil Phyllobius maculicornis Germ. was monitored in the field following experimental reduction of population density. As density decreased, a tendency for size-assortative mating emerged. Body size of mating and nonmating animals was not significantly different within the sexes. However, on average, mating females were significantly larger than nonmating females when population density decreased, indicating male selection under low density conditions. When mating pairs were isolated in the laboratory, durations of the copulations were positively associated with female body size. Males in this way put extra effort into fertilizing eggs of large females. INTRODUCTION For insect species in which males perform no parental care, the number of inseminated females is the prime determinant of male fitness (Trivers, 1972). Sexual selection theory predicts intrasexual competition among the low investment sex (typically males), and usually the competitive ability of male insects is related to their body size (Mason, 1980; McLain, 1982; Johnson, 1983; Thornhill and Alcock, 1983; Lawrence, 1986; Pinto and Mayor, 1986; Simmons, 1986). Theory also includes the possibility that females choose among possible mates, thereby enforcing selective pressures on males (Majerus, 1986). Since female body size in insects usually is positively correlated with fecundity (Freeman, 1981; Evans, 1982; O'Neill, 1984; Butlin and Day, 1985; Juliano, 1985), males are expected to prefer large females. Such a choice has been demonstrated in tropical weevils (Johnson and Hubbell, 1984). Several studies of insects have revealed size-assortative mating patterns, in which body size of male and female is correlated in mating pairs (Thornhill and Alcock, 1983; Partridge and Halliday, 1984). Few studies, however, offer more detailed analyses of variations in recorded mating patterns. Numerous selective pressures obviously influence fitness of insects (e.g, habitat changes, food availability, parasite/predator incidence, or the dynamics within the population itself). Differences in male behavior and mating success, for example, may depend upon local sex ratios (Lawrence, 1986), interspecific interference (McLain, 1981), weather conditions (Banks and Thomson, 1985) or differences in female oviposition behavior (Fincke, 1985). Density-dependent differences in mating patterns have been reported in soldier beetles. Positive phenotypic assortative mating, based on various body size measurements, was more pronounced in low density populations (McLain, 1982, 1984). In general, the level of male competition and the variance in male fitness are thought to be positively related to increased density, as the disparity between successful and unsuccessful males increases (Wade, 1980). The occurrence of alternative male mating patterns at high densities is reported in several insect orders (Alcock et al., 1977; Greenfield and Shelly, 1985). I measured the direct effects of experimental changes in population density on mating pattern in a field study of the weevil Phyllobius maculicornis Germ. Density manipulations were conducted in the field, and additional observations of mating behavior were performed in the laboratory. The basic hypothesis tested was an expected change in mating pattern as density was reduced. MATERIALS AND METHODS An isolated population of Phyllobius maculicornis was monitored during the mating