One Size Fits All? Relationships Between the Size and Degree of Variation in Genitalia and Other Body Parts in Twenty Species of Insects and Spiders

Abstract

Hypotheses regarding the function of elaborate male genitalia were tested in a sample of insects and spiders by comparing their allometric values (slopes in log-log regressions on indicators of body size) with those of other body parts. Male genitalia consistently had lower slopes than other body parts. Perhaps as a consequence of this pattern, genitalic size also tended, though less consistently, to have lower coefficients of variation than did the size of other body parts. The morphological details of coupling between males and females in several species clearly indicated that selection favoring mechanical fit is not responsible for these trends. Sexual selection on male courtship structures that are brought into contact with females in precise ways may favor relatively low allometric values, in contrast to the high values seen in the other sexually selected characters (usually visual display devices) that have been studied previously, because a female's own size will influence her perception of the contact courtship devices of a male.

Daily production of spermatophores, sperm number and spermatophore size in two eriophyoid mite species

“…Similarly, a low coefficient of variation, on average 6.5% for various genital traits, was found across 20 species of insects and spiders (Eberhard et al 1998), and 5.9% for seven genitalic traits of the water strider Gerris incognitus Drake and Hottes (Arnqvist and Thornhill 1998). Also House and Simmons (2003) found a comparably low level of variation in the genital sclerites of the dung beetle Onthophagus taurus (Schreber).…”

“…Spermatophores can be regarded not only as a packaging for spermatozoa but also as male genitalia that fit to the genitalia of females and enable female insemination (Eberhard 1985;Proctor et al 1995;Peretti 2003). In eriophyoid mites, both the head and the stalk of a spermatophore appear to play important roles in the act of female self-insemination.…”

“…ejaculate volume, sperm number, testis size or copula duration) tend to have high coefficients of phenotypic variation (CV). This does not refer, however, to genitalia that, similarly to non-sexual traits, usually have low coefficients of phenotypic variation (Pomiankowski and Møller 1995;Eberhard et al 1998;Simmons and Kotiaho 2002;Arnaud et al 2001).…”

“…Also House and Simmons (2003) found a comparably low level of variation in the genital sclerites of the dung beetle Onthophagus taurus (Schreber). The low variability in genital traits could be explained by a stabilizing sexual selection favoring an intermediate, standard size of male and female genitalia where 'one size fits all' (Eberhard et al 1998). One cannot exclude, however, the balance between sexual and natural selection, in which variation in genital traits is generated only to the extent to which the mechanics of coupling and sperm transfer is not disrupted (House and Simmons 2003).…”

Quantification of cranial and tomiodont dimorphism in Testudines using the Midland Painted Turtle, Chrysemys picta marginata

“…To evaluate dispersion of data points around the RMA best fit regression line, the CV 0 was calculated for each putatively non-sexual, putatively sexual, and known sexual trait. The CV 0 accounts for variation in y when x is held constant and is suitable for evaluating variation in allometric slopes for sexual and non-sexual traits (Eberhard et al 1998; also see Tasikas et al 2009;Schulte-Hostedde et al 2011). Generally, low phenotypic variance and isometry or negative allometry are expected from putatively non-sexual traits if they are truly non-sexual.…”