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

TLDR

Sexual selection on male courtship structures that contact females in precise ways may favor relatively low allometric values, contrasting with the high values of other sexually selected visual displays, because a female’s size influences her perception of the male’s contact devices. The authors tested hypotheses about elaborate male genitalia by comparing their allometric slopes from log‑log regressions on body size indicators with those of other body parts in a sample of insects and spiders. Male genitalia consistently showed lower allometric slopes and, to a lesser extent, lower coefficients of variation than other body parts, and morphological coupling details in several species indicate that mechanical fit selection does not explain these trends.

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.