Abstract
Male insects with large weapons such as horns and elongate mandibles would be expected to invest more on such structures than other parts of the body for advantages in male to male competition for mating. In male genitalia, however, intermediate size provides a better fit for more females than small or large sizes, and such a male would leave more offspring regardless of their body size. These predictions were tested using a static allometry analysis between body size and other trait sizes. Acanthacorydalisasiatica is a large dobsonfly (Megalotera) and males have conspicuously large mandibles used as weapons. We examined the hypothesis that the male mandibles of this sexually dimorphic species are sexually selected to enlarge, whereas the male genitalia are stable to be intermediate regardless of a great variation in body size. The results, as predicted, showed positive allometry between male body size and mandible length but negative allometry between male body size and ectoproct length (a male grasping structure). Sperm are transferred through a small spermatophore attached externally to the female genital opening, so it may be evolutionarily unnecessary to develop an enlarged male genital size. In contrast, there may be a trade-off between male mandible size and wing length, because of negative allometry between body size and wing length in males but isometry between them in females.
Highlights
Many groups of animals develop secondary sexual traits mostly in males but a few in females
coefficient of variation (CV) was largest in ML and smallest in GL, suggesting the mandible length varies individually, but the ectoproct length is rather stable
Males of Acanthacorydalis species combat each other for access to females or limited resources that attract females (Liu et al 2015). They use mandibles as weapons for their combat and the exaggerated mandibles may be favored by sexual selection, because males had longer mandibles than females and the allometry between body and mandible sizes was positive in males but isomeric in females
Summary
Many groups of animals develop secondary sexual traits mostly in males but a few in females (reviewed by Emlen 2008; Rico-Guevara and Hurme 2019). There are usually isometric relationships between body size and other body parts In this case, the body proportion does not differ between small and large individuals. Male sexually selected traits such as horns, mandibles, and visual display devices have generally positive allometry with body size (reviewed by Emlen and Nijhout 2000; Kodric-Brown et al 2006; Bonduriansky 2007; Voje 2016; Eberhard et al 2018; O’Brien et al 2018), the presence or absence of positive allometry cannot be used to infer the presence or absence of sexual selection if the function of the traits is unknown (e.g., Bonduriansky 2007). If intermediate-sized genitalia provide a better fit or better tactile stimulation for more females in the population than small genitalia or large genitalia, males with intermediate-sized genitalia would leave more offspring regardless of their body size (Eberhard et al 1998)
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