The complexity of mating decisions in stalk‐eyed flies

Abstract

All too often, studies of sexual selection focus exclusively on the responses in one sex, on single traits, typically those that are exaggerated and strongly sexually dimorphic. They ignore a range of less obvious traits and behavior, in both sexes, involved in the interactions leading to mate choice. To remedy this imbalance, we analyze a textbook example of sexual selection in the stalk‐eyed fly (Diasemopsis meigenii). We studied several traits in a novel, insightful, and efficient experimental design, examining 2,400 male–female pairs in a “round‐robin” array, where each female was tested against multiple males and vice versa. In D. meigenii, females exhibit strong mate preference for males with highly exaggerated eyespan, and so we deliberately constrained variation in male eyespan to reveal the importance of other traits. Males performing more precopulatory behavior were more likely to attempt to mate with females and be accepted by them. However, behavior was not a necessary part of courtship, as it was absent from over almost half the interactions. Males with larger reproductive organs (testes and accessory glands) did not make more mating attempts, but there was a strong tendency for females to accept mating attempts from such males. How females detect differences in male reproductive organ size remains unclear. In addition, females with larger eyespan, an indicator of size and fecundity, attracted more mating attempts from males, but this trait did not alter female acceptance. Genetic variation among males had a strong influence on male mating attempts and female acceptance, both via the traits we studied and other unmeasured attributes. These findings demonstrate the importance of assaying multiple traits in males and females, rather than focusing solely on prominent and exaggerated sexually dimorphic traits. The approach allows a more complete understanding of the complex mating decisions made by both males and females.

Handicap theory is applied to females but not males in relation to mate choice in the stalk-eyed fly Sphyracephala detrahens

“…3B) was measured, and its correlation with eye span was examined (coefficient of determination = 0.44, P value = 0.0018). The results showed that eye span and reproductive capacity were correlated in both sexes, as shown in previous studies in other stalk-eyed fly species 15,19,25,26 .…”

Limits to environmental masking of genetic quality in sexual signals

“…If some degree of stress is typical of ecological conditions in nature, sexual selection could often be stronger than currently estimated from laboratory experiments—usually carried out under low stress conditions of ad libitum food and constant temperature and without predators, parasites or ecological competitors. We note that our experimentation used stress from a unimodal environment variable (food availability), controlling all other physical and biotic factors, and that we used a simple measure of male signalling, leaving aside other, more subtle aspects of male behaviour used in female evaluation of their partners (Chapman et al, ). Thus, the majority of environments probably lie between the low and high stress regimes, which is consistent with the considerable range in eyespan observed among wild‐caught stalk‐eyed flies (Cotton et al, ).…”

“…After eclosion, flies of each cross were collected and frozen at −20°C. All males were measured for eyespan (the distance between the outermost tips of the eyes, Chapman et al, 2017;David et al, 1998) and thorax (the distance between the centre of the most posterior point of the head to the joint between the meta-thoracic legs and the thorax, Meade et al, 2017;Rogers, Denniff, Chapman, Fowler, & Pomiankowski, 2008) to a tolerance of 0.01 mm, using a video camera mounted on a monocular microscope and imagej image capture software v.1.46 (Schneider, Rasband, & Eliceiri, 2012). The repeatability of these morphological trait measurements is very high at >99% (David et al, 1998).…”

Does meiotic drive alter male mate preference?

“…While there was no difference in the preference of SR and ST males, we did find that eyespan affected male preference for large females, with large eyespan males showing the strongest preference and small eyespan males exhibiting no preference at all. Vision is the dominant sensory mode for assessment of potential mates in stalk-eyed flies (Chapman et al 2005; Chapman et al 2017), and both stereoscopic vision and visual acuity are improved as eyespan increases (Burkhardt and de la Motte, 1983; de la Motte and Burkhardt, 1983). It follows that males with larger eyespan will be better able to distinguish differences between females and express stronger preference.…”