Faculty Opinions recommendation of Interactions among mechanisms of sexual selection on male body size and head shape in a sexually dimorphic fly.

1. The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reversed sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. 2. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this discrepant body of evidence in the primary literature. 3. Using pre-registered methods and hypotheses, we performed a systematic review and meta-analysis combining published (n = 19 studies, k = 106 effect sizes) and unpublished data (n = 17, k = 242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand sources of heterogeneity across effects. 4. Meta-analysis revealed an overall positive correlation between male size and reproductive performance (r = 0.23, 95% confidence interval: 0.10 – 0.35, n = 36, k = 348, 4514 males, three Gambusia species). Despite high heterogeneity, the large-male advantage appeared robust across all measures studied (i.e. female choice, mating success, paternity, sperm quantity and quality), but was considerably larger for female choice (r = 0.43, 95% confidence interval: 0.28 – 0.59, n = 14, k = 43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status, and environmental conditions. We found evidence of publication bias; however, its effect on our estimates was attenuated by including a substantial amount of unpublished effects, highlighting the importance of unpublished (open) data for more accurate meta-analytic estimates. 5. In addition to positive selection on male size, our study suggests that we need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.

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Allometry and morphological trait relationship in the sexually dimorphic Chinese dobsonfly, Acanthacorydalis asiatica (Wood-Mason, 1884) (Megaloptera, Corydalidae)

ZooKeys ◽

10.3897/zookeys.854.32897 ◽

2019 ◽

Vol 854 ◽

pp. 119-129

Author(s):

Chengquan Cao ◽

Pei Yu ◽

Fumio Hayashi

Keyword(s):

Body Size ◽

Male Genitalia ◽

Wing Length ◽

The Body ◽

Sexually Dimorphic ◽

Male Body ◽

Male Body Size ◽

Intermediate Size ◽

Mandible Length ◽

Dimorphic Species

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.

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Evolution of sexual dimorphism and Rensch’s rule in the beetle genusLimnebius(Hydraenidae): is sexual selection opportunistic?

PeerJ ◽

10.7717/peerj.3060 ◽

2017 ◽

Vol 5 ◽

pp. e3060 ◽

Cited By ~ 4

Author(s):

Andrey Rudoy ◽

Ignacio Ribera

Keyword(s):

Sexual Selection ◽

Body Size ◽

General Pattern ◽

Strong Support ◽

Male Body ◽

Rensch’S Rule ◽

Male Body Size ◽

Rensch's Rule ◽

The Individual ◽

Male Genital

Sexual size dimorphism (SSD) is widespread among animals, with larger females usually attributed to an optimization of resources in reproduction and larger males to sexual selection. A general pattern in the evolution of SSD is Rensch’s rule, which states that SSD increases with body size in species with larger males but decreases when females are larger. We studied the evolution of SSD in the genusLimnebius(Coleoptera, Hydraenidae), measuring SSD and male genital size and complexity of ca. 80% of its 150 species and reconstructing its evolution in a molecular phylogeny with 71 species. We found strong support for a higher evolutionary lability of male body size, which had an overall positive allometry with respect to females and higher evolutionary rates measured over the individual branches of the phylogeny. Increases in SSD were associated to increases in body size, but there were some exceptions with an increase associated to changes in only one sex. Secondary sexual characters (SSC) in the external morphology of males appeared several times independently, generally on species that had already increased their size. There was an overall significant correlation between SSD, male body size and male genital size and complexity, although some lineages with complex genitalia had low SSD, and some small species with complex genitalia had no SSD. Our results suggest that the origin of the higher evolutionary variance of male body size may be due to lack of constraints rather than to sexual selection, that may start to act in species with already larger males due to random variation.

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