Abstract
Common scientific wisdom assumes that spider sexual dimorphism (SD) mostly results from sexual selection operating on males. However, testing predictions from this hypothesis, particularly male size hyperallometry, has been restricted by methodological constraints. Here, using geometric morphometrics (GMM) we studied for the first time sex-differential shape allometry in a spider (Donacosa merlini, Araneae: Lycosidae) known to exhibit the reverse pattern (i.e., male-biased) of spider sexual size dimorphism. GMM reveals previously undetected sex-differential shape allometry and sex-related shape differences that are size independent (i.e., associated to the y-intercept, and not to size scaling). Sexual shape dimorphism affects both the relative carapace-to-opisthosoma size and the carapace geometry, arguably resulting from sex differences in both reproductive roles (female egg load and male competition) and life styles (wandering males and burrowing females). Our results demonstrate that body portions may vary modularly in response to different selection pressures, giving rise to sex differences in shape, which reconciles previously considered mutually exclusive interpretations about the origins of spider SD.
Highlights
Sexual dimorphism (SD), defined as size or morphological differences between the sexes (Hedrick & Temeles, 1989) is a large source of phenotypic variation in animals
As a relevant topic in evolutionary biology ever since Darwin (Lande, 1980; Lande & Arnold, 1985; Hedrick & Temeles, 1989; Fairbairn, Blanckenhorn & Szekely, 2007), the evolution of SD is known to result from genetic correlations between the sexes and sex-differential selection pressures, natural selection related to reproductive sex roles or life styles and sexual selection, related to mate competition (Lande, 1980; Lande & Arnold, 1985; Hedrick & Temeles, 1989; Cox, Skelly & John-Alder, 2003; Foellmer & Fairbairn, 2005a; Fairbairn, Blanckenhorn & Szekely, 2007)
Using shape analysis geometric morphometrics (GMM), we have quantified for the first time that, (1) spider males and females unambiguously differ in body shape, (2) such shape differences relate to two main features: the carapace-toopisthosoma ratio and the carapace shape, (3) a portion of spider shape variation is allometric, but sexual shape differences are completely independent on size, and (4) the shape allometric intercepts are sexually dimorphic, whereas the slopes— scaling—are not
Summary
Sexual dimorphism (SD), defined as size or morphological differences between the sexes (Hedrick & Temeles, 1989) is a large source of phenotypic variation in animals. Spiders follow the arthropod female-biased pattern of sex differences in size, but spider sexual size dimorphism (SSD) spans from moderately male-biased (Alderweireldt & Jocque, 1991a; Schutz & Taborsky, 2003; Aisenberg, Viera & Costa, 2007) to the most extremely female-biased terrestrial patterns (Foellmer & Fairbairn, 2005a; Vollrath, 1998). Spider SSD fails to follow a common trend of phenotypic variation resulting from sexual selection, namely the higher variation of male size over evolutionary times. This trend would lead to a reduced SSD the larger the body size in such a female-biased taxon (i.e., Rensch’s Rule (Rensch, 1959)). Spider female size tends to vary more than male’s within and across taxa (Abouheif & Fairbairn, 1997), which might be consistent with a prevalent role of fecundity selection in the evolution of spider SSD (Head, 1995; Coddington, Hormiga & Scharff, 1997)
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