Sexual dimorphism in the size and shape of the raptorial pedipalps of Giant Whip Spiders (Arachnida: Amblypygi)

Abstract

Sexual dimorphism in the form of elaborate crests, horns and swellings can be a clear indicator of the differing evolutionary pressures to which males and females are subject. However, dimorphism can also be expressed in more subtle shape differences not outwardly obvious to the observer. Whip spiders (Amblypygi) possess a unique pair of spined pedipalps hypothesized to primarily function in prey capture, but also serving multiple other functions. Little is known regarding the intraspecific shape variation of these limbs and its potential causes. Because a role during courtship and male contest has also been hypothesized, sexual selection may contribute to shape differences. As such, we hypothesize that sexual dimorphism will be present in the size and shape of amblypygid pedipalps, with male contest selecting for longer and thicker pedipalps, and larger spines in males. This study aims to test this hypothesis, by quantifying the contribution of ontogeny and sexual dimorphism to shape within the raptorial pedipalps of Damon variegatus. Discriminant function analysis using GMM landmark data reveals statistically significant sexual shape dimorphism in both the tibia and femur of the pedipalp. Contrary to our hypothesis, males display a more gracile pedipalp morphology with reduced spination. Sex differences in the allometric slope and overall size were also found in a number of linear appendicular metrics using Type-II regression. Males have statistically longer pedipalp tibiae, whip femora, and leg two femora. We propose that males have evolved a longer pedipalps in the context of display contest rather than physical aggression. The elongation of structures used in display-based contest and courtship found herein further emphasizes the contribution of visual cues to the evolution of morphology more broadly.

Assessing the patterns and drivers of shape complexity in the amblypygid pedipalp

“…Hypothesis 3 Females will possess higher palpal complexity than males. This is likewise informed by McLean et al (2020), in which relative spine length was found to be significantly higher in female Damon variegatus than males.…”

“…Hypothesis 2 Across species, shape complexity will decrease with relative pedipalp length (normalized to body size), trading off shape complexity for segment elongation. Previous GMM analysis within Damon variegatus found relative spine length to decrease with increasing pedipalp length (McLean et al, 2020), yet the degree to which this intraspecific pattern in static allometry holds true across the order is unclear.…”

“…However, modern morphometric techniques can be used to elucidate previously undocumented shape dimorphism. For the first time, McLean et al (2020) quantified shape variation within the pedipalps of a single amblypygid species using 2D geometric morphometric (GMM) analysis and identified significant sexual dimorphism within the spines, and relative width of the central shaft. That study discussed the potential importance of display-based conflict and divergent reproductive roles in driving pedipalp sexual dimorphism (McLean et al, 2020) and highlighted the need to quantify shape variation across the order more broadly.…”

“…For the first time, McLean et al (2020) quantified shape variation within the pedipalps of a single amblypygid species using 2D geometric morphometric (GMM) analysis and identified significant sexual dimorphism within the spines, and relative width of the central shaft. That study discussed the potential importance of display-based conflict and divergent reproductive roles in driving pedipalp sexual dimorphism (McLean et al, 2020) and highlighted the need to quantify shape variation across the order more broadly. Indeed, studies using modern statistical shape analysis techniques have led to a clearer understanding of shape evolution in invertebrate weapons, and the link between shape and function.…”

“…Common shape analysis methods such as GMM require the manual/semi automatic placement of "landmarks" upon homologous features that are readily identifiable across all individuals in a sample. Within a given amblypygid taxon, spination is remarkably consistent in terms of gross pattern, and McLean et al (2020) therefore proceeded with manual landmarking on the most prominent four spines. Between species however, spination is highly irregular, homology is difficult to determine by visual inspection, and the toolkit of evolutionary developmental biology has yet to be brought to bear on the genetic basis of pedipalp morphology.…”