Sexual dimorphism in forelimb lengths of intertidal jumping spiders (Terralonus californicus) and implications for current theory on allometric patterns and trait function
Date
2022
Authors
Hentschel, Devin
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Abstract
Sex-specific selection theory suggests that secondary sexual characteristics can vary between sexes in their mean, their variance, and their allometric scaling due to differences in optimal resource investment and condition. The intertidal jumping spider, Terralonus californicus, is an unstudied species with the potential to exhibit a wide range of possible sexual dimorphisms due to its high degree of intrasexual agonistic encounters and female mate choice. From qualitative behavioural examinations, I observed that males appeared to use forelimbs as both a 'weapon' and an 'ornament.' Therefore, I predicted that forelimb length (FL) would exhibit dimorphism between males and females via differences in mean, variance, and allometric scaling both between sexes and between traits. I measured the cephalothorax width, forelimb length, and hindlimb length of mature males and females using images taken under a dissection microscope along with a calibrator of known distance to tabulate measurements in millimeters. I found that FL differed significantly between males and females (T = 4.17, p < 0.001) and that the variance was significantly higher in male FL than in females (F = 1.81, p = 0.0485). A one-way ANCOVA found that only cephalothorax width had a significant effect on forelimb length (F = 90.1, p < 0.001), while comparisons of regression slopes found no difference between male FL and HL (b = 0.91 ± 0.15 for FL and 0.84 ± 0.19 for HL). These results confirmed my first two predictions but did not provide support my latter two predictions for a positive allometric scaling in male forelimb length, contradicting theory on patterns of allometry and function. This study provided empirical evidence for sexual dimorphism in the forelimbs of a unique species that had received no attention in the current body of literature, warranting future research into mechanisms of both sexual selection and intersexual ecological niche differentiation that may act on the forelimbs of this unstudied local species.