Research Article: Small size does not confer male agility advantages in a sexually-size dimorphic spider

Date Published: May 15, 2019

Publisher: Public Library of Science

Author(s): Shakira G. Quiñones-Lebrón, Matjaž Gregorič, Matjaž Kuntner, Simona Kralj-Fišer, Joseph Clifton Dickens.

http://doi.org/10.1371/journal.pone.0216036

Abstract

Selection pressures leading to extreme, female-biased sexual size dimorphism (SSD) in spiders continue to be debated. It has been proposed that males of sexually size dimorphic spiders could be small because gravity constrains adult agility (locomotor abilities). Accordingly, small males should achieve higher vertical climbing speeds and should be more prone to bridge. The curvilinear model of the gravity hypothesis predicts a negative relationship between vertical climbing speed and male body size only over a threshold of 7.6 mm, 42.5 mg. Because males of most species with extreme SSD fall well below this threshold, the relationship between male size and agility at this scale remains vague. Here, we tested three hypotheses on how male size, mass and age (after maturation) relate to vertical climbing and bridging ability in Nephilingis cruentata, a highly sexually dimorphic orb-weaver with males well below the size threshold. We placed males of different sizes and adult ages in a vertical platform and recorded their climbing speeds. Contrary to the original study testing male bridging ability as binary variable, we measured the duration of the crossing of the bridging thread, as well as its sagging distance. Male body size and mass positively related to the vertical climbing speed and to the distance of the sagging thread during bridging, but had no influence on the bridging duration. The detected positive correlation between male size/mass and vertical climbing speed goes against our first prediction, that small males would have vertical climbing advantage in Nephilingis cruentata, but agrees with the curvilinear model. Against our second prediction, small males were not faster during bridging. Finally, in agreement with our third prediction, threads sagged more in heavier males. These results suggest that small male size confers no agility advantages in Nephilingis cruentata.

Partial Text

Among terrestrial animals, spiders (Araneae) exhibit the most extreme cases of female-biased sexual size dimorphism, i.e. females are much larger than males [1–3], with cases of females being up to 500 times heavier than males [1]. Despite considerable research efforts, the explanations for the evolution of sexual size dimorphism (SSD) in spiders remain controversial [4,5].

We tested the climbing speed and the bridging ability on a total of 46 males with a high variability of body measures (Table 1). There was considerable variation in all the tested parameters of male ability in both climbing and bridging trials (Table 1).

Previous studies proposed that males of sexually size dimorphic spiders have persisted at small, ancestral sizes due to gravity that have limited adult agility over the size threshold of 7.6 mm and 42.5 mg. Namely, small male size should confer advantages in the contexts of vertical climbing and bridging. Yet, male size in the majority of those groups with extreme SSD lies below these thresholds and thus the relationship between male size and agility at this scale remains to be explored. We tested three hypotheses on whether and how male size, mass and senescence relate to vertical climbing and bridging ability in Nephilingis cruentata. We found that male body size and mass positively related to the vertical climbing speed and to the distance of the sagging thread during bridging, but had no influence on the bridging duration (Table 2, Fig 1). The detected positive correlation between male size/mass and vertical climbing speed goes against the prediction that small males would have vertical climbing advantage in Nephilingis cruentata but agrees with the prediction from the curvilinear GH [30] (Fig 1A). Against our prediction, small males were not faster during bridging (Fig 1B). Finally, in the line with our third prediction, threads sagged more in heavier males (Fig 1C). Our results thus imply that small male size confers no agility advantages in Nephilingis cruentata.

 

Source:

http://doi.org/10.1371/journal.pone.0216036

 

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