Re-assessment of rensch’s rule in Centrobolus

Abstract

Sexual size dimorphism (SSD) was investigated in the helminthomorph genus Centrobolus (=Chersastus). Width, length and mass were used to compare the interspecific variation for 20 species. Interspecific variation in volume was calculated in these species and an allometric coefficient of 0.7 found. The allometric equation generated for the genus also the inverse of Rensch's rule as there was a positive correlation between SSD and body size (R=0.70485; P=0.00109; n=18 spp) in this diplopod genus. This SSD is thought to have evolved through intersexual competition driven by sexual selection.

Centrobolus anulatus (Attems, 1934) reversed sexual size dimorphism

“…Previous studies on SSD in invertebrates and these results consistently give a positive correlation and break the rule [5,10,[12][13][14][15][16][17][18][19]36] . Figure 1 shows the finding for Centrobolus anulatus where the regression of log male volume on log female volume was highly significant with a positive slope of 0.86207; showing females get larger than males with an increase in body size [2][3][7][8] .…”

A review of studies on the fire millipede genus centrobolus (diplopoda: trigoniulidae)

“…The inverse of Rensch's rule was found in Centrobolus based on the positive relationship between SSD and body size [31] . Body mass in southern African worm-like millipedes positively related to copulation duration [32] . Sexual size dimorphism in southern African worm-like millipedes inversely related to copulation duration because larger males copulate for shorter and larger females copulate for longer [33] .…”

Sexual dimorphism in pill millipedes (Diplopoda)

“…The regression of SSD on body size indicates a positive correlation in Sphaerotherium and rejected Rensch's rule as was the pervading case for many taxa having female-biased SSD . Mean volume ratios in Sphaerotherium suggest relatively high variance of SSD ranging from 1.49 in S. tenuitarse to 5.36 in S. punctulatum which were both larger than helminthomorph diplopods [54] . The inclusion of many more taxa as new species from this genus is suggested to show support the trend of larger females as seen here.…”

“…Rensch's rule may be explained as sexual selection [17][18][19][20][21] . The macro-evolutionary pattern is being resolved in the class Diplopoda [53][54] . In the present study, SSD in the genus Sphaerotherium was investigated.…”