# Beyond Rensch’s Rule: Prevalent Female-Biased Size Dimorphism and Its Allometric Scaling in Cassidinae Beetles

## TL;DR

This study examines sexual size dimorphism in Cassidinae beetles and finds that females are consistently larger than males, challenging a well-known evolutionary pattern called Rensch’s rule.

## Contribution

The study provides empirical evidence that female-biased size dimorphism in Cassidinae beetles does not conform to Rensch’s rule, suggesting alternative evolutionary mechanisms.

## Key findings

- Females are consistently larger than males across ten Cassidinae species, with body weight showing the greatest disparity.
- Allometric scaling of SSD in these beetles does not support Rensch’s rule, indicating distinct evolutionary pathways.
- Genital dissection enabled accurate sex identification, revealing subtle but significant SSD patterns.

## Abstract

In the animal world, many species exhibit differences in body size between males and females—a phenomenon known as sexual size dimorphism (SSD). A well-known macroevolutionary pattern, Rensch’s rule, predicts how SSD scales with body size depending on which sex is larger. This study investigated SSD in tortoise and hispine beetles (Cassidinae), a group of leaf beetles from the Nanling Mountains of southern China, where males and females are extremely similar. Using genital dissection for reliable sex identification, we measured body and wing dimensions across ten species. Females are consistently larger than males—a pattern often associated with fecundity selection in insects, although direct reproductive data are lacking. Importantly, the allometric scaling of SSD showed no significant support for Rensch’s rule, suggesting that body size evolution in these morphologically conserved beetles may follow distinct evolutionary pathways.

Body size is a key trait influencing life history and ecological adaptation, and sexual size dimorphism (SSD) reflects divergent selective pressures acting on males and females. In morphologically conserved insect groups such as Cassidinae leaf beetles, the external similarity between sexes often impedes accurate dimorphism assessment. To address this, we conducted a systematic morphometric study of ten Cassidinae species from the Nanling Mountains—the largest east–west mountain system in southern China—where we definitively assigned sex via genital dissection. We measured body weight, body length, body width, length–width ratio, and corresponding wing traits. Across all species, SSD was consistently female biased, with statistically significant but subtle differences in most traits; body weight exhibited the greatest relative disparity. While this pattern aligns with the fecundity advantage hypothesis, direct fecundity data were not collected. Crucially, interspecific allometric analyses revealed that the scaling of male and female body sizes was statistically indistinguishable from that of isometry, providing no significant support for Rensch’s rule in this female-biased system. Our findings offer foundational insights into SSD evolution in cryptically dimorphic, herbivorous beetles and highlight the need for phylogenetically informed studies across broader geographic and taxonomic scales.