Evidence for Male Horn Dimorphism and Related Pronotal Shape Variation in Copris lunaris (Linnaeus, 1758) (Coleoptera: Scarabaeidae, Coprini)

Abstract

Male horn dimorphism is a rather common phenomenon in dung beetles, where some adult individuals have well-developed head horns (i.e., major males), while others exhibit diminished horn length (i.e., minor males). We focused on horn dimorphism and associated head and pronotum shape variations in Copris lunaris. We examined the allometric relationship between horn length (i.e., cephalic and pronotal horns) and maximum pronotum width (as index of body size) by fitting linear and sigmoidal models for both sexes. We then asked whether head and pronotum shape variations, quantified using the geometric morphometric approach, contributed to this allometric pattern. We found that female cephalic and pronotal horn growth showed a typical isometric scaling with body size. Horn length in males, however, exhibited sigmoidal allometry, where a certain threshold in body size separated males into two distinct morphs as majors and minors. Interestingly, we highlighted the same allometric patterns (i.e., isometric vs. sigmoidal models) by scaling horn lengths with pronotum shape, making evident that male horn dimorphism is not only a matter of body size. Furthermore, the analysis of shape showed that the three morphs had similar heads, but different pronota, major males showing a more expanded, rounded pronotum than minor males and females. These morphological differences in C. lunaris can ultimately have important functional consequences in the ecology of this species, which should be explored in future work.

Size-dependent seasonal activity for males of the dung beetle Copris acutidens (Coleoptera: Scarabaeidae)

“…Sexual dimorphism is striking in this genus; all males bear a single, curved head horn and often also bear prothoracic horns, whereas females have only rudimentary horns. Some species exhibit male dimorphism in horn size (Eberhard and Gutiérrez 1991;Sugiura et al 2007;Pizzo et al 2015;Akamine 2016a;Kerman et al 2018). However, behaviours associated with this dimorphism and its adaptive aspects have received little attention.…”

Dung beetle distress signals may be correlated with sex and male morph: a case study on Copris lunaris (Coleoptera: Scarabaeidae, Coprini)

“…We used proW as a reliable approximation for body size (Knell 2009). We defined male morphs based on the switchpoint value reported in Kerman et al (2018): males with proW higher than the switchpoint value were considered majors while the opposite was true for minors (Akamine 2019). Horn lengths (cHL and pHL) were used to corroborate the assignment of males to each morph.…”

“…Male dimorphism is relatively common in insect, and it is known (among others) also in some insect taxa which exihibit complex sound communication as the Orthoptera (Kelly 2005;Kelly and Adams 2010). However, several of the best-known example of male dimorphism are those found in Scarabaeidae genera such as Onthophagus (Emlen et al 2005;McCullough and Simmons 2016) and Copris (Sugiura et al 2007;Akamine 2016Akamine , 2019Kerman et al 2018), where some males have large bodies with long and sturdy horns (i.e. majors), while others possessing smaller body sizes with short, highly diminished horn structures (i.e.…”

“…Here we focused on the effect of sex and male polymorphism on sound production, by reporting morphological measurements of the stridulatory organ and acoustic analysis of distress calls in a horn dimorphic species, Copris lunaris (Linnaeus, 1758). In the genus Copris, male dimorphism has been described in more than one species (Kerman et al 2018), and defensive stridulations are known to occur frequently in semi-natural contexts (Klemperer 1982b(Klemperer , 1986. Although Klemperer (1982bKlemperer ( , 1986 notes the presence of stridulations in Copris in experimentally monitored nesting chambers, detailed acoustic examinations regarding the structural properties of these calls are only available for C. hispanus and C. incertus (Palestrini et al 1991(Palestrini et al , 2000.…”

Male Horn Lack of Allometry May be Tied to Food Relocation Behaviour in Lifting Dung Beetles (Coleoptera, Scarabaeidae, Eucraniini)

“…The laminar, expanded armatures that originate from the clypeal portion of the head are the most conspicuous morphological character in the Eucraniini. Such heavy modifications leading to the development of exaggerated horns [12,13] are usually considered an expression of phenotypic plasticity, as already ascertained for many other Scarabaeinae [14,15,16,17,18].…”