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

Abstract

This study aimed to determine differences in activities between two male morphs of the dung beetle Copris acutidens Motschulsky (Coleoptera: Scarabaeidae) during the reproductive period and to examine the size distribution of reproductive males that stayed in nests. The activities of two male morphs distinguished by a threshold value of body size were compared with those by horn length. Regardless of body size or horn length, earlier activity of minor males was observed during the reproductive period. The sex ratio showed the greatest female bias when minor males were the more abundant than major males, indicating that minor males were the most active when competition was the weakest and these could avoid direct combat with major males. In morphs distinguished by horn length, more major males than minor males stayed in nests with females although the major males became the most active from the middle of the reproductive period. Thus, longer horns may directly confer a competitive advantage to males, enabling them to stay in nests with females, whereas early activity of minor males does not always indicate the effect of horn length directly. Therefore, this behaviour may occur regardless of whether the morphs differ in body size or horn length.

Population structure of Phanaeus vindex (Coleoptera: Scarabaeidae) in SE Michigan

“…As predicted by Eberhard (1982) , minor males of Copris acutidens were more abundant than major males early in the year and coincided partially with female bias ( Akamine 2019 ). In contrast to this, dominance of minor P. vindex males occurred in all months and did only occasionally coincide with female dominance ( Fig.…”

“…9AB ). In O. taurus , House et al (2011) found the same phenological coincidence of female and minor bias early in the year ( Eberhard 1982 , Akamine 2019 ), and postulated that females produce an excess of daughters when mating to minor males while producing a balanced sex ratio with major males. O. taurus is a species with a relatively fast ontogeny and phenological plasticity allowing it to sustain at least 2 generations per year in temperate climate regions ( Wassmer 1994 , 2020a ).…”

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.…”

“…Minor males avoid direct competition with major males either by employing a sneaker tactic (Moczek and Emlen, 2000;Simmons et al 2007;Moczek 2009aMoczek , 2009bBuzatto et al 2015), or by increasing their activity levels later in the breeding season when majors are less abundant and low in density (Akamine 2019). Hence, we should expect to see no functional need for minors to implement intense stridulations during intraspecific contests.…”

“…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.…”