Exaggerated allometric structures in relation to demographic and ecological parameters inLucanus cervus(Coleoptera: Lucanidae)

Abstract

Enlarged weapons and ornamental traits under sexual selection often show a positive allometric relationship with the overall body size. The present study explores the allometry of mandibles and their supporting structure, the head, in males of the European stag beetle, Lucanus cervus. This species shows a remarkable dimorphism in mandible shape and size that are used by males in intraspecific combats. Stag beetles were captured, measured, weighed, and released in the framework of a capture-mark-recapture study. The relationship of mandible length (ML) and head width in respect to the overall body size was described by a segmented regression model. A linear relationship was detected between ML and head width. The scaling relationships for both ML and head width identified the same switchpoint, highlighting the advantages of using combined results of weapons and their supporting structures in such analysis. These results led to a more consistent distinction of males in two morphologies: minor and major. The survival probability of individuals was dependent on the morphological class and was higher for minor males than for major. Elytron length and body mass of the individuals did not show any significant variation during the season. Differences in predatory pressure were detected between morphs by the collection and analysis of body fragments due to the predatory activity of corvids. Morphological differences and shift in demographic and ecological parameters between the two classes suggested that selection continues to favor intrasexual dimorphism in this species throughout a trade-off mechanism between costs and benefits of carrying exaggerated traits.

Guidelines for the monitoring of Lucanus cervus

“…Using a traditional morphometric approach, Romiti et al (2015) highlighted the presence of two morphological classes: large males (which invest significantly more in mandibles and head size) and minor males. Furthermore, a geometric morphometric approach revealed that smaller males show highly variable mandibles (usually less curve dorsoventrally, thickset, with barely outlined teeth and a head without prominent basal edges) whereas larger males exhibit less variable mandibles (slender and equipped with fully developed median teeth, and deep, divaricate apical forks, supported by wide head basal edges) (Romiti et al 2016).…”

Latitudinal cline in weapon allometry and phenology of the European stag beetle

“…BM was not measured for beetles sampled in CRO. Mandible length and elytron length were measured as described in Chiari et al (2014), Romiti et al (2015) and . Mandible length (ML) was measured as the linear distance from clypeus to the apex of the outer tooth of the fork and this can be considered a good proxy for the actual mandible size (Romiti et al 2016).…”

“…For the allometric analysis of male mandibles, the elytron length was used as an independent variable, mainly for three reasons: (i) its proven high correlation coefficient with mandible length (Romiti et al 2015); (ii) the comparability with previous morphometric studies which used linear distances for L. cervus (Harvey and Gange 2006, Hardersen et al 2011, Chiari et al 2014, Romiti et al 2015 and (iii) to avoid scale dependence issues in comparing traits with different units (e.g., mm vs g) which could affect the data despite the log transformation (Warton et al 2006). The pooled dataset was used to analyse the allometric relationship between the mandible length (LnML) and the elytron length (LnEL) of L. cervus males.…”

“…Recently, in studying the exaggerated rostrum of the New Zealand giraffe weevil Lasiorhynchus barbicornis (Fabricius, 1775), Painting et al (2014) linked the latitudinal cline in body size with allometry (sensu Huxley and Teissier 1936). The European stag beetle, Lucanus cervus (Linnaeus, 1758), like many other species of Lucanidae, shows remarkable sexual dimorphism in mandible size and shape (Franciscolo 1997) and a great variation amongst males for this characteristic (Clark 1977, Harvey et al 2011, Hardersen et al 2011, Romiti et al 2015. The mandibles of L. cervus males can be considered as secondary sexual characteristics (SSC) that are used as weapons in intra-sexual combats (Darwin 1871, Arrow 1951, Mathieu 1969, Otte and Staymen 1979, Lagarde et al 2005.…”

“…Species with relatively short mandibles exhibit a steeper allometric coefficient (slope of the regression) as size increases; on the other hand, a limit for the development of such exaggerated structures is evident in species with relatively long mandibles. L. cervus males use their mandibles in male-to-male combat (Clark 1977, Lagarde et al 2005, Fremlin 2009) and major males are equipped with enlarged and more complex mandibles (Romiti et al 2015(Romiti et al , 2016. The analysis of the geographical variation of allometric trajectories allows researchers to obtain more robust conclusions about the relative investment in these SSC.…”