Stag Beetle Battle Behavior and its Associated Anatomical Adaptations

Abstract

Stag beetle battle behavior and its associated anatomical adaptationsMale stag beetles battle for females with their impressive, oversized mandibles. We describe their fighting behavior, which is essential to understand the evolution and morphology of their weaponry. Our behavioral analysis reveals several anatomical structures that are important for fighting, and our morphological investigations show how these may be adapted for their functions. Stag beetle fightsare much more variable than other armed beetles' battles. They spend considerable time and effort in dislodging their opponent, that clings to the substrate with its tarsal claws. These tarsal claws are also indispensable to maintain balance in the most spectacular battles, when they lift a rival high in the air. The male claws are highly curved and have an increased height for this purpose. The prothoracic muscles are hypertrophied to support the lifting movement. The largest beetle wins in 85% of the fights and the smaller the difference in mandible length is between the rivals, the longer the battles can last. The long mandibles enable males to reach the opponent's legs in order to dislodge it. For this purpose, they bite with all parts of their mandibles, even though the distal part is more vulnerable for failure and transfers less bite force.Blindfolded experiments prove that visual information is not a requisite for a successful battle.Various weapons, evolved by sexual selection and used in male-male combats, are found in a very wide range of taxa: trilobite spines; crustacean chelipeds; stag beetle mandibles; scarab horns; humps, saws and spines of fishes; horns, spines and tusks of frogs; crests, frills, plates and spines of dinosaurs; antlers of deer, elks and moose; horns of bovids and rhinoceros (for a review, see (Emlen 2008)). Knowledge of the nature of fighting tactics and styles is primordial to understand the evolution and morphology of these armaments (Eberhard 1977;Caro et al 2003;Emlen et al 2005;Emlen 2008;Gotoh et al 2012). However, the fighting behavior, as such, has received surprisingly little scientific attention (Emlen 2008). Only for ungulates has the relationship between fighting style,

Jaw morphology and fighting forces in stag beetles

“…3B). This agrees with the observation that in several stag beetle species, males with the longest jaws are more likely to win battles (Goyens et al, 2015b;Inoue and Hasegawa, 2012;Lagarde et al, 2005;Mills et al, 2016), probably owing to a longer reach to the rival's legs when trying to dislodge it from the substrate (Goyens et al, 2015b). Also, the in-lever grows disproportionately fast with increasing muscle force (Fig.…”

“…However, accelerations measured from high-speed video recordings of fighting C. metallifer males (J.G., unpublished results) show that these dorso-ventral forces are very small compared with the bite forces. Concomitantly, their jaws are more resistant to bending caused by biting than by dorso-ventral loadings (Goyens et al, 2015a). Our FE analyses of the H. parryi (specimen 7) and L. cervus (specimen 9) specimens show high material stresses under natural loading conditions (see Fig.…”

“…Furthermore, they may have invested in large and heavy musculature to bite their rivals forcefully. Concomitantly, species with high bite forces probably needed to evolve a more robust jaw morphology to enable them to take a firm grip on rivals and to avoid mechanical failure (Goyens et al, 2015a).…”

“…Owing to their longer jaws, they can reach further forward in aggressive battles to grab their opponent and to detach him from the substrate (Goyens et al, 2015b). Once detached, the opponent is pushed away or lifted above the winner's head and thrown backwards onto the substrate (Goyens et al, 2015b;Shiokawa and Iwahashi, 2000). In addition to the stag beetles' jaws being elongated, their enlarged jaw-closer muscles enable high bite forces (Goyens et al, 2014a;Shiokawa and Iwahashi, 2000).…”

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

“…The differential development of weaponry between minor and major males could be explained as a balance between the benefit derived from a given amount of investment and its cost (Gadgil 1972). The advantages of bearing enlarged weapons becomes particularly evident during stag beetles male-male interaction, giving an enhanced capacity for dislodging the opponent, both in interspecific and intraspecific contests (Hongo andOkamoto 2013, Lagarde et al 2005), thus shortening the battle duration (Goyens et al 2015a) and finally resulting in increased mating opportunities. However, exaggerated mandibles can be costly in terms of impairing locomotion and stability (Goyens et al 2015b) and constraining flight ability (Goyens et al 2015c).…”