Functional mechanics of beetle mandibles: Honest signaling in a sexually selected system

Mills, Maria R.; Nemri, Rahmi S.; Carlson, Emily A.; Wilde, W. H.; Gotoh, Hiroki; Lavine, Laura Corley; Swanson, Brook O.

doi:10.1002/jez.1961

Cited by 31 publications

(39 citation statements)

References 28 publications

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“…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.…”

Section: Morphological Adaptationssupporting

confidence: 90%

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Jaw morphology and fighting forces in stag beetles

Goyens

Dirckx

Aerts

2016

Journal of Experimental Biology

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The jaws of different species of stag beetles show a large variety of shapes and sizes. The male jaws are used as weapons in fights, and they may exert a very forceful bite in some species. We investigated in 16 species whether and how the forcefulness of their bite is reflected in their jaw morphology. We found a large range of maximal muscle forces (1.8-33 N; factor of 18). Species investing in large bite muscles also have disproportionately large jaw volumes. They use this additional jaw volume to elongate their jaws, increasing their chances of winning in battles. The fact that this also decreases the mechanical advantage is largely compensated for by elongated in-levers. As a result, high muscle forces are correlated with elevated bite forces (0.27-7.6 N; factor of 28). Despite the large difference in the forcefulness of their bite, all investigated species experience similar Von Mises stresses in their jaws while biting (29-114 MPa; factor of 4.0; calculated with finite element simulations). Hence, stag beetles have successfully adapted their jaw anatomy according to their bite force in fights

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Section: Morphological Adaptationssupporting

confidence: 90%

“…2) in Amira. By multiplying the muscle force by the mechanical advantage (in-lever divided by jaw length), we estimated the specimens' bite forces (Goyens et al, 2014a;Mills et al, 2016). We established scaling relationships between morphological variables after log-log transformation with the reduced major axis.…”

Section: Fe Simulations Of Jaw Bitingmentioning

confidence: 99%

Jaw morphology and fighting forces in stag beetles

Goyens

Dirckx

Aerts

2016

Journal of Experimental Biology

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“…As a result, males manage to retain their safety factor at approximately the same level as that of female jaws, despite their strongly increased bite muscle force. The safety factors that we measured (5.2-7.2) are comparable to other measurements of male C. metallifer stag beetle jaws (2.05-12.7) and to those of rhinoceros beetle horns (6.5) [17,18].…”

Section: Intersexual Differencessupporting

confidence: 87%

Role of stag beetle jaw bending and torsion in grip on rivals

Goyens

Dirckx

Piessen

et al. 2016

J. R. Soc. Interface.

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In aggressive battles, the extremely large male stag beetle jaws have to withstand strongly elevated bite forces. We found several adaptations of the male Cyclommatus metallifer jaw morphology for enhanced robustness that conspecific females lack. As a result, males improve their grip on opponents and they maintain their safety factor (5.2 -7.2) at the same level as that of females (6.8), despite their strongly elevated bite muscle force (3.9 times stronger). Males have a higher second moment of area and torsion constant than females, owing to an enhanced cross-sectional area and shape. These parameters also increase faster with increasing bending moment towards the jaw base in males than in females. Male jaws are more bending resistant against the bite reaction force than against perpendicular forces (which remain lower in battles). Because of the triangular cross section of the male jaw base, it twists more easily than it bends. This torsional flexibility creates a safety system against overload that, at the same time, secures a firm grip on rivals. We found no structural mechanical function of the large teeth halfway along the male jaws. Therefore, it appears that the main purpose of these teeth is a further improvement of grip on rivals.

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“…; Mills et al. ). This trend is grounded in simple lever physics where increases/decreases in one component of a lever require proportional change in another to maintain performance.…”

Section: Discussionmentioning

confidence: 97%

“…; Mills et al. ). However, space constraints within the animal or structure may place an upper bound on this growth, ultimately limiting the performance of the weapon system.…”

Section: Discussionmentioning

confidence: 97%

Selection on an extreme weapon in the frog‐legged leaf beetle ( Sagra femorata )

2017

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Biologists have been fascinated with the extreme products of sexual selection for decades. However, relatively few studies have characterized patterns of selection acting on ornaments and weapons in the wild. Here, we measure selection on a wild population of weapon-bearing beetles (frog-legged leaf beetles: Sagra femorata) for two consecutive breeding seasons. We consider variation in both weapon size (hind leg length) and in relative weapon size (deviations from the population average scaling relationship between hind leg length and body size), and provide evidence for directional selection on weapon size per se and stabilizing selection on a particular scaling relationship in this population. We suggest that whenever growth in body size is sensitive to external circumstance such as nutrition, then considering deviations from population-level scaling relationships will better reflect patterns of selection relevant to evolution of the ornament or weapon than will variation in trait size per se. This is because trait-size versus body-size scaling relationships approximate underlying developmental reaction norms relating trait growth with body condition in these species. Heightened condition-sensitive expression is a hallmark of the exaggerated ornaments and weapons favored by sexual selection, yet this plasticity is rarely reflected in the way we think about-and measure-selection acting on these structures in the wild.

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Functional mechanics of beetle mandibles: Honest signaling in a sexually selected system

Cited by 31 publications

References 28 publications

Jaw morphology and fighting forces in stag beetles

Jaw morphology and fighting forces in stag beetles

Role of stag beetle jaw bending and torsion in grip on rivals

Selection on an extreme weapon in the frog‐legged leaf beetle ( Sagra femorata )

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