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      Allometry and Fighting Behaviour of a Dimorphic Stag Beetle Cyclommatus mniszechi (Coleoptera: Lucanidae)

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      , , *
      Insects
      MDPI
      body size, male–male competition, mandible, sequential analysis, sexual selection, weapon

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          Abstract

          Male stag beetles (Coleoptera: Lucanidae) use their mandibles as weapons to compete for resources and reproduction. Mandible size in stag beetles can be associated with different behaviours and the outcome of male contests. We investigated the allometric relationship between mandible and body size in males of the stag beetle Cyclommatus mniszechi to uncover distinct morphs. The results divided male C. mniszechi into majors and minors with the switch point of mandible length at 14.01 mm. The allometric slope of mandibles was positive for both morphs but was steeper for the minors. We also characterised the fighting behaviour of the different morphs in size-matched contests using sequential analyses. Males matched each other’s behaviour in contests with many physical contacts, no injury and a progression from low towards high aggression and rare de-escalation. Major and minor males employed the same behavioural elements in contests, but major males were more likely to escalate directly into more aggressive phases and minor males tended to stay within phases. This finding suggests that major males may compete more aggressively than minor males in contests.

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          The Evolution of Animal Weapons

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            The development and evolution of exaggerated morphologies in insects.

            We discuss a framework for studying the evolution of morphology in insects, based on the concepts of "phenotypic plasticity" and "reaction norms." We illustrate this approach with the evolution of some of the most extreme morphologies in insects: exaggerated, sexually selected male ornaments and weapons, and elaborate social insect soldier castes. Most of these traits scale with body size, and these scaling relationships are often nonlinear. We argue that scaling relationships are best viewed as reaction norms, and that the evolution of exaggerated morphological traits results from genetic changes in the slope and/or shape of these scaling relationships. After reviewing literature on sexually selected and caste-specific structures, we suggest two possible routes to the evolution of exaggerated trait dimensions: (a) the evolution of steeper scaling relationship slopes and (b) the evolution of sigmoid or discontinuous scaling relationship shapes. We discuss evolutionary implications of these two routes to exaggeration and suggest why so many of the most exaggerated insect structures scale nonlinearly with body size. Finally, we review literature on insect development to provide a comprehensive picture of how scaling relationships arise and to suggest how they may be modified through evolution.
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              Sexual selection and allometry: a critical reappraisal of the evidence and ideas.

              One of the most pervasive ideas in the sexual selection literature is the belief that sexually selected traits almost universally exhibit positive static allometries (i.e., within a sample of conspecific adults, larger individuals have disproportionally larger traits). In this review, I show that this idea is contradicted by empirical evidence and theory. Although positive allometry is a typical attribute of some sexual traits in certain groups, the preponderance of positively allometric sexual traits in the empirical literature apparently results from a sampling bias reflecting a fascination with unusually exaggerated (bizarre) traits. I review empirical examples from a broad range of taxa illustrating the diversity of allometric patterns exhibited by signal, weapon, clasping and genital traits, as well as nonsexual traits. This evidence suggests that positive allometry may be the exception rather than the rule in sexual traits, that directional sexual selection does not necessarily lead to the evolution of positive allometry and, conversely, that positive allometry is not necessarily a consequence of sexual selection, and that many sexual traits exhibit sex differences in allometric intercept rather than slope. Such diversity in the allometries of secondary sexual traits is to be expected, given that optimal allometry should reflect resource allocation trade-offs, and patterns of sexual and viability selection on both trait size and body size. An unbiased empirical assessment of the relation between sexual selection and allometry is an essential step towards an understanding of this diversity.
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                Author and article information

                Journal
                Insects
                Insects
                insects
                Insects
                MDPI
                2075-4450
                23 January 2020
                February 2020
                : 11
                : 2
                : 81
                Affiliations
                Department of Life Science, National Taiwan Normal University, No. 88, Sec. 4, Tingzhou Rd., Taipei 11677, Taiwan; 60543017s@ 123456ntnu.edu.tw (Z.-Y.C.); yuyinghs@ 123456ntnu.edu.tw (Y.H.)
                Author notes
                [* ]Correspondence: treehopper@ 123456ntnu.edu.tw ; Tel.: +886-2-7734-6309
                [†]

                These authors have contributed equally to this work.

                Author information
                https://orcid.org/0000-0003-1472-5080
                Article
                insects-11-00081
                10.3390/insects11020081
                7074643
                31979391
                e48f2798-2d36-414f-a31f-a790edeecb74
                © 2020 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 02 December 2019
                : 20 January 2020
                Categories
                Article

                body size,male–male competition,mandible,sequential analysis,sexual selection,weapon

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