Mate securing tactics and the cost of fighting in the Japanese horned beetle,Allomyrina dichotoma L. (Scarabaeidae)

Evolution of sexual dimorphism and male dimorphism in the expression of beetle horns: phylogenetic evidence for modularity, evolutionary lability, and constraint

Beetle horns are enlarged outgrowths of the head or thorax that are used as weapons in contests over access to mates. Horn development is typically confined to males (sexual dimorphism) and often only to the largest males (male dimorphism). Both types of dimorphism result from endocrine threshold mechanisms that coordinate cell proliferation near the end of the larval period. Here, we map the presence/absence of each type of dimorphism onto a recent phylogeny for the genus Onthophagus (Coleoptera: Scarabaeidae) to explore how horn development has changed over time. Our results provide empirical support for several recent predictions regarding the evolutionary lability of developmental thresholds, including uncoupled evolution of alternative phenotypes and repeated fixation of phenotypes. We also report striking evidence of a possible developmental constraint. We show that male dimorphism and sexual dimorphism map together on the phylogeny; whenever small males have horns, females also have horns (and vice versa). We raise the possibility that correlated evolution of these two phenomena results from a shared element in their endocrine regulatory mechanisms rather than a history of common selection pressures. These results illustrate the type of insight that can be gained only from the integration of developmental and evolutionary perspectives.

Integrating micro- and macroevolution of development through the study of horned beetles

A major challenge in evolutionary developmental biology is to understand how developmental evolution on the level of populations and closely related species relates to macroevolutionary transitions and the origin of evolutionary novelty. Here, I review the genetic, developmental, endocrine, and ecological basis of beetle horns, a morphological novelty that exhibits remarkable diversity both below and above the species level. Integrating from a variety of approaches three major insights emerge: the origin of beetle horns relied at least in part on the redeployment of already existing genetic, developmental and endocrine mechanisms. At the same time little to no phylogenetic distance appeared to have been necessary for the evolution of diverse modifier mechanisms that permit substantial modulation of trait expression at different time points during development in different species, sexes, alternative male morphs or even different tissue regions of the same individual. Lastly, at least a subset of these modifier mechanisms can evolve rapidly in geographically isolated populations, apparently driven by relatively simple, and probably ubiquitous, changes in ecological conditions. I discuss the implications of these results for our understanding of the genesis of morphological novelty and diversity.

Horn possession reduces maneuverability in the horn-polyphenic beetle, Onthophagus nigriventris

Alternative male morphologies are common in a wide range of organisms and particularly extreme in horned beetles. Here, large males (majors) commonly develop extravagant weaponry such as horns or enlarged mandibles, whereas small males (minors) develop only rudimentary traits. In some taxa, including the genus Onthophagus , the transition from minors to majors occurs over a very small range of body sizes causing intermediate morphologies to be rare or absent from natural populations. Several studies have shown that majors use horns as weapons during male combat over females and that the possession of horns increases male fighting success, and presumably fitness. However, the advantages of a hornless morphology, if any, have remained elusive. Here the alternative male morphs are examined in the horn-polyphenic beetle Onthophagus nigriventris . In particular, the hypothesis was tested that lack of horns in minors increases their maneuverability inside tunnel systems in which these males sneak matings from major males. Using a simple behavioral assay the effects of horn possession on maneuverability were quantified inside an artificial tunnel. Minors were found to be significantly more mobile compared to majors. No such differences were found in mobility between similarly small and large females, which always lack horns. This suggests that mobility differences observed among male morphs are due to the presence or absence of horns rather than differences in body size. This notion was further supported in a second experiment in which surgical removal of horns significantly improved maneuverability, while subsequent re-attachment of horns reversed this effect. These results suggest that lack of horns increases male maneuverability inside tunnels and may thus be advantageous in the context of the particular social niche inhabited by minor males. The results are discussed in the context of the evolutionary ecology of horn-polyphenic beetles.

Male horn dimorphism in the scarab beetle, Onthophagus taurus: do alternative reproductive tactics favour alternative phenotypes?

In a variety of organisms morphological variation is discrete rather than continuous. Discrete variation within a sex has attracted particular interest as it is thought to reflect the existence of alternative adaptations to a heterogeneous selection environment. The beetle Onthophagus taurus shows a dimorphism for male horns: males that exceed a critical body size develop a pair of long, curved horns on their heads, while smaller males remain hornless. In this study we report on the alternative reproductive tactics used by males with these two morphologies, and present experimental and behavioural data suggesting that these alternative tactics selectively favour discretely different male phenotypes. Horned males aggressively defended tunnel entrances containing breeding females. Fights involved the use of horns, and males with longer horns were more likely to win fights. In contrast, hornless males employed nonaggressive sneaking behaviours when faced with competitively superior males. Sneaking behaviours appeared to require high degrees of manoeuvrability inside tunnels to access and mate with females despite the presence of a guarding male. Comparisons of running performances of males with identical body sizes but different horn lengths suggest that the possession of horns reduces male agility inside tunnels. Thus, horn possession confers a clear advantage to males using fighting behaviours to access females, whereas hornlessness may be favoured in males that rely primarily on sneaking behaviours. Combined, the two alternative reproductive tactics used by male O. taurus appear to favour opposite horn phenotypes, which may explain the paucity of intermediate morphologies in natural populations of O. taurus. Copyright 2000 The Association for the Study of Animal Behaviour.

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.

Hormonal control of male horn length dimorphism in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

Male dung beetles (Onthophagus taurus) facultatively produce a pair of horns that extend from the base of the head: males growing larger than a threshold body size develop long horns, whereas males that do not achieve this size grow only rudimentary horns or no horns at all. Here we characterize the postembryonic development of these beetles, and begin to explore the hormonal regulation of horn growth. Using radioimmune assays to compare the ecdysteroid titers of horned males, hornless males, and females, we identify a small pulse of ecdysteroid which is present in both hornless males and females, but not in horned males. In addition, we identify a brief period near the end of the final (third) larval instar when topical applications of the juvenile hormone analog methoprene can switch the morphology of developing males. Small, normally hornless, males receiving methoprene during this sensitive period were induced to produce horns in 80% of the cases. We summarize this information in two models for the hormonal control of male dimorphism in horn length.