@article {1568,
	title = {Hormonal control of male horn lenght dimorphism in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)},
	journal = {Journal of Insect Physiology},
	volume = {45},
	year = {1999},
	pages = {45-53},
	abstract = {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.},
	keywords = {Horned beetle, Juvenile hormone, male dimorphism, Polyphenism, Threshold},
	author = {Emlen, Douglas J. and Nijhout, H. Frederik}
}
@article {1569,
	title = {The development and evolution of exaggerated morphologies in insects},
	journal = {Annual Review of Entomology},
	volume = {45},
	year = {2000},
	pages = {661-708},
	abstract = {We discuss a framework for studying the evolution of morphology in insects, based on the concepts of {\textquoteleft}{\textquoteleft}phenotypic plasticity{\textquoteright}{\textquoteright} and {\textquoteleft}{\textquoteleft}reaction norms.{\textquoteright}{\textquoteright} 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.},
	keywords = {allometry, castes, Polyphenism, sexual selection, threshold traits},
	author = {Emlen, Douglas J. and Nijhout, H. Frederik}
}
@article {1966,
	title = {Horn possession reduces maneuverability in the horn-polyphenic beetle, Onthophagus nigriventris},
	journal = {Journal of Insect Science},
	volume = {6},
	number = {21},
	year = {2006},
	pages = {1-10},
	abstract = {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 genusOnthophagus, 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 beetleOnthophagus 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. },
	keywords = {ALTERNATIVE REPRODUCTIVE TACTICS, Horned beetle, phenotypic plasticity, Polyphenism, sexual selection},
	author = {Madewell, Richard and Moczek, Armin P.}
}
@article {2026,
	title = {Horn polyphenism in the beetle Onthophagus taurus: larval diet quality and plasticity in parental investment determine adult body size and male horn morphology},
	journal = {Behavioral Ecology},
	volume = {9},
	number = {6},
	year = {1998},
	pages = {636-641},
	keywords = {alternative phenotypes, facultative parental investment, Onthophagus, phenotypic plasticity, Polyphenism},
	author = {Moczek, Armin P.}
}
@article {2027,
	title = {Facultative paternal investment in the polyphenic beetle Onthophagus taurus; the role of male morphology and social context},
	journal = {Behavioral Ecology},
	volume = {10},
	number = {6},
	year = {1999},
	pages = {641-647},
	keywords = {beetles, facultative parental investment, honest indicators, Onthophagus, phenotypic plasticity, Polyphenism, sexual selection},
	author = {Moczek, Armin P.}
}
@article {2029,
	title = {The behavioral ecology of threshold evolution in a polyphenic beetle},
	journal = {Behavioral Ecology},
	volume = {14},
	number = {6},
	year = {2003},
	pages = {841-854},
	abstract = {Facultative expression of alternative male morphologies is thought to allow individual males to select the phenotype with the highest fitness gain given their competitive status relative to other males with which they compete for females. Choice of, or switching between, morphs commonly relies on developmental threshold responses. Evolutionary changes in developmental threshold responses are thought to provide an important avenue for phenotypic diversification and the evolution of morphological and behavioral novelties. However, the extent to which alternative male phenotypes and their underlying threshold responses actually evolve in natural populations is unclear. Likewise, the ecological factors that shape the evolution of threshold responses in natural populations are unexplored for most organisms, as are the consequences of such modifications for patterns of morphological diversity. I examined the ecological basis of rapid threshold evolution in exotic populations of the horn-polyphenic dung beetle Onthophagus taurus. Male O. taurus vary continuously in body size as a function of larval feeding conditions. Only males that exceed a critical threshold body size develop a pair of long horns on their heads, whereas males below this threshold remain hornless. Populations in two exotic ranges of this species, the eastern United States and western Australia, have diverged in the mean threshold body size, which has resulted in the evolution of highly divergent and novel horn length{\textendash}body size allometries in these populations. Populations in a third and previously unstudied exotic range of O. taurus in eastern Australia exhibit threshold body sizes roughly intermediate between the eastern U.S. and western Australian populations. I tested three hypothesis to explain how differences in ecological and demographic factors can drive allometric divergences between populations, using data derived from comparative, standardized sampling of a large number of populations in each exotic range. Results suggest that differences in the intensity of both intra- and interspecific competition have contributed to the evolution of divergent thresholds in these populations. My results do not support the hypothesis that shifts in threshold body sizes to larger body sizes are a consequence of increases in the mean body size of competing males. I discuss my results in the context of Onthophagus mating systems and the evolutionary implications of threshold evolution.},
	keywords = {adaptive phenotypic plasticity, developmental threshold, exotic species, horned beetles, male, Onthophagus, Polyphenism, resource competition, sex ratio evolution, status-dependent selection, threshold evolution, {\textendash}male competition},
	author = {Moczek, Armin P.}
}
