@article {1560,
	title = {A new method for observing underground and soil surface behaviors},
	journal = {The Coleopterists Bulletin},
	volume = {47},
	number = {3},
	year = {1993},
	pages = {243-244},
	type = {scientific note},
	author = {Emlen, Douglas J.}
}
@article {1561,
	title = {Alternative reproductive tactics and male-dimorphism in the horne beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae)},
	journal = {Behavioral Ecology and Sociobiology},
	volume = {41},
	year = {1997},
	month = {Aug},
	pages = {335-341},
	keywords = {alternative reproductive behavior, horned beetles, male competition, male dimorphism, Onthophagus},
	author = {Emlen, Douglas J.}
}
@article {1562,
	title = {Diet alters male horn allometry in the beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae)},
	journal = {Proceedings of the Royal society of London B},
	volume = {264},
	year = {1997},
	pages = {567-574},
	abstract = {Darwin considered the horns of male beetles to be among the most striking examples of sexual selection. As with antlers in deer or elk, beetle horns scale positively with male body size, with the result that large males have disproportionately longer horns than small males. It is generally assumed that such scaling relationships ({\textquoteleft} static allometries {\textquoteright}) are insensitive to short-term changes in the environment, and for this reason they are regularly used as diagnostic attributes of populations or species. Here I report breeding experiments on horned beetles that demonstrate that the scaling relationship between male horn length and body size changes when larval nutrition changes. Males reared on a low-quality diet had longer horn lengths at any given body size than sibling males reared on a high-quality diet. Such {\textquoteleft} allometry plasticity {\textquoteright} may explain seasonal changes observed in this same scaling relationship in a natural population. These experiments demonstrate that scaling relationships of sexually selected traits can respond facultatively to variation in the environment, thereby revealing a new mechanism by which males regulate the production of exaggerated secondary sexual traits. },
	author = {Emlen, Douglas J.}
}
@article {1563,
	title = {Integrating development with evolution: a case study with beetle horns},
	journal = {Bioscience},
	volume = {50},
	number = {5},
	year = {2000},
	month = {May},
	pages = {403-418},
	author = {Emlen, Douglas J.}
}
@article {1564,
	title = {Costs and the diversification of exaggerated animal structures},
	journal = {Science},
	volume = {291},
	year = {2001},
	month = {Feb},
	pages = {1534-1536},
	abstract = {Sexual selection can favor production of extravagant ornaments and weapons in the contest for access to the opposite sex. Existing explanations for the diversity of sexually selected structures focus on reproductive benefits con- ferred by particular ornament or weapon morphologies. Here, I show that costs of weapon production also may drive patterns of weapon evolution. In beetles, production of horns reduces the size of neighboring morphological structures (antennae, eyes, or wings, depending on the location of the horns), and these tradeoffs reveal unexpected functional associations between ecology and horn morphology. This study illustrates a critical but overlooked role of costs in sexual selection and has implications for understanding the evolution of animal morphology.},
	author = {Emlen, Douglas J.}
}
@article {1567,
	title = {Diversity in the weapons of sexual selection: horn evolution in the beetle genus Onthophagus (Coleoptera: Scarabaeidae)},
	journal = {Evolution},
	volume = {59},
	number = {5},
	year = {2005},
	month = {Jan},
	pages = {1060-1084},
	abstract = {Both ornaments and weapons of sexual selection frequently exhibit prolific interspecific diversity of form. Yet, most studies of this diversity have focused on ornaments involved with female mate choice, rather than on the weapons of male competition. With few exceptions, the mechanisms of divergence in weapon morphology remain largely unexplored. Here, we characterize the evolutionary radiation of one type of weapon: beetle horns. We use partial sequences from four nuclear and three mitochondrial genes to develop a phylogenetic hypothesis for a worldwide sample of 48 species from the dung beetle genus Onthophagus (Coleoptera: Scarabaeidae). We then use these data to test for multiple evolutionary origins of horns and to characterize the evolutionary radiation of horns. Although our limited sampling of one of the world{\textquoteright}s most species-rich genera almost certainly underestimates the number of evolutionary events, our phylogeny reveals prolific evolutionary lability of these exaggerated sexually selected weapons (more than 25 separate gains and losses of five different horn types). We discuss these results in the context of the natural history of these beetles and explore ways that sexual selection and ecology may have interacted to generate this extraordinary diversity of weapon morphology. },
	keywords = {Adaptive radiation, beetle horns, character divergence, male competition, Onthophagus, phylogeny, sexual selection, weapons.},
	author = {Emlen, Douglas J. and Marangelo, Jennifer and Ball, Bernard and Cunningham, Clifford}
}
@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 {1570,
	title = {Phylogenetic evidence for an association between tunneling behavior and the evolution of horns in dung beetles (Coleoptera : Scarabaeidae : Scarabaeinae)},
	journal = {The Coleopterists Bulletin},
	volume = {60},
	number = {5},
	year = {2006},
	note = {Times Cited: 0Suppl. S},
	month = {Dec},
	pages = {47-56},
	type = {monograph},
	abstract = {Dung beetles employ numerous behavioral strategies to sequester dung away from other insects, and these have been broadly grouped into two categories: species that dig tunnels beneath the dung (tunnelers) and species that roll dung on the surface of the soil (rollers). Many species also are armed with rigid exoskeletal outgrowths called horns. Horns function as weapons, and horn sizes can be extreme. One widespread pattern within dung beetles is that tunneling species often have horns, whereas rolling species almost always do not, suggesting that residing (and fighting) inside tunnels at the dung deposition site may be an important ecological prerequisite for the evolution of horns in dung beetles. Here, we test explicitly for an historical association between tunneling behavior and the evolution of horns using a recent phylogeny for the scarabaeine dung beetles. We show that all eight of the independent gains of horns included in our analyses occurred on branches of the phylogeny reconstructed as tunneling, and that one of the three evolutionary losses of horns occurred on a branch that had lost tunneling behavior. We interpret this as evidence for a biologically meaningful association between tunneling behavior and the evolution of enlarged or exaggerated weapons such as horns, supporting the ideas of Eberhard and others that beetle horns may be most "beneficial" when used within the confines of restricted spaces such as burrows or tunnels.},
	keywords = {ALTERNATIVE REPRODUCTIVE TACTICS, body-size, DIMORPHISM, FEMALE CHOICE, FIDDLER-CRAB, FIGHTS, FOOD RELOCATION, onthophagus-taurus, SELECTION, SEXUAL, TRADE-OFFS},
	isbn = {0010-065X},
	url = {<Go to ISI>://000243611100005 },
	author = {Emlen, Douglas J. and Philips, T. Keith}
}
@article {2034,
	title = {Male horn dimorphism in the scarab beetle, Onthophagus taurus: do alternative reproductive tactics favour alternative phenotypes},
	journal = {Animal Behavior},
	volume = {59},
	year = {2000},
	pages = {459-466},
	abstract = {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 dimor- phism 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. },
	author = {Moczek, Armin P. and Emlen, Douglas J.}
}
@article {2035,
	title = {Threshold evolution in exotic populations of a polyphenic beetle},
	journal = {Evolutionary Ecology Research},
	volume = {4},
	year = {2002},
	pages = {587-601},
	abstract = {Polyphenic development is thought to play an important role in the evolution of phenotypic diversity and morphological novelties, yet the evolution of polyphenisms has rarely been docu- mented in natural populations. Here we compare the morphologies of male dung beetles (Onthophagus ttaurus; Coleoptera: Scarabaeidae) from populations introduced to Australia and the eastern United States. Males in this species express two alternative morphologies in response to larval feeding conditions. Males encountering favourable conditions grow larger than a threshold body size and develop a pair of horns on their heads, whereas males that encounter poor conditions do not reach this threshold size and remain hornless. Australian and US populations did not differ in overall body size ranges, but exhibited significant differences in the location of the critical body size threshold that separates alternative male morphs. Australian males remained hornless at much larger body sizes than males in US populations, resulting in substantial and significant differences in the average body size{\textendash}horn length allometry between exotic populations, as well as significant differences in morph ratios. The phenotypic divergence observed between field populations was maintained in laboratory populations after two generations under identical environmental conditions, suggesting a genetic basis to allometric divergence in these populations. Divergence between exotic O. taurus populations was of a magnitude and kind typically observed between species. We use our results to examine potential causes of allometric divergence in onthophagine beetles, and discuss the evolutionary potential of threshold traits and polyphenic development in the origin of morphological and behavioural diversity.},
	keywords = {adaptive phenotypic plasticity, alternative tactics, developmental threshold, exotic species, horn polyphenism, Onthophagus, status-dependent selection, threshold evolution.},
	author = {Moczek, Armin P. and Hunt, John and Emlen, Douglas J. and Simmons, Leigh W.}
}
