Abstract
When male animals engage in intrasexual contests then any alternative tactics they use can be associated with dimorphisms in the expression of weapons. Some species have recently been found to exhibit trimorphism in their weaponry, suggesting that the processes leading to their evolution and maintenance of these polymorphisms can be more complex than previously thought. Here, we describe the extraordinary diversity of polymorphism within the genus Odontolabis: there are dimorphic species (O. siva and O. platynota), trimorphic species (O. cuvera, as previously described, and O. sommeri s.stricto) and, uniquely, tetramorphic species, with males of O. sommeri lowei and O. brookeana showing four clearly differentiated male morphs: small “Gammas”, “Alphas” which express large, long mandibles, “Betas” which have long mandibles with different morphology and “Boltcutters”, with short, wide mandibles. Such polymorphisms are usually thought of as being maintained as a status-dependent conditional strategy, but we found only one size threshold: in most cases males develop into Gamma males below a certain size but there is no relationship between morph and body size amongst the larger, ‘weaponised’ morphs. We suggest that the complex polymorphisms in these animals are probably maintained by a combination of a conditional strategy and a genetic polymorphism.
Highlights
Intrasexual phenotypic dimorphisms in male animals are widespread and known often to be associated with the use of alternative tactics for acquiring mates[1,2]
This species exhibits a threshold body size below which males develop into minor morphs and above which they mature as horned major morphs, and minor beetles rely on ‘sneak’ tactics to acquire matings whereas majors engage in aggressive contests to gain access to and monopolise females[5]
In this study we examine the morphology of six species of Odontolabis using both linear measurements and geometric morphometrics, in order to determine 1) whether other species of Odontolabis display trimorphism, whether they show different forms of polymorphism or whether they are monomorphic; 2) whether the relationship between morph and size indicates a threshold based morph determination mechanism and 3) whether the morphology of the head and prothorax covaries with morph in these species in a comparable manner to that described from Lucanus cervus
Summary
Intrasexual phenotypic dimorphisms in male animals are widespread and known often to be associated with the use of alternative tactics for acquiring mates[1,2]. On the basis of this limited set of information, we can cautiously infer that the dimorphism in L. cervus at least is likely to be similar to the well-described male dimorphism found in, for example, Onthophagus taurus This species exhibits a threshold body size below which males develop into minor morphs and above which they mature as horned major morphs, and minor beetles rely on ‘sneak’ tactics to acquire matings whereas majors engage in aggressive contests to gain access to and monopolise females[5]. This model predicts the existence of trimorphisms and thresholds consistent with observations from Oxysternon dung beetles, but whether the trimorphisms known in lucanids are consistent with a two-threshold, three morph model is not currently clear
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