Diverse and complex male polymorphisms in Odontolabis stag beetles (Coleoptera: Lucanidae)

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.

Divergence in Gut Bacterial Community Structure between Male and Female Stag Beetles Odontolabis fallaciosa (Coleoptera, Lucanidae)

“…Adults of this species exhibit dramatically sexual dimorphism. In addition, the adult males show rare phenotypic trimorphism [ 30 , 31 ]. Three types of male morphs, that is large-, medium-, and small-sized males, coexist and display large variations in mandible shape and body size.…”

Morph-specific investment in testes mass in a trimorphic beetle, Proagoderus watanabei

“…On the basis of the scaling relationships of weapons and body size we find that P. watanabei males group into three separate morphs, and that those males with large head horns and pronotal horns, the alpha morph, invest relatively less in testes mass than do either the beta males (large head horns but no pronotal horn) or the gamma males (small head horns, no pronotal horn). Previous studies of trimorphic arthropod males have analysed differences in the scaling relationships of single weapon traits only ( Rowland & Emlen, 2009; Painting et al ., 2015; Matsumoto & Knell, 2017; Powell et al ., 2020). The males of many beetle species carry horns on both their heads and their pronota (Emlen et al ., 2005; Emlen, 2008; Knell, 2011), and we suggest that further study of some of these might reveal similar complex polymorphisms to the one described here.…”

“…In many cases these alternative tactics are themselves associated with distinct morphological differences between males. In the insects these differences are common in the Coleoptera and especially in the superfamily Scarabaeoidea, where male dimorphism is frequently found in the Lucanidae (Matsumoto & Knell, 2017), the Dynastinae (McCullough et al ., 2015), and the Scarabaeinae (Emlen et al ., 2005; Simmons, Emlen & Tomkins, 2007). In these animals one morph, usually called the “major” morph typically invests heavily in weaponry such as horns or enlarged mandibles while the other, the “minor” morph bears reduced or no weaponry, leading to non-linear scaling relationships between weapon size and body size (Knell, 2009; McCullough et al ., 2015).…”

“…In recent years, a number of examples of arthropods which appear to have more than two male morphs have been described. Three different male morphs have been found in some species of Philotrypesis fig wasp (Jousselin, van Noort & Greeff, 2004), a number of dung beetle species (Rowland & Emlen, 2009), stag beetles (Rowland & Emlen, 2009), (Iguchi, 2013; Matsumoto & Knell, 2017), a weta (Kelly & Adams, 2010), a weevil ((Rowland & Emlen, 2009) and two species of harvestman (Painting et al ., 2015; Powell et al ., 2020). In two lucanid species there is even evidence for four separate male morphs (Iguchi, 2013; Matsumoto & Knell, 2017).…”

Assessing the patterns and drivers of shape complexity in the amblypygid pedipalp

“…Similar studies in beetles have used a mixture of GMM and traditional morphometric methods to investigate links between mandible shape and function in intrasexual contest (Goczał et al, 2019;Matsumoto & Knell, 2017;Mills et al, 2016;Romiti et al, 2017). In particular, authors have noted that changes in mandible curvature and dentition in Coleoptera correlate with increasing mandible size (Matsumoto & Knell, 2017;Romiti et al, 2017). As mandible length is a strong predictor of the number of fights won, these shape changes are thought to play a key role in contest (Romiti et al, 2017).…”