All the better to bite you with! Striking intrasexual differences in cheliceral size define two male morphs in an Amazonian arachnid

Abstract

When two mating tactics can be successfully used by different-sized males, disruptive selection may favour morphological divergence between males, resulting in intrasexual dimorphism. Here we characterize intrasexual dimorphism based on cheliceral size in males of the harvestman Paecilaemula lavarrei. We also describe how males of the two morphs use their chelicerae in intra-and intersexual interactions, and investigate microhabitat use to infer their mating tactics. The distribution of body and cheliceral traits was markedly bimodal, indicating the existence of a major morph (majors) with large body and cheliceral size, and a minor morph (minors) with small body and cheliceral size. Male-female interactions were similar between morphs, but only majors initiated male-male contests by biting the legs of rivals using their chelicerae. Males and females preferred tree trunks with large perimeters and containing burrows that could be used as shelters. Female spatial distribution was aggregated, whereas males of both morphs were randomly distributed. Majors seem to fight for reproductive territories that concentrate females, and minors probably rely on territory invasion to sneak copulations. Little overlap in the distribution of body and cheliceral size between morphs suggests that male dimorphism in P. lavarrei may not be a polyphenism, and it is associated with different mating tactics.

Ecology and behavior of the troglobitic harvestman Jimeneziella decui Avram, 1970 (Arachnida: Opiliones)

“…Our observations on male-male interactions suggest that the robust male is more aggressive than the slender male, which never initiated an attack to the robust male, but on some occasions responded to the attacks of robust males. A similar case occurs in Paecilaemula lavarrei Goodnight & Goodnight, 1942 (Cosmetidae) which inhabits the Amazon forest (Solano-Brenes et al 2018). Laboratory observations performed in this study, although with a small sample size, found that only majors initiated a contest, biting the opponent, and both minors and majors responded to the attacks with bites on the opponent's legs.…”

“…Currently, the dorsal scutum length, measuring rigid parts alone, is preferred over the total body length as a reference of the size (Acosta et al 2007). Some families in the order Opiliones have females that are smaller than males, and the intersexual dimorphism is evident not only in body size but also in other traits, such as leg armature, leg length, cheliceral size, and cheliceral shape (e.g., Buzatto & Machado 2008;Willemart et al 2009;Buzatto et al 2011;Zatz et al 2011;Painting et al 2015;Solano-Brenes et al 2018), as it seems to occur, in part, in J. decui. found that the evolution of intrasexual male dimorphism in gonyleptids is strongly correlated with intersexual dimorphism.…”

“…Male dimorphism in a given trait is very likely to evolve in the context of sexual dimorphism, as was found in several gonyleptid harvestmen ). On the other hand, ethological studies of Neotropical species have suggested that the ornate falciform weaponry of Laniatores may be linked to reproductive behavior (Willemart et al 2009;Zatz et al 2011;Solano-Brenes et al 2018).…”

Description of Nilgirius pygoprominulus sp. n. (Opiliones, Assamiidae, Trionyxellinae) from China, with notes on its sexual dimorphism

“…Sexual dimorphism appears in various forms in Opiliones , including differences in body size, length and armature of chelicerae, legs and pedipalpi and glandular openings on legs, pedipalpi and chelicerae (e.g. Dasilva and Kury 2007 ; Buzatto and Machado 2008 ; Willemart et al 2009 ; Willemart et al 2010 ; Buzatto et al 2011 ; Zatz et al 2011 ; Painting et al 2015 ; Solano-Brenes et al 2018 ). The typical sexual dimorphism of assamiids is reinforced in the appendages of male, for example, enlarged leg II and leg IV, swollen cheliceral hand ( Sharma et al 2011 ; Kury et al 2022a ) and filiform pedipalpi in Filopalpinae ( Martens 2022 ).…”

Reproductive biology of harvestmen (Arachnida: Opiliones): a review of a rapidly evolving research field

“…Alternative reproductive tactics are widespread among the Laniatores, but also occur in some species of Eupnoi ( Buzatto and Machado 2014 ). Alternative reproductive tactics are usually coupled with male polymorphism, i.e., the presence of discrete male morphs in the same population that can be recognized by differences in body size and/or size of fighting-related traits ( Buzatto and Machado 2008 ; Zatz et al 2011 ; Painting et al 2015 ; Solano-Brenes et al 2018 ; Powell et al 2020 ; Palaoro et al 2022 ). In Serracutisoma proximum (Laniatores: Gonyleptidae), for instance, there are 2 male morphs that differ in the length of the second pair of legs ( Buzatto et al 2011 ).…”