The Dilemma of Choosing a Reference Character for Measuring Sexual Size Dimorphism, Sexual Body Component Dimorphism, and Character Scaling: Cryptic Dimorphism and Allometry in the Scorpion Hadrurus arizonensis

Abstract

Sexual differences in morphology, ranging from subtle to extravagant, occur commonly in many animal species. These differences can encompass overall body size (sexual size dimorphism, SSD) or the size and/or shape of specific body parts (sexual body component dimorphism, SBCD). Interacting forces of natural and sexual selection shape much of the expression of dimorphism we see, though non-adaptive processes may be involved. Differential scaling of individual features can result when selection favors either exaggerated (positive allometry) or reduced (negative allometry) size during growth. Studies of sexual dimorphism and character scaling rely on multivariate models that ideally use an unbiased reference character as an overall measure of body size. We explored several candidate reference characters in a cryptically dimorphic taxon, Hadrurus arizonensis. In this scorpion, essentially every body component among the 16 we examined could be interpreted as dimorphic, but identification of SSD and SBCD depended on which character was used as the reference (prosoma length, prosoma area, total length, principal component 1, or metasoma segment 1 width). Of these characters, discriminant function analysis suggested that metasoma segment 1 width was the most appropriate. The pattern of dimorphism in H. arizonensis mirrored that seen in other more obviously dimorphic scorpions, with static allometry trending towards isometry in most characters. Our findings are consistent with the conclusions of others that fecundity selection likely favors a larger prosoma in female scorpions, whereas sexual selection may favor other body parts being larger in males, especially the metasoma, pectines, and possibly the chela. For this scorpion and probably most other organisms, the choice of reference character profoundly affects interpretations of SSD, SBCD, and allometry. Thus, researchers need to broaden their consideration of an appropriate reference and exercise caution in interpreting findings. We highly recommend use of discriminant function analysis to identify the least-biased reference character.

Sexual dimorphism in the Arachnid orders

“…Kjellesvig-Waering (1966) found males of Tityus tritatis to be longer in overall body length than females. We note that this length metric is likely a poor proxy for total body size, as the metasoma of male scorpions (segments comprising the tail exclusive of the telson) is often elongated ( Koch, 1977 ; Carlson, McGinley & Rowe, 2014 ; Fox, Cooper & Hayes, 2015 ); a trait most marked in the genera Centruoides, Hadogenes, Isometrus and Hemiscorpius ( Polis, 1990 ). This elongation is achieved by lengthening of existing metasomal segments relative to females ( Carlson, McGinley & Rowe, 2014 ), rather than the addition of segments.…”

Sexual dimorphism and reproductive behavior in the Brazilian scorpion Tityus pusillus (Scorpiones, Buthidae)

“…; Fox et al. ). Here, we showed that SD in the scorpion Tityus pusillus was characterized by variations in body size, with females having a larger prosoma and mesosoma and a larger metasoma, and males having wider pedipalp chelae.…”

“…; Fox et al. ). Thus, the larger body size of adult females in T. pusillus may support the survival of their juveniles, as is believed to be the case in other scorpions (Booncham et al.…”

“…; Fox et al. ). As a result, males and females exhibit different behaviors during foraging, defense, escape, and male–female sexual interactions (Gefen ; Carlson et al.…”

“…Various reproductive processes have been reported in many scorpion species, with most studies relating SD to mating behavior (Jiao & Zhu ), or highlighting taxonomic descriptions (Fox et al. ). SD can help us differentiate between traits arising from natural selection (e.g., via feeding and defense) and sexual selection (via reproduction) through variations within and between sexes.…”