Multi-modal locomotor costs explain sexual size but not shape dimorphism in a leaf-mimicking insect

Abstract

In most arthropods, adult females are larger than males, and male competition is a race to quickly locate and mate with scattered females (scramble competition polygyny). In this context, smaller males may be favored due to more efficient locomotion leading to higher mobility during mate searching while larger males may benefit from increased speed and higher survivorship. Understanding how body size affects different aspects of the locomotor performance of males is therefore essential to shed light on the evolution of this widespread mating system. Using a combination of empirical measures of flight performance and substrate adhesion, and modelling of body aerodynamics, we show that large body size impairs both flight and landing (attachment) performance in male leaf insects (Phyllium philippinicum), a species where relatively small and skinny males fly through the canopy in search of large sedentary females. Smaller males were more agile in the air, ascended more rapidly during flight, and had a lower risk of detaching from the substrates on which they walk and land. Our models revealed variation in body shape affected body lift and drag, but tradeoffs with weight meant that effects were negligible, suggesting that flight costs do not explain the evolution of strong sexual dimorphism in body shape in this species.

Beyond winglets: evolutionary scaling of flight-related morphology in stick insects (Phasmatodea)

“…In the direction of flight gain, a positive static margin may be necessary for flapping flight in fully-winged insects. So far, kinematics of wing flapping and flight performance have only been documented in a leaf insect (Boisseau et al, 2022). These predictions need to be confirmed using actual body-leg postures and body kinematics, along with stability analysis, from flight of phasmids with various sized wings.…”

“…The processes of wing reduction and flight transition in phasmids remain largely understudied, despite the hypothesis that relaxed selection for flight (i.e., aerodynamic utility) underpins short-winged or wingless species within an arboreal context (Zeng et al, 2020). A recent study showed ascending flight in leaf insects, a group of leaf-mimicking phasmids (Boisseau et al, 2022).…”

Supplementary material 7 from: Cumming RT, Le Tirant S (2022) Three new genera and one new species of leaf insect from Melanesia (Phasmatodea, Phylliidae). ZooKeys 1110: 151-200. https://doi.org/10.3897/zookeys.1110.80808

“…1A) and females that are largely stationary, spending their lives in the canopy (Fig. 1B; Wedmann et al 2007;Boisseau et al 2021). This strong sexual dimorphism, coupled with their impressive camouflage means that leaf insects are unfortunately rather rare in collections despite having a significant range throughout most of Southeast Asia, and unless confirmed through molecular analyses or from captive rearing, pairing up the sexes is not always possible and can lead to significant identifications errors (Cumming et al 2020a;Brock et al 202).…”