Climbing to Reach Females: Romeo Should Be Small

Abstract

The race for reaching mates by the time they are receptive, or sexual selection by scramble competition, has received little attention. We argue that smaller males are favored in species in which the male must climb to reach females located in high habitat patches. This new explanation we term the "gravity hypothesis" of sexual size dimorphism (SSD). We show that a simple biomechanical model of animal movement predicts that: (1) selection should favor a comparatively smaller size in the searching sex when searching involves climbing; and (2) this effect should be stronger in larger species than in smaller species. In reaching high habitats, smaller, faster searchers will be favored either through sexual selection by scramble competition and/or by escaping predation easier by running faster on vertical surfaces. Different spider species are found at a wide range of heights. We compiled a dataset of spider taxa and arranged their habitats according to four height categories, ranked from soil surface to trees. We show that, after controlling for phylogeny, both predictions of the gravity hypothesis of SSD are met. Thus, it appears that the constraint imposed by gravity on climbing males is a selective factor in determining male dwarfism.

Selection for predation, not female fecundity, explains sexual size dimorphism in the orchid mantises

“…The repeated origin of extreme SSD across arachnids suggests multiple evolutionary pathways, but the underlying causes remain unclear36. Arachnid SSD is typically thought to be caused by female investment in fecundity through size increase4202136, but it has also resulted from male size selection via limitations to male locomotion and silk use (i.e., Gravity Hypothesis37 and Bridging Gravity Hypothesis38) or selective mortality favouring smaller individuals4. These scenarios indicate arachnid lineages with selective pressures extending beyond female fecundity.…”

Overcoming an evolutionary conflict: Removal of a reproductive organ greatly increases locomotor performance

“…First, a large body of literature shows that male spiders, on reaching adulthood, shift from being sedentary to actively searching for females. Male spiders frequently travel long distances, often vertically, to find a mate, because females can be dispersed widely (31,46). Second, some simple considerations show that Tidarren males may be forced to move long distances because of their very small size.…”

“…Because adult male spiders actively move about in search of females (28)(29)(30)(31) and participate in often intense intermale competition (29,32), any decrement in locomotor ability would have important negative consequences for their reproductive fitness. This proposed explanation for the origin of the pedipalp removal behavior has remained untested; no studies have examined the potential functional consequences of locomotion with these large pedipalps.…”

Evolution and maintenance of sexual size dimorphism: aligning phylogenetic and experimental evidence

“…Thus, future experimental studies might profitably focus on selection pressures that could disfavor large male body size, including those that may arise through conflict with female interests, dispersal between webs and long-term survival. Several authors have suggested that small male size is favored in response to sexual cannibalism (Elgar, 1991), protandry (Danielson-Francois et al, 2012;Elwood and Prenter, 2013), or gravity (Moya-Larano et al, 2002Corcobado et al, 2010), but these ideas have attracted either little attention or no empirical support (Table 1; see also Prenter et al, 2010). Perhaps the low variance in male mating success that frequently characterizes the mating system of these spiders limits the opportunity for selection on male size.…”