The form of sexual selection arising from male–male competition depends on the presence of females in the social environment

Abstract

Sexual selection arises from social interactions, and if social environments vary so too should sexual selection. For example, male–male competition often occurs either in the presence or in the absence of females, and such changes in the social environment could affect the form and strength of sexual selection. Here we examine how the presence of a female influences selection arising from male–male competition in a leaf‐footed cactus bug, Narnia femorata, which has a resource defence mating system. Males compete for territories on cacti because females lay eggs on the cactus plants. Females are not always present when this competition first occurs; however, the presence or absence of the female matters. We found that both the form and strength of selection on male traits, those traits that influenced success in intrasexual competition, depended on the social context. When a female was not present, male size and the area of the sexually dimorphic hind legs was only marginally important to winning a contest. However, males with larger overall size and leg area were more likely to win in the presence of a female. There was also positive quadratic selection on these traits when a female was present with both the largest and the smallest males winning. The implication is unexpected alternative strategies when females are present. Our results support the notion that sexual selection should be studied under all relevant social contexts.

Seasonal Effects on the Population, Morphology and Reproductive Behavior of Narnia femorata (Hemiptera: Coreidae)

“…hind femora width or HFW) were estimated using Mitutoyo digital calipers (maximum accuracy 0.01 mm) as insects were held immobilized. Pronotum width has been shown to be an excellent proxy for body size for N. femorata in laboratory studies [34,35]. Hind femora width is simple to measure, sexually dimorphic, and representative of overall hind femora area [45].…”

“…Female mate choice and male-male competition are affected by resource availability and quality in N. femorata in laboratory settings [34,35]. Females prefer the odor of males that develop on cactus fruit and are even more likely to choose males when red ripe fruit is present [34,35].…”

“…However, the duration is highly temperature dependent [29,33]. Male N. femorata grapple with one another using their spiny enlarged hind legs to acquire a territory—a cactus pad with ripe or unripe fruit [34]. Females fly among cactus patches to mate, feed, and oviposit.…”

“…A developmental diet with cactus fruit is very important for juvenile N. femorata [34]. Juveniles can feed on the cladodes (pads) of Opuntia spp., but without any fruit during the final stages of development they mature into tiny adults, males have greatly reduced weapon size, and sexual dimorphism is diminished [8,32,45].…”

Leaf-Footed Bugs (Coreidae)

“…In the presence of cactus fruit, females prefer the odor of males who developed on fruit although no preference is exhibited in the absence of fruit odors (Addesso et al 2014 ). Both male-male competition and female choice operate in this species; enlarged femora in males are used in combat (Miller 2013 ) to defend territories on cactus, but male body size and leg area only strongly affect the outcome of male-male competition in the presence of females (Procter et al 2012 ).…”