Developmental Nutrition Affects the Structural Integrity of a Sexually Selected Weapon

Abstract

Males in many species engage in physical combat over access to mates, and sexual selection has led to the evolution of weapons to enhance contest performance. The size of these often-elaborate structures is known to be exquisitely sensitive to nutrition. However, we know very little about the degree to which nutrition affects other attributes of animal weapons that can be crucial to fighting. In this study, we investigated the impact of natural dietary variation on weapon structural integrity in a fighting insect, Narnia femorata (Hemiptera: Coreidae). Males in this species display their enlarged, spiny hind legs to other males, and these legs serve as weapons in aggressive physical contests where they are used to strike and squeeze opponents. N. femorata feeds on the fruit of prickly pear cactus and sets up territories on this plant. In North Central Florida the prickly pear Opuntia mesacantha spp. lata blooms and begins to produce fruits in April and May. N. femorata has multiple, overlapping generations while the green fruits slowly ripen over the next several months. We examined insects reaching adulthood at two nearby time points in this range, June and July, to test the influence of the nutrition provided by ripening green cactus fruit on weapon size and its ability to resist puncture. We also raised insects on cactus with red, ripe fruit for comparison. We found a striking effect of cactus fruit phenology on weapons. Insects raised with the more mature green fruit (those in the second cohort) had 71% larger weapon area and 4.4 times greater puncture resistance than those raised on the early green fruit (those in the first cohort). In contrast, insects raised on red, ripe fruit were moderate in size, had high puncture resistance, and they changed little phenotypically from the first to second cohort. Increased structural integrity of the hind femur weapon was associated with the increased body size that came with better nutrition. This pattern highlights that cuticle thickness increased or its material properties changed when weapons were larger. Importantly, effects of nutrition on puncture resistance also transcended size. Insects of the same size had greater structural integrity if they received superior nutrition. Sexually selected weapons are often used as visual signals to conspecifics before fights, and this work hints that the size of the weapons may be a poor signal of weapon performance when nutrition is variable.

High-quality host plant diets partially rescue female fecundity from a poor early start

“…We isolated 4th instar juveniles into a 32 oz deli cup with a cactus pad and fruit within 0–24 h after eclosion between 30 April and 12 June 2018. We restricted our experiment to this timeframe because the early-season cactus fruit should be of especially low quality at this time [ 21 ]. Recent findings have shown that mid-season fruit rapidly switches from a low-quality resource to a high-quality resource over a short time period [ 21 ].…”

“…We restricted our experiment to this timeframe because the early-season cactus fruit should be of especially low quality at this time [ 21 ]. Recent findings have shown that mid-season fruit rapidly switches from a low-quality resource to a high-quality resource over a short time period [ 21 ]. We randomly assigned juveniles to either an early-season ( N = 209) or a late-season ( N = 206) cactus fruit diet ( table 1 ), and they fed ad libitum .…”

“…They access the nutrients inside these plants with their long, straw-like, piercing-sucking mouthparts [27]. Since N. femorata feed on these cacti across the entire growing season, the seasonal changes that these plants undergo induce a variety of changes in the N. femorata phenotype [19][20][21], which we explore further in this study.…”

“…Interestingly, N. femorata can develop and reproduce throughout the cactus growing season from early in the season when the cacti are just beginning to flower to the time when all cactus fruit has fully ripened and turned red [ 17 ]. Previous studies show that N. femorata raised on cactus pads with unripe fruit grow smaller bodies [ 19 – 21 ], males grow smaller testes relative to body size [ 20 ] and males raised on the season's earliest unripe fruit have smaller weapons that are less puncture-resistant [ 21 ], though we do not know the specific nutrients that these bugs require. This research highlights that the cactus fruit that appears early in the breeding season is lower quality compared with fruit that has had the chance to mature and ripen.…”

Evolution of stridulatory mechanisms: vibroacoustic communication may be common in leaf-footed bugs and allies (Heteroptera: Coreoidea)

“…Coreoidea are morphologically diverse, include some agriculturally important species [ 33 ] and have been investigated in evolutionary ecology and behavioural studies primarily centered on sexual selection (e.g. [ 34 , 35 ]). Stridulatory signals and/or structures have been observed in both sexes for relatively few species within three families: Alydidae, Coreidae and Rhopalidae.…”