The exploitation of sexual signals by predators: a meta-analysis

Sexual signals are often central to reproduction, and their expression is thought to strike a balance between advertising to mates and avoiding detection by predatory eavesdroppers. Tests of the predicted predation costs have produced mixed results, however. Here we synthesized 187 effects from 78 experimental studies in a meta-analytic test of two questions; namely, whether predators, parasites and parasitoids express preferences for the sexual signals of prey, and whether sexual signals increase realized predation risk in the wild. We found that predators and parasitoids express strong and consistent preferences for signals in forced-choice contexts. We found a similarly strong overall increase in predation on sexual signallers in the wild, though here it was modality specific. Olfactory and acoustic signals increased the incidence of eavesdropping relative to visual signals, which experienced no greater risk than controls on average. Variation in outcome measures was universally high, suggesting that contexts in which sexual signalling may incur no cost, or even reduce the incidence of predation, are common. Our results reveal unexpected complexity in a central viability cost to sexual signalling, while also speaking to applied problems in invasion biology and pest management where signal exploitation holds promise for bio-inspired solutions.

Phylogeny and secondary sexual trait evolution in Schizocosa wolf spiders (Araneae, Lycosidae) shows evidence for multiple gains and losses of ornamentation and species delimitation uncertainty

Members of the Nearctic spider genus Schizocosa Chamberlin, 1904 have garnered much attention in behavioral studies and over many decades, a number of species have developed as model systems for investigating patterns of sexual selection and multimodal communication. Many of these studies have employed a comparative approach using putative, but not rigorously tested, sister species pairs that have distinctive morphological traits and attendant behaviors. Despite past emphasis on the efficacy of these presumably comparative-based studies of closely related species, generating a robust phylogenetic hypothesis for Schizocosa has been an ongoing challenge. Here, we apply a phylogenomic approach using anchored hybrid enrichment to generate a data set comprising over 400 loci representing a comprehensive taxonomic sample of 23 Nearctic Schizocosa. Our sampling also includes numerous outgroup lycosid genera that allow for a robust evaluation of genus monophyly. Based on analyses using concatenation and coalescent-based methods, we recover a well-supported phylogeny that infers the following: 1) The New World Schizocosa do not form a monophyletic group; 2) Previous hypotheses of North American species require reconsideration along with the composition of species groups; 3) Multiple longstanding model species are not genealogically exclusive and thus are not "good" species; 4) This updated phylogenetic framework establishes a new working paradigm for studying the evolution of characters associated with reproductive communication and mating. Ancestral character state reconstructions show a complex pattern of homoplasy that has likely obfuscated previous attempts to reconstruct relationships and delimit species. Important characters presumably related to sexual selection, such as foreleg pigmentation and dense bristle formation, have undergone repeated gain and loss events, many of which have led to increased morphological divergence between sister-species. Evaluation of these traits in a comparative framework illuminates how sexual selection and natural selection influence character evolution and provides a model for future studies of multimodal communication evolution and function.

Courtship behavior and coloration influence conspicuousness of wolf spiders (Schizocosa ocreata (Hentz)) to avian predators

Signalers must balance the benefits of detection by intended receivers with the costs of detection by eavesdroppers. This trade-off is exemplified by sexual signaling systems, in which signalers experience sexual selection for conspicuousness to mates as well as natural selection for crypsis to predators. In this study, we examined how courtship behavior and body coloration influenced the conspicuousness of males to avian predators in the well-studied brush-legged wolf spider system (Schizocosa ocreata (Hentz)). We focused on three behaviors (courtship, walking, and freezing) and two coloration schemes (natural coloration and idealized background-matching coloration). We presented captive blue jays (Cyanocitta cristata) with video playbacks of male spiders in a presence-absence detection task and characterized conspicuousness by measuring response latency and detectability. We found that any type of motion significantly increased detectability, and that body coloration and behavior interacted to determine detectability while the spiders were in motion. Among spiders in motion, courting spiders were detected faster than walking spiders. Stationary (frozen) spiders, in contrast, were rarely detected. These results illustrate that male S. ocreata can be both highly conspicuous and highly cryptic to avian predators. Thus, while we find that courtship is conspicuous to avian predators in this system, we suggest that behavioral plasticity may mitigate some of the predation costs of the sexual signal.

Frequent misdirected courtship in a natural community of colorful Habronattus jumping spiders

Male courtship display is common in many animals; in some cases, males engage in courtship indiscriminately, spending significant time and energy courting heterospecifics with whom they have no chance of mating or producing viable offspring. Due to high costs and few if any benefits, we might expect mechanisms to evolve to reduce such misdirected courtship (or ‘reproductive interference’). In Habronattus jumping spiders, males frequently court heterospecifics with whom they do not mate or hybridize; females are larger and are voracious predators, posing a severe risk to males who court indiscriminately. In this study, we examined patterns of misdirected courtship in a natural community of four sympatric Habronattus species (H. clypeatus, H. hallani, H. hirsutus, and H. pyrrithrix). We used direct field observations to weigh support for two hypotheses (differential microhabitat use and species recognition signaling) to explain how these species reduce the costs associated with misdirected courtship. We show that, while the four species of Habronattus do show some differences in microhabitat use, all four species still overlap substantially, and in three of the four species individuals equally encountered heterospecifics and conspecifics. Males courted females at every opportunity, regardless of species, and in some cases, this led to aggression and predation by the female. These results suggest that, while differences in microhabitat use might reduce misdirected courtship to some extent, co-existence of these four species may be possible due to complex communication (i.e. species-specific elements of a male’s courtship display). This study is the first to examine misdirected courtship in jumping spiders. Studies of misdirected courtship and its consequences in the field are limited and may broaden our understanding of how biodiversity is maintained within a community.

Choosing a mate in a high predation environment: Female preference in the fiddler crab Uca terpsichores

The interplay between a receiver's sensory system and a sender's courtship signals is fundamental to the operation of sexual selection. Male courtship signals that match a female receiver's preexisting perceptual biases can be favored yet the message they communicate is not always clear. Do they simply beacon the male's location or also indicate his quality? We explored this question in a species of fiddler crab Uca terpsichores that courts under elevated predation risk and that mates and breeds underground in the safety of males' burrows. Sexually receptive females leave their own burrows and are thereby exposed to avian predators as they sequentially approach several courting males before they choose one. Males court by waving their single greatly enlarge claw and sometimes by building a sand hood next to their burrow entrance. Hoods are attractive because they elicit a risk-reducing orientation behavior in females, and it has been suggested that claw waving may also serve primarily to orient the female to the male. If the wave communicates male quality, then females should discriminate mates on the basis of variation in elements of the wave, as has been shown for other fiddler crabs. Alternatively, variation in elements of the claw waving display may have little effect on the display's utility as a beacon of the location of the male and his burrow. We filmed courting males and females under natural conditions as females responded to claw waving and chose mates. Analysis of the fine-scale courtship elements between the males that females rejected and those they chose revealed no differences. When predation risk during courtship is high, males' courtship displays may serve primarily to guide females to safe mating and breeding sites and not as indicators of male quality apart from their roles as beacons.