Determinants of Natural Mating Success in the Cannibalistic Orb-Web Spider Argiope bruennichi

Monogynous mating systems (low male mating rates) occur in various taxa and have evolved several times independently in spiders. Monogyny is associated with remarkable male mating strategies and predicted to evolve under a male-biased sex ratio. While male reproductive strategies are well documented and male mating rates are easy to quantify, especially in sexually cannibalistic species, female reproductive strategies, the optimal female mating rate, and the factors that affect the evolution of female mating rates are still unclear. In this study, we examined natural female mating rates and tested the assumption of a male-biased sex ratio and female polyandry in a natural population of Argiope bruennichi in which we controlled female mating status prior to observations. We predicted variation in female mating frequencies as a result of spatial and temporal heterogeneity in the distribution of mature females and males. Females had a low average mating rate of 1.3 and the majority copulated only once. Polyandry did not entirely result from a male-biased sex-ratio but closely matched the rate of male bigamy. Male activity and the probability of polyandry correlated with factors affecting pheromone presence such as virgin females' density. We conclude that a strong sex ratio bias and high female mating rates are not necessary components of monogynous mating systems as long as males protect their paternity effectively and certain frequencies of bigyny stabilise the mating system

Flowering, Pollination, and Fruit Set in Tongue Orchids \u27Cryptostylis\u27 spp

Volume: 123Start Page: 128End Page: 13

Convergent evolution of sexual deception via chromatic and achromatic contrast rather than colour mimicry

The Orchidaceae is characterised by the repeated evolution of sexual deception, one of the most specialised pollination strategies. In orchids, sexual deception involves long-range pollinator attraction via mimicry of female insect sex pheromones. At close range, visual signals involving colour mimicry, contrast to the background, and exploitation of pollinator sensory biases could attract pollinators, but remain largely untested. Here we focus on a remarkable system in which species from two only distantly related sexually deceptive orchid genera with strikingly different flowers (Drakaea livida and three species of Caladenia) share the same pollinator, males of the thynnine wasp Zaspilothynnus nigripes. We used spectral reflectance measurements and modelling to investigate pollinator perception of colour, including the first examination of overall colour patterns in flowers via colour pattern geometry analyses. Rather than closely matching the colours of female Z. nigripes, these orchids had strong chromatic and achromatic contrast against their backgrounds. For Caladenia, the sepals and petals show high contrast, while in D. livida, which has diminutive petals and sepals, it is the labellum that contrasts strongly against the background. Despite varying in colour, the Caladenia species all had strong within-flower contrast between a UV-bright central target (column and labellum) and a corolla of radiating stripes (petals and sepals). The colour pattern geometry analyses also indicated that the orchids’ overall colour patterns are highly conspicuous against their backgrounds. Contrast, UV, and target patterns could all enhance detection, and exploit pollinators’ innate preferences. Since colour contrast may function with a range of colours and floral forms, attracting pollinators via contrast rather than visual mimicry may be a critical but previously overlooked process facilitating the evolution of sexual deception

Flowers, phenology and pollination of the endemic New Zealand greenhood orchid <i>Pterostylis brumalis</i>

<p>Greenhood orchids are most diverse in New Zealand and Australia, with at least some species pollinated by sexual deception of male fungus gnats. Preliminary investigation of endemic New Zealand <i>Pterostylis brumalis</i> via field phenology, hand-pollination and insect trapping revealed <i>P. brumalis</i> is self-compatible but relies on insects for cross pollination. Natural fruit-set was low, averaging 2.6% across two populations. High fruit-set in hand-pollinated flowers (66.7%) indicates severe pollinator limitation. In comparison, only one collected insect, a female <i>Mycetophila vulgaris</i>, bore orchid pollinia. Potential pollinator attractants (colour and scent) were analysed. UV reflectance may enhance pollinator detection of the flower against the background, but visual modelling generated conflicting results: a categorical fly vision model indicated excellent camouflage against herbivory, but a generic tetrachromat vision model suggested flowers are easily distinguishable against the background. Both models have flaws that create issues with drawing conclusions, and identification of the pollinator is required to understand their visual system further. Scanning electron microscopy (SEM) revealed putative scent-releasing osmophores (lateral sepals) and uniform papillate trichomes (labellum) that may mechanically direct pollinators into the floral trap. The trap is a pressure sensitive labellum that temporarily traps the visitor inside the flower.</p

Cryptic female choice within the genus Argiope : a comparative approach

The orb-web spider genus Argiope (Araneae) offers an excellent opportunity to detect cryptic female choice and to identify the species-specific traits that might lead to its evolution and maintenance. All studied species of the genus Argiope are characterized by low male mating rates. While males of some species are strictly mono- or bigynous, others plastically switch between these two strategies. All studied species show sexual cannibalism during copulation. Generally, males die after their second copulation, but the probability of surviving the first copulation differs considerably between species and so does copulation duration. Males of most species break off pieces of their genitalia during copulation that act as mating plugs, but how often this occurs and how effective these plugs are is highly variable. Females that mate multiply can influence the relative paternity success of males through their partial control of copulation duration and their likely control of sperm storage. There is evidence that females cryptically favor small males over large ones, unrelated males over siblings, and males that courted over those that do not. We will sketch variation within and between species in mating systems and related traits, and we will discuss how this relates to cryptic female choice. We will review the existing evidence for cryptic female choice and suggest future avenues in elucidating possible mechanisms that facilitate cryptic female choice and the cues that females may base their choices on.23 page(s