Mate availability does not influence mating strategies in males of the sexually cannibalistic spiderArgiope bruennichi

Abstract

BackgroundSexual selection theory predicts that male investment in a current female should be a function of female density and male competition. While many studies have focused on male competition, the impact of female density on male mating investment has been widely neglected. Here, we aimed to close this gap and tested effects of mate density on male mating decisions in the orb-web spider Argiope bruennichi. Males of this species mutilate their genitalia during copulation, which reduces sperm competition and limits their mating rate to a maximum of two females (bigyny). The mating rate is frequently further reduced by female aggression and cannibalization. Males can reduce the risk of cannibalism if they jump off the female in time, but will then transfer fewer sperm. An alternative solution of this trade-off is to copulate longer, commit self-sacrifice and secure higher minimal paternity. The self-sacrificial strategy may be adaptive if prospective mating chances are uncertain. In A. bruennichi, this uncertainty may arise from quick changes in population dynamics. Therefore, we expected that males would immediately respond to information about low or high mate availability and opt for self-sacrifice after a single copulation under low mate availability. If male survival depends on information about prospective mating chances, we further predicted that under high mate availability, we would find a higher rate of males that leave the first mating partner to follow a bigynous mating strategy.MethodWe used naïve males and compared their mating decisions among two treatments that differed in the number of signalling females. In the high mate availability treatment, males perceived pheromone signals from four adult, virgin females, while in the low mate availability treatment only one of four females was adult and virgin and the other three were penultimate and unreceptive.ResultsMales took more time to start mate searching if mate availability was low. However, a self-sacrificial strategy was not more likely under low mate availability. We found no effects of treatment on the duration of copulation, the probability to survive the first copulation or the probability of bigyny. Interestingly, survival chances depended on male size and were higher in small males.DiscussionOur results do not support the hypothesis that mate density variation affects male mating investment, although they clearly perceived mate density, which they presumably assessed by pheromone quantity. One reason for the absence of male adjustments to mating tactics could be that adaptations to survive female attacks veil adaptations that facilitate mating decisions.

An Economic Dilemma between Molecular Weapon Systems May Explain an Arachno-Atypical Venom in Wasp Spiders (Argiope bruennichi)

“…We selected the wasp spider Argiope bruennichi, which features a wasp-like banding pattern that may have evolved as a tool to lure prey [37]. This species is used as a model organism for the investigation of sexual dimorphism, chemical ecology, reproductive behavior, microbiome analysis and range expansion linked to climate change [38][39][40][41][42][43][44][45][46][47]. Its venom has been extracted for bioactivity assays but has not been analyzed in detail [27].…”

Males of a sexually cannibalistic spider chemically assess relative female quality

“…Previous studies suggest that males rather use a threshold rule than a best-of-n strategy [51,53,54]. Under best-of-n strategy, males should either sample several females before they mate or they should immediately visit the largest female, if they are able to distinguish females of different body size via chemical cues.…”

“…Under best-of-n strategy, males should either sample several females before they mate or they should immediately visit the largest female, if they are able to distinguish females of different body size via chemical cues. However, in several previous studies, males showed neither of these mate searching patterns [51,54]. In contrast, males adjusted their mating strategy (monogyny, bigyny) to female body size even though no alternative The test frames were custom-made and allowed for stationing females on fixed positions within the arena.…”

“…We presented the females at different distances relative to the male starting position ( Fig. 1b) with the aim to mimic a natural situation as well as the conditions in previous studies [51,52,54]. The distribution of different sized females among these positions was randomised so that distance and female size could be analysed independently.…”

An economic dilemma between weapon systems may explain an arachno-atypical venom in wasp spiders (Argiope bruennichi)

“…We selected the wasp spider Argiope bruennichi, which features a wasp-like banding pattern that may have evolved as a mimetic warning trait (Bush et al, 2008) This species is used as a model organism for the investigation of sexual dimorphism, chemical ecology, reproductive behavior, microbiome analysis and range expansion linked to climate change (e.g. Chinta et al, 2010;Welke and Schneider, 2010;Zimmer et al, 2012;Wilder and Schneider, 2017;Cory and Schneider, 2018;Ganske and Uhl, 2018;Sheffer et al, 2019;Krehenwinkel and Tautz, 2013;Krehenwinkel and Pekar, 2015;. Its venom has been extracted for bioactivity assays but has not been analyzed in detail (Friedel and Nentwig, 1989).…”