1. A number of strategies that divert attacks of visually guided predators, such as birds, have evolved multiple times in animals. Detritus web decorations built by certain orb-web spider species are thought to deflect avian predator attacks away from spiders and towards their web decorations. Still, empirical evidence for this function and its adaptive significance is lacking. The orb-web spider, Cyclosa monticola, adorns its web using a linear detritus decoration consisting of moults, egg sacs, prey remains and leaf litters. 2. In the present study, we investigated whether detritus decorations constructed by C. monticola spiders divert attacks of avian predators away from spiders. We first employed colour modelling to compare spider bodies and detritus decoration colouration from the perspective of domestic chicks and blue tits. We then experimentally tested the deflection hypothesis in the laboratory using naïve chicks as predators. We put the chicks in a cage containing a web either with (S+) or without (S−) a spider and either with (D+) or without (D−) detritus decoration (a total of four types of webs: S+D+, S+D−, S−D+ and S−D−) under both natural habitat background and white background. 3. We found that the colour of C. monticola spiders is indistinguishable from that of their detritus decorations for both chicks and blue tits with both backgrounds. Laboratory predation experiments showed that with both backgrounds, chicks attacked the spiders much less frequently when their decorations were present on the webs (S+D+; natural habitat: 20%, white background: 30%) than when their decorations were absent (S+D−; natural habitat: 95%, white background: 85%), resulting in greater spider survival advantage. 4. We also found that the rate of attack of spiders and their decorations was not random; the decorations were more likely to be attacked than spiders, regardless of the ratio of surface area of decorations to spider bodies when both spiders and decorations were presented (S+D+). Therefore, our results support a deflection, rather than concealment hypothesis for web decorations. | 2111 Functional Ecology MA et Al.
Animals may consider both biotic and abiotic factors in foraging site choice. Among the biotic factors, food availability and predation risk are two widely reported important factors in determining foraging site fidelity. Their earlier investments, such as those retreat-building species' efforts in retreats construction, however, have been largely ignored. The orb-web spider Cyclosa monticola constructs a long column of masquerading detritus decoration in its web for predator avoidance purpose. This detritus decoration also functions as a retreat. However, the role of the detritus decoration for its foraging site fidelity is unknown. By manipulating three factors, presence of detritus decoration, prey availability and predation risk to the spider webs in the field, we show that the self-constructed detritus decoration is as important as prey availability and predation risk in mediating foraging site fidelity. In addition, the web area also has a significant impact to the foraging site fidelity of the spider, those with larger webs were more likely to leave after being manipulated. However, other factors such as spider body size, decoration length and rain all have no significant impact. Our study may strengthen the current understanding of the movement and foraging of animals, especially those building retreats.
Sexual conflict is common in animals, and female sexual cannibalism represents an extreme form of sexual conflict. Males in many species have evolved a variety of strategies to circumvent or decrease the risk of female sexual cannibalism. Opportunistic mating, by which a male mates with a female when she is disturbed or when she is feeding or undertaking moulting, is one of such kinds of strategies, and widely occurs in many animals, especially in spiders. However, whether the occurrence of male opportunistic mating depends on the intensity of female sexual cannibalism remains largely unexplored. We predicted a positive correlation between them. In this study, we tested this prediction by performing a series of mating trials in the laboratory using three species of web-building spiders with different intensities of female sexual cannibalism: Nephila pilipes, Nephilengys malabarensis, and Parasteatoda tepidariorum. We found that the occurrence of male opportunistic mating was positively, though not statistically significantly, correlated with the intensity of female sexual cannibalism, thus supporting our hypothesis. All together, we provide evidence that male opportunistic mating may have evolved to respond to the selection pressure posed by female sexual cannibalism.
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