Nuptial gifts beat pheromones

Unlike many other species, male hunting spiders do not use chemical signals such as sex pheromones to attract a mate. Instead, they make their mark by uniquely exploiting a female hunting spider's interest in food. Research led by Cristina Tuni of the Ludwig Maximilian University in Munich in Germany now shows that male hunting spiders wrap morsels of food in their silk and offer these as gifts to prospective mates. The study is published in Springer's journal Behavioural Ecology and Sociobiology.

Many previous studies have examined the role that sex-specific pheromones or semiochemicals play in the evolution of a spider's reproduction cycle. Pheromones can be emitted both from a spider's body and from its silk. This form of chemical messaging allows for long-distance air-borne, or direct communication between spiders, and enables them to locate a mate, distinguish between males and females, or decide on the suitability of a potential partner.

Among web-building species, females living on webs rely on their pheromones to be carried through the air so that potential males can determine their whereabouts and learn more about their sexual maturity and mating status. Webless wandering species such as the hunting spider (Pisaura mirabilis) often depend on so-called draglines which hang, for example, from branches. These draglines are important not only for movement, but as part of the process of searching and attracting mates. Males also use their silk in another way: they are among only a few species that offer nuptial gifts of prey wrapped in dense layers of silk to females, to be eaten during copulation.

Tuni and her colleagues conducted a series of experiments on around 100 spiders (Pisaura mirabilisi) to test whether the silk that male and female hunting spiders produce is an important part of mating, and if sex pheromones are always released.

The study revealed that male and female spiders do not have the same reaction to silk. Males were attracted to the draglines that females produced. According to the researchers, this suggests that there are chemical cues attached to these silk draglines, and these likely serve as a form of female advertisement. Signalling through draglines may also be a way for females to supplement their own efforts to find food, because it lures would-be gift-carrying mates.

Quite unexpectedly, the researchers found that females had no interest in the draglines that males produced, nor the silk that they used to wrap nuptial gifts in. This suggests that male hunting spiders do not release chemical signals.

"This suggests that males rather may be uniquely exploiting females' interest in food through their gift-giving behaviour," says co-author Michelle Beyer, who adds that females might also have learnt to ignore chemical signals, because males deceive them about the quality of the food hidden in the silk-wrapped gifts presented to them.

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Reference: Beyer, M. et al (2018). Does silk mediate chemical communication between the sexes in a nuptial feeding spider? Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-018-2454-1

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https://www.springer.com/gp/about-springer/media/research-news/all-english-research-news/nuptial-gifts-beat-pheromones/15499442 http://dx.doi.org/10.1007/s00265-018-2454-1