Arachnid Science: Article Reviews

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Roberts, J.A. & G.W. Uetz. 2005. Information content of female chemical signals in the wolf spider, Schizocosa ocreata: Male discrimination of reproductive state and receptivity. Animal Behaviour 70: 17-233.

The use of chemical messengers such as pheromones has not been examined in invertebrate predators. Can spiders communicate with chemicals to beacon their species identity, sex, age, or reproductive status to others? These authors tested the ability of male Schizocosa ocreata spiders to sense a chemical message contained on or in the silk strands of female webs. In other species, chemical signals associated with silk have been shown to induce courtship behaviors in males. Because sexual cannibalism is common in S. ocreata, there is a high selection pressure on male discrimination of female receptivity. It therefore seems likely that males of this species have developed this discrimination ability too. Male mating success and survival rest on the choice of when to contact the female—ideally, timing his arrival on the scene when she displays little aggression (i.e., at least 1 week following adult molt) and is most receptive sexually.

Silk created within the preceding 24 hours by females of different ages (subadult to 6 weeks postadult molt) were used for testing. As the male spiders were exposed to the experimental silk or the control filter paper, the researchers looked for behavioral differences in two courtship rituals: jerky tapping and chemoexploration. Jerky tapping was characterized by irregular locomotion, foreleg tapping and, sometimes, chelicerae banging against the substrate. Chemoexploratory behaviors involved cruising around the environment while actively brushing the substrate with the anterior portion of the pedipalps.

Males presented with the silk of unmated adult females showed the shortest latency to begin courtship and exploratory behaviors. They also engaged in more periods of jerky tapping and chemoexploration and these periods were of longer duration than silk from females that are too young, too old, or likely to be aggressive based on time since last molt. Males were most responsive to silk from females that were 3-5 weeks postadult, the optimal time for a male to attempt mating.

As days of the breeding season go by, males adopt a more conservative criterion for approaching and courting females. Schizocosa ocreata is a monogamous species; females copulate with a second mate in fewer than 10% of pairings. Another recent study by one of these authors demonstrated that already mated females are the most likely to cannibalize. After mating, females pose a high risk to another would-be suitor.

These findings are consistent with the idea that male S. ocreata spiders, and perhaps male spiders in general, can discriminate reproductive cues potential mates leave behind in their silk. Such discrimination leads to higher quality mates and less risk of injury or death.

For more information: www.pubs.royalsoc.as.uk

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