A project undertaken at Macquarie University, and supervised by Lizzie Lowe.
Animals are locked in a perpetual evolutionary arms race, where prey must elude predators, and predators must overcome prey defenses. Predator prey interactions have driven the evolution of a huge diversity of anti-predator traits. In this project, we took advantage of the natural history of mud dauber wasps to evaluate the effectiveness of different anti-predatory strategies adopted by spiders, the prey of mud dauber wasps.
Female wasp stocking her nest with a jumping spider. She has built six nests, laying a single egg in each and stocking them with spiders she has captured and paralysed. Once the nest is full of spiders, she closes it and leaves her offspring to hatch, eat the spiders, pupate and emerge from the nest as an adult. Photograph: Jim McLean
Mud dauber wasps construct a small cylindrical nest from mud, capture (and paralyse) spiders to stock the nest (figure 1), and lay a single egg inside the nest. The egg hatches into a larva that consumes the spiders provided by its mother, then pupates before emerging from the nest as an adult. Traditionally, it has been difficult to objectively measure the rates at which spiders are preyed upon, however, the unusual life-history of the mud dauber wasps provides us with a unique opportunity. By collecting and identifying the spiders within the wasps’ nests, we can determine all of the prey collected by each wasp.
By comparing the relative abundances of spiders found in wasp nests to those in the environment, we were able to determine whether some spiders, such as those that are strongly camouflaged (“cryptic”) are better protected from predation by these visually hunting wasps. Indeed, we expected to find that cryptic spiders benefit from a lower rate of predation than more conspicuous spiders. Surprisingly, that was not what we found.
Rather, we found that individual wasps seemed to become particularly efficient at capturing certain types of prey. For example, the highly cryptic two-tailed spiders (Hersiliidae) occurred at much higher frequencies in the wasps’ nests than expected, suggesting that crypsis is not an effective anti-wasp strategy. Rather than finding that a particular anti-predatory strategy is effective against mud dauber wasps, we found that individual wasps appear to “learn” to locate certain types of spiders with higher than expected efficiency. This suggests that the wasps adopt a “search image” to guide their search for prey, probably based on the spiders they encounter early on during their hunting. Predators that hunt based on a search image tend to increase the survival of rare prey, since they are unlikely to be encountered early, and hence predators are unlikely to form a search image for rare prey. Consequently, rare prey or even new forms experience a survival benefit over more common forms, which tends to favour an evolutionary increase in prey diversity.
Our findings suggest that the hunting strategy adopted by wasps, rather than favouring the evolution of any one type of anti-predatory defense, favours a diversity of defenses, contributing to the development and maintenance of prey diversity.