Testing the optimal foraging theory

This project has investigated the behavioural responses of orb-web spiders to varying foraging conditions, based on the prediction generated by foraging theory. Accordingly, animals adjust their foraging behaviour with respect to the costs and benefits associated with foraging. In orb-web spiders (Araneidae), a prerequisite to fulfil these assumption is finding a suitable habitat, applying efficient foraging strategies, and the ability to recognise and avoid potential risks. The nocturnal orb-web spider Larinioides sclopetarius occurs in clumped dispersion patterns near water and in the vicinity of artificial lights. These spiders have conquered urban habitats to their advantage, as insects, highly abundant near water, are attracted by the artificial lights. In order to exploit this overabundance of food, spiders actively search for web sites adjacent to artificial light, and this response is genetically predetermined in these animals. Moreover, L. sclopetarius synchronise their foraging activities to those of their potential prey. In addition to the obvious benefit of choosing lit habitats, there are also costs associated with these. The feeding conditions of spiders foraging near light is better, the females grow larger and produce larger eggs, compared to conspecifics feeding apart from light. However, the spider densities near light are very high, resulting in space competition, where larger individuals actively defend their web sites, while weaker competitors are displaced to suboptimal unlit sites. In accordance with the optimal foraging theory, these spiders trade-off cost and profits: smaller spiders with a weaker body condition are less risk-aversive, they more likely embark on intraspecific contests. Similarly, spiders in poorer body condition accept a larger spectrum of prey sizes, while satiated individuals ignore large, potentially dangerous insects as well as prey, which are too small and thus energetically unprofitable. Spiders must also be able to recognise variable environmental conditions to alter their behaviour adequately. For example, the squally winds in their natural habitats is channelled rapidly between the buildings and may dislodge the spiders from their webs. Confronted with wind to their ventral side, L. sclopetarius respond by switching to the other web side, which brings them higher stability at the hub. Therefore, these spiders do not simply react on danger by escape behaviour, but also respond by an adequate and complex change of position in the orb-web. However, spiders are also able to change the design of their webs according to their energy requirements based on experience. Orb-webs are typically vertical oriented and asymmetrical, which means that the region above the hub is smaller and contains less silk than the region below. The adaptive value of asymmetric webs may be a greater foraging success. Prey is detected earlier and captured more quickly when entangled in the lower web region, because spiders typically sit head down in the hub of the web facing the lower web region. The degree of asymmetry in the webs of L. sclopetarius may be influenced by gravitational forces during web construction; it increases with increasing spider size and spider weight. Moreover, experience enables spiders to construct a more efficient trap. It was shown for the first time, that long-term memory may contribute to the behaviour of spiders. Web-building experienced L. sclopetarius constructed more asymmetric webs than conspecifics deprived of any prior building experience. Spiders may also make use of short-term memory. Based on previous prey capture experience, they do not only alter parameters such as web size and mesh height., but they also vary specific regions of the web. L. sclopetarius optimised web design by increasing those web regions which previously intercepted most prey. Finally, these spiders also trade off the costs and benefits of foraging versus the increased exposure to predators during foraging. L. sclopetarius is either found in the hub or in a retreat above the web. Sitting in the hub, spiders may suffer from exposure to predators and unfavourable weather conditions. However, residing at the hub allows spiders to detect and capture prey more quickly than spiders hidden in a retreat. When the retreat was experimentally reversed, the flexibility of these spiders allowed them to create an untypical designed web. The asymmetry of the web was reversed reducing the retreat-hub distance. This in turn allows a spider to arrive prey more quickly and to escape to the retreat more quickly in dangerous situations.