Age-related sequential web building in the colonial spider Metepeira incrassata (Araneidae): an adaptive spacing strategy

Colonial orb-weaving spiders provide insight into the proximate mechanisms by which social animals space themselves within a group. We examined mechan isms for the temporal patterns of web building that determine individual po sitions in Metepeira incrassata (Araneidae) colonies. The spiders display a characteristic age-related sequence of daily web building, with larger spi ders completing their webs significantly earlier than smaller ones. We used data on behavioural interactions, web building, prey capture and predator attacks to evaluate four hypotheses. (1) Larger spiders are better competit ors and pre-empt optimal spatial positions. (2) Smaller spiders reduce comp etition with larger individuals by building webs later. (3) Prey captured b y different size classes is available at different times. (4) Differential predation risk determines web-building times. Large individuals dominated b ehavioural interactions. Disturbances by larger spiders during web construc tion significantly delayed the completion of smaller individuals' webs and precipitated movements to new web sites. One prediction of the first hypoth esis, that spatial needs translate into earlier building, was confirmed by significantly earlier web building by mature females with egg sacs (which a re unable to move their egg sacs:) compared with same-sized females without eggs (which can change locations freely). Experiments to determine whether the presence of large spiders inhibited the web building of smaller indivi duals were equivocal. Prey availability and risk of predation are not facto rs affecting web-building patterns. Sequential web building appears to be a result of both larger spiders competing to pre-empt space from one another and smaller individuals attempting to reduce conflict during web construct ion. Sequential web building is a proximate mechanism that influences spaci ng among colonial orb-weaving spiders and helps shape the typical hierarchi cal size distribution of spiders within the colony. Similar spacing mechani sms may be seen in colonial birds and marine invertebrates. (C) 2000 The As sociation for the Study of Animal Behaviour.