Effects of climate and prey availability on foraging in a social spider, Stegodyphus mimosarum (Araneae, Eresidae)

Tropical areas with favorable climatic conditions, high prey availability and large prey size are assumed to favor sociality in spiders. Notwithstanding, the three social species of Stegodyphus (Eresidae) inhabit arid and semi-arid habitats with marked daily and seasonal variation in climate. The nests of the social spider Stegodyphus mimosarum Pavesi commonly occur in dry Acacia savanna in southern Africa. We investigated the abiotic conditions to which the nests of S. mimosarum are exposed and the changes in availability of potential insect prey at different times of year and over the daily cycle. We used these data to determine the extent to which prey availability and climatic conditions explain seasonal and daily variation in the activity of the spiders. Data were collected during four sampling periods a year over two years from nests of S. mimosarum located on the Mkomazi River Bridge (KwaZulu-Natal, South Africa). We measured ambient and nest temperatures and in a sample of nests, spider growth rate, prey availability, foraging activity and activity on the web at night. Spiders had two periods of increased growth rate occurring in early and late summer, at times of year when ambient temperature rarely falls below 20 °C. Temperatures inside the nest were generally higher than ambient throughout the day and night. Foraging response, measured as the numbers of individuals responding to the vibrations of a tuning fork, was significantly higher by night than by day. In summer, foraging response decreased with increasing temperature during the day, whereas in winter, there was a positive correlation between foraging response and temperature at night. Potential prey, measured as mean numbers of insects trapped in a sample of webs, were more abundant during the day than at night, despite the fact that the spiders were most active on the web at night. Nocturnal insects, however, were larger than diurnal ones and spiders handled significantly more large prey both during the day and at night. Correlation and partial correlation analyses indicate that ambient temperature and windspeed play a direct role in influencing foraging and other activity on the web. Nonetheless, the predominance of nocturnal activity in both summer and winter could not be explained by climatic conditions and prey availability alone. Some other factor (e.g., predation or parasitism) may be involved.