THE INFLUENCE OF FOOD-SUPPLY ON FORAGING BEHAVIOR IN A DESERT SPIDER

We tested the alternative hypotheses that foraging effort will increas e (energy maximizer model) or decrease (due to increased costs or risk s) when food supply increased, using a Namib desert burrowing spider, Seothyra henscheli (Eresidae), which feeds mainly on ants. The web of S. henscheli has a simple geometrical configuration, comprising a hori zontal mat on the sand surface, with a variable number of lobes lined with sticky silk. The sticky silk is renewed daily after being covered by wind-blown sand. In a field experiment, we supplemented the spider s' natural prey with one ant on each day that spiders had active webs and determined the response to an increase in prey. We compared the fo raging activity and web geometry of prey-supplemented spiders to non-s upplemented controls. We compared the same parameters in food-deprived and supplemented spiders in captivity. The results support the ''cost s of foraging'' hypothesis. Supplemented spiders reduced their foragin g activity and web dimensions. They moulted at least once and grew rap idly, more than doubling their mass in 6 weeks. By contrast, food-depr ived spiders increased foraging effort by enlarging the diameter of th e capture web, We suggest that digestive constraints prevented supplem ented spiders from fully utilizing the available prey. By reducing for aging activities on the surface, spiders in a prey-rich habitat can re duce the risk of predation. However, early maturation resulting from a higher growth rate provides no advantage to S. henscheli owing to the fact that the timing of mating and dispersal are fixed by climatic fa ctors (wind and temperature). Instead, large female body size will inc rease fitness by increasing the investment in young during the period of extended maternal care.