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.