DIFFERENTIAL MORTALITY AND RELATIVE MATERNAL INVESTMENT IN DIFFERENT LIFE STAGES IN STEGODYPHUS LINEATUS ( ARANEAE , ERESIDAE )

The general reproductive pattern in spiders is to produce large clutches with small egg s although some species produce a few, large eggs . The spider Stegodyphus lineatus (Eresidae) is unusual in that it lays a single extremely small clutch and has small eggs . The female feeds the young afte r hatching and eventually the young completely consume her. Differential mortality risks in different life stages may provide an explanation of relative materna l investment in eggs versus hatchlings . By monitoring natural populations of S . lineatus, sources and rate s of mortality were assessed during the reproductive period . Complete failure of reproduction was mor e likely during egg development than after the spiderlings had hatched. This was partially explained by the presence of a parasitoid wasp that only attacked females before the young had hatched . Overall attac k rates were similiar in the egg stage and in the hatchling stage ; however, the spiderlings suffered les s mortality because they were mobile . By allocating the major reproductive effort to hatchlings rather than eggs, these spiders may pass throug h the risky egg stage relatively fast ; and overall development and growth can be maximized . Small egg siz e might be an adaptation to high predation pressure during the reproductive period . Small clutch size can be explained by the importance of early growth for offspring fitness . Parental care is considered to increase the survival probability, and often the reproductive value, of offspring (Clutton-Brock 1991) . Accordingly, parental care is usually associated with a high investment per individua l young (Shine 1978 ; Sargent et al . 1987; Stearns 1992) . Assuming that a female has a limited amount of energy to invest in reproduction, an increase in investment per individual young will reduce the number of offspring that can be produced . Thus one may expect that animals with brood care will have relatively few but large offspring . This is indeed the case in many oviparous ectotherms, wher e the amount of parental care and propagule size are positively correlated (Shine 1978 ; Gros s & Sargent 1985 ; Sargent et al . 1987 ; Clutton Brock 1991 ; Roff 1992) . Large propagules can be achieved in two different ways: resources can be invested in offspring either by providing them with yol k at the egg stage or enhancing growth by feed'Present address : Max-Planck-Institut fur Verhalten sphysiologie, Seewiesen, D-82319 Starnberg, Germany. ing the young after hatching . The first form of resource allocation is the more common one in oviparous ectotherms in which parenta l care is mostly egg guarding and is associated with relatively large eggs (Shine 1978) . However, extended brood care does occur in many species ; and differences in the allocation of resources into the various developmenta l phases may be a result of different predation pressures in the different stages, with selection pressures similar to those proposed for th e evolution of egg size per se (Shine 1978, Sargent et al . 1987) . In the "safe-harbor" hypothesis, Shine (1978) proposed that in oviparous ectotherms that large eggs should evolve if predation pressure is high during the juvenile stage, and small eggs should be selected for if mortality is relatively high in the egg stage . In many species, total clutch mass is a function of female body mass (Craig 1987 ; Anderson 1990; Marshall & Gittleman 1994) . Variation in clutch mass mostly results from differences in clutch size rather than in egg size (Marshall & Gittleman 1994) . Most spiders lay several large clutches with small egg s