Functional Ecology 2001 15, 24–28 © 2001 British Ecological Society 24 Blackwell Science, Ltd Mortality risk of rapid growth in the spider Nephila clavipes L. E. HIGGINS*† and M. A. RANKIN‡ *Department of Entomology, University of Massachusetts, Amherst MA 01003, USA, and ‡Department of Zoology, University of Texas at Austin, Austin, TX 78712, USA Summary 1. Female Nephila clavipes from univoltine populations have greatly reduced repro- ductive success if they grow slowly and reach maturity late in the growing season. Although such fitness costs are expected to select for rapid increases in mass, several authors have presented models and data describing physiological costs of rapidly increasing mass. 2. In order to test the hypothesis that there are inherent costs of rapid growth (increasing mass), laboratory-reared juveniles of the orb-weaving spider Nephila clavipes were randomly assigned to receive daily feedings ranging from 2·5% to 23% of their initial mass. 3. Spiders receiving higher amounts of food were more likely to die at or immediately before the next moulting cycle. 4. These results indicate that there may be inherent physiological costs of rapidly increasing mass. In opportunistic feeders such as spiders that tend to gorge when prey are abundant, this could present a real cost to a common foraging strategy. Key-words : Aranea, fitness costs, growth rates, plasticity, Tetragnathidae Functional Ecology (2001) 15, 24–28 Introduction Although variation in growth rate has traditionally been assumed to reflect non-adaptive phenotypic re- sponses to the availability of food (e.g. Roff 1980, 1983; Stearns & Koella 1986), experimental work with some insects has demonstrated adaptive variation in rate of growth (reviewed in Gotthard et al. 1994). Low rate of increasing mass in larval arthropods increases the duration of each larval stage, and is associated with both increased risk of prereproductive mortality (Higgins & Rankin 1996) and reproductive costs of delayed maturation (Stearns & Koella 1986; Higgins & Rankin 1996; Higgins 2000). Although very high rates of growth have been often assumed to increase fitness, allowing animals to avoid the above-mentioned costs of slow growth, there is increasing evidence that there are ecological and physiological costs asso- ciated with rapidly increasing mass (Stockhoff 1991; Wiklund et al. 1991; Gotthard et al. 1994; Abrams et al. 1996; Arendt 1997). Ecological costs of rapid growth are primarily assumed to reflect risk of predation and parasitism associated with the increased foraging necessary to increase mass more rapidly (Abrams et al. 1996). Such costs are postulated to underlie observed patterns of declining investment into foraging when prey are abundant (Lubin & Henschel 1996). The best demonstrated physiological cost of rapid growth is decreased resistance to starvation and other environ- mental stresses (Stockhoff 1991; Gotthard et al. 1994). One can postulate other physiological costs of rapidly increasing mass. For example, rapidly increasing mass may decrease resource allocation to other functions, such as development (Arendt 1997). Under these circumstances, high rates of growth alone could increase mortality in the absence of external environ- mental stresses. In populations of the orb-weaving spider Nephila clavipes (Linnaeus) (Araneae: Tetragnathidae) inhabit- ing strongly seasonal environments, there are strong fitness costs associated with slow growth and develop- ment in females (Higgins 2000). The duration of each juvenile instar is negatively correlated with the rate of increasing mass, and the number of juvenile instars is variable (Higgins 1992, 1993). Size at matur- ity is determined in large part in this species by the number of juvenile instars (Higgins & Rankin 1996), and rapidly growing females pass through several instars and reach maturity at a large size earlier than slowly growing females. There is a strong repro- ductive advantage to early maturation: large, early maturing females have disproportionally greater reproductive success, producing multiple large clutches. †Author to whom correspondence should be addressed.