Sexual Differences in Metabolic Rates of Spiders

In general, spiders are considered to exhibit resting metabolic rates about half of those measured for other poikilothermic animals of equal mass (e .g ., Anderson 1970; Greenstone & Bennett 1980; Anderson & Prestwich 1982 ; Anderson 1987; Paul et al. 1989; Anderson 1996). However, these metabolic rates were all compared to Hemmingsen's poikilotherm mass-scaling equation (1960), which has recently been shown to systematically overestimate metabolism in small animals (Lighton & Fielden 1995) . Thus, almost any data on standard metabolic rates result in low values of metabolic rate compared to this equation . More directly, Lighton & Fielden (1995) further showed that metabolic rates for spiders (22 genera) do not differ from those of ants [Formicidae (10 genera)] and beetles [Tenebrionidae (8 genera)] of comparable size . However, under prolonged starvation spider metabolic rates may be below the standard metabolic rate (Ito 1964; Nakamura 1972 ; Anderson 1974), thus making them well adapted to environments with unpredictable food availability . Almost all studies on spider metabolic rates have used only adult females (Table 1) . This may be due mainly to the very influential paper on the field by Anderson (1970) . He reasoned that using juveniles or males may complicate the data because the growth of the juveniles or the relatively high activity patterns of males may affect oxygen consumption. However, given the many differences in life-history characteristics between female and male spiders in general (e .g ., size, longevity, reproductive efforts), it might be possible that there are also ecologically significant differences in energy consumption between females and males . Indeed, Edgar (1971) reported differences in female and male growth efficiency (ratio food consumed/weight increase) in Pardosa lugubris (Walkenaer) ; and Bromhall (1987), having found a significant difference in heart-rates between males and females of