The life history of most animals is composed of three components: growth, maturation and reproduction (Vollrath 1987). Traditionally, life-history theory assumes that growth rates are maximized and vary only in response to the environment (Roff 1980; Gebhardt & Stearn 1988). However, repeated findings show that growth rates may vary even under similar conditions and that they are not always maximized but rather optimized (Gotthard 2000; Metcalfe & Monaghan 2001). The term growth compensation refers to an animal's capacity to grow faster than normal after a period of environmentally induced growth depression to recover an original body weight or growth trajectory (Jobling, Jørgensen & Sikavupio 1993; Broekhuizen et al. 1994; Nicieza & Metcalfe 1997; Metcalfe & Monaghan 2001). The very existence of compensatory growth responses demonstrates that under normal conditions individuals grow at a rate below that achievable (Broekhuizen et al. 1994; Morgan & Metcalfe 2001).
Growth compensation is seen across a wide range of taxa (Arendt 1997). It is normally achieved through ‘hyperphagia’ (increased feeding activity above the level of control animals) and is usually short-lived, although it can last for months (Jobling & Miglavs. 1993; Jobling & Baardvik 1994; Nicieza & Metcalfe 1997). Compensatory growth is mainly observed among animals that have suffered from deficiencies during early life stages, but it has also been observed among animals born late in the breeding season or animals that have experienced atypically low temperatures (Nicieza & Metcalfe 1997; Nylin & Gotthard 1998). During a stress period, affected animals can preserve irreplaceable tissue and thereby save key functions at the expense of other body functions that are less crucial to long-term fitness (Metcalfe & Monaghan 2001).
Organisms often seem to recover as a result of growth compensation, though the processes during the compensatory phase most likely carry a variety of costs and can have permanent effects on the adult individuals and even on their offspring (Metcalfe & Monaghan 2001). Costs can act at the physiological/cellular level, which are usually paid in the longer term, whereas ecological costs of compensatory responses tend to be more immediate (Gotthard, Nylin & Wiklund 1994; Abrams et al. 1996; Arendt 1997; Metcalfe & Monaghan 2001). The ecological costs may be higher susceptibility to predators and parasites because of the need for increased foraging activity (Abrams 1991; Werner & Anholt 1993; Gotthard et al. 1994; Abrams et al. 1996; Gotthard 2000). The physiological costs are often decreased resistance to starvation and other environmental stresses (Gotthard et al. 1994). Since high growth rates carry fitness costs (Higgins & Rankin 2001), the optimal solution to a setback could be to avoid these costs by delaying a life-history event rather than to pay the costs of keeping to the original time schedule (Abrams et al. 1996; Arendt 1997; Nylin & Gotthard 1998; Grand 1999; Gotthard 2000).
In temperate latitudes many animals do not have the option to set back development since the mating period is seasonal (Miyashita 1986; Vollrath 1987), unless they change from an annual to a biennial life cycle (Schaefer 1987). Gotthard et al. (1994) found that fast-growing individuals of the Speckled Wood Butterfly Pararge aegeria (Linnaeus) do not become heavier than slow-growing individuals but instead shorten their growth period. This indicates that there may be strong selection for being the ‘right’ size as well as having a short development time. Growth compensation is often incomplete, with affected animals never fully catching up in size, though sometimes animals (notably fish) may exhibit a higher net growth rate and in that way over-compensate to become even larger than control animals (Broekhuizen et al. 1994; Hayward, Noltie & Wang 1997).
We tested the ability of a Wolf Spider Pardosa prativaga (L. Koch) to compensate for early nutritional stress in an experiment with four types of stressful treatments. The stress factors used were: starvation; toxic prey, the collembolan Folsomia candida (Willem); deterrent prey, the aphid Rhopalosiphum padi (Linnaeus); and nutrient-deficient prey, using the fruit fly Drosophila melanogaster (Meigen).
The expectation was that starvation and toxic prey would be strong stress factors, deterrent prey intermediate, and the nutrient-deficient prey the least stressful treatment. The most stressful treatments were expected to cause the highest mortality and the deepest growth depression. Since coping with the stress factors is likely to be energetically costly this should result in a longer latency period until recovery starts. All other things being equal, a slower recovery speed during the subsequent compensatory growth phase might also be expected. However, the investment in compensatory growth may be traded off against the ecological and physiological costs associated with high feeding and growth rates, or may be dependent on the time constraint for reaching a certain size (Gotthard et al. 1994), and recurrent stress treatment may enhance the response, leading to overcompensation (Hayward et al. 1997). Therefore, the speed of recovery may not be simply related to the stressfulness of the treatments and exact predictions cannot be made.
Folsomia candida is considered toxic to the Wolf Spider Schizocosa because survival of hatchlings fed these collembola was lower than that of starved controls (Toft & Wise 1999). It could therefore be expected that F. candida would be a harder stress factor than starvation. Starvation was expected to be very stressful even though polyphagous predators often live with high hunger levels in the field (Juliano 1986; Bilde & Toft 1998), and spiders in particular are known to be adapted to periods of starvation (Wise 1993). Though hunger is a naturally occurring condition, spiders are vulnerable to starvation, especially during early life stages (Anderson 1974). As adults, the spiders are less sensitive (Miyashita 1968b). Spiders may survive starvation by decreasing their basal metabolic rate and by maintaining a relatively motionless sit-and-wait foraging strategy (Miyashita 1969; Anderson 1974; Wise 1993).
Rhopalosiphum padi was expected to be less stressful than starvation or F. candida but more than fruit flies. Rhopalosiphum padi has earlier been ranked as a low-quality prey to a range of generalist arthropod predators (Toft 1995; Bilde & Toft 1997). It is deterrent, as it induces specific feeding aversions (Toft 1997), but it is not toxic and may even have positive effects in mixed diets (Toft 1995). Drosophila melanogaster was expected to be the least stressful prey. It is a highly preferred prey on which most ground beetles and spiders rapidly become satiated, but its nutritional quality is generally insufficient to allow complete development (Mayntz & Toft 2001).