The effects of food availability on reproductive success in a false widow spider

All animals require food to survive, grow, reproduce, and thus optimize fitness in nature. Food availability can profoundly affect demographic parameters such as longevity and fecundity. Here, we compared reproductive parameters in the false widow spider, Steatoda grossa (Araneae, Theridiidae), when the availability and size of prey (the house cricket, Acheta domesticus) were manipulated. Adult mated female spiders that were fed weekly (constant prey treatment) produced more progeny during their lifetime than females fed every 3 weeks (intermittent prey treatment). Furthermore, the monitoring of fecundity schedules showed that over the first 10 egg sacs, the mean number of neonate spiderlings per egg sac was around 40% higher in constantly fed than intermittently fed spiders. Time intervals between egg sac productions were generally higher when prey availability was lower. Some females lost more than 50% of their body mass after the production of the first egg sac, although reproductive investment tended to decrease thereafter. The amount of prey offered to females significantly affected mass gain between reproductive events, but fewer progeny were produced by females per egg sac, as well as cumulatively over the first three egg sacs, when they were only fed small prey. Starved females that had not produced egg sacs in several months exhibited total reproductive recovery when fed. Our results demonstrate the importance of prey attributes on S. grossa reproduction. Furthermore, females invest remarkably large amounts of resources during each reproductive cycle and over the course of a lifetime.

Methods
This study aims to investigate the effects of food availability on reproductive success. The experiment was divided into two parts: a lifetime experiment, which investigates a long-term effect on the quantity of viable offspring, reproductive intervals, and female body mass changes, and a three-month experiment with more divided treatments, which explains the details of the reproductive performance from the first 3 egg sacs. We titled these two experiments “long-term experiment” and “short-term experiment” respectively.

In the long-term experiment, adult females were fed one cricket per week before they were mated according to the protocol described in the Animal collection and rearing section. Following mating, they were divided into two feeding groups: 1) Intermitted feeding (n = 36), where spiders received one cricket every 3 weeks, and 2) Constant feeding (n = 57), where spiders received one cricket every 2-3 days. Crickets for feeding weighed between 150 and 200 mg. We recorded the number of spiderlings hatched from each egg sac and the duration of intervals between egg sac production until the females died. For females’ body mass monitoring, we randomly selected 10 virgin females from the rearing and mated them. Crickets (weighing between 150mg to 200mg) were offered once a week regularly, and one extra cricket after producing an egg sac. The body mass changes one day before and after producing egg sacs were recorded for the first five egg sacs.

In the short-term experiment, in which we varied the amount of food that the females were provided with before and after mating, adult females were divided into five groups (with 8 to 11 individuals per group), each receiving varying quantities and frequencies of crickets. The food treatments are as follows:

 two large crickets per week (~240mg per week);

 one large cricket per week (~120mg per week);

 one medium cricket per week (~60mg per week);

 one medium cricket every 2 weeks (~30mg per week);

 one small cricket every 2 weeks (~5mg per week).

We used the approximate food mass provided to spiders as treatment names. Female spiders were exposed to these feeding regimes 2 months before mating and following matings until the females had produced 3 egg sacs or after 2 months if no egg sacs were produced. Body mass of the females was measured at the beginning of the experiment, 50 days after initiation of the feeding regime, and 7 days after mating. Cannibalism behavior was checked within 48 hours after mating. When an egg sac was produced, the date and its mass were recorded. The number of spiderlings hatching, the number of eggs that did not develop into spiderlings, and the mean weight of spiderlings per egg sac were recorded. If females did not produce egg sacs after 2 months (treatment 5mg), they were offered crickets weekly and monitored for 1 month or until their time of reproduction. Females that died during experimental months were excluded from the experiment, and data from these females were removed.