The relative body size at which predators are willing to attack prey, a key trait for predator-prey interactions, is usually considered invariant. However, this ratio can vary widely among individuals or populations. Identifying the range and origin of such variation is key to understanding the strength and constraints on selection in both predators and prey. Still, these sources of variation remain largely unknown. We filled this gap by measuring the genetic, maternal and environmental variation of the maximum prey-to-predator size ratio (PPSRmax) in juveniles of the wolf spider Lycosa fasciiventris using a paternal half-sib split brood design, in which each male was paired with two different females and the offspring reared in two different food environments: poor and rich. Each juvenile spider was then sequentially offered crickets of decreasing size and the maximum prey size killed was determined. We also measured body size and body condition of spiders upon emergence and just before the trial. We found low, but significant heritability (h2=0.069) and dominance and common environmental variance (d2+4c2=0.056). PPSRmax was also partially explained by body condition (during trial) but there was no effect of the rearing food environment. Finally, a maternal correlation between body size early in life and PPSRmax indicated that offspring born larger were less predisposed to feed on larger prey later in life. Therefore, PPSRmax, a central trait in ecosystems, can vary widely and this variation is due to different sources, with important consequences for changes in this trait in the short and long terms.

Spider collection

Individuals of Lycosa fasciiventris were collected from June 23^rd to July 27^th 2015 in four different localities within the Almeria province (South-East Spain), in dry temporal washes (“ramblas”): 1) around Paraje las Palmerillas, Estación Experimental de Cajamar (36.7917°N, 2.6891°O); 2) near Boca de los Frailes village (36.8036°N, 2.1386°O); 3) near Carboneras village (36.9667°N, 2.1019°O) and 4) near Almanzora river (37.3414°N, 2.0078°O).  Individuals were then kept separately in the laboratory in a container (22 x 18 x 18 cm) with the bottom filled with 2-3 cm of soil collected from the sampling sites. Two wooden blocks (10 x 8 x 1 cm and 3 x 5 x 1 cm) were added to each tank to provide shelter. Only sub-adult virgin females were used to form the laboratory population. All individuals (adult and sub-adult males, and sub-adult females) were fed once a week with size-matched crickets (Gryllus assimilis; Fabricius 1775) purchased from a pet supply online store Exofauna, Spain (available in: https://exofauna.com). Spiders had access to water ad libitum through a 40 ml vial filled with water and covered with cotton. Tanks were placed in a climate chamber with simulated outdoor climatic conditions (day and night temperature cycles and photoperiod with light fluorescent tubes of 54 W, mimicking natural sunshine, and a relative humidity from 50 to 65%). Climatic conditions were adjusted to the preceding weekly average conditions in the Almeria province, with day-night temperature and light oscillations (temperature: 18.7-34.3 °C; light-dark photoperiod: 17:7-16:8 hours).

Breeding design

To assess genetic, maternal and environmental variation in individual prey-to-predator size ratio (PPSR), we performed a paternal half-sib split-brood design (Roff 1997; Lynch and Walsh 1998), in which 52 males (sires) were each mated with two virgin females (dams). Each week, offspring were provided with fruit flies (Drosophila melanogaster; Meigen, 1830) originated from cultures produced in the laboratory. Flies were fed with a nitrogen rich medium supplemented with high quality dogfood, which highly improves spider survival (Jensen et al. 2011). Maternal families were constituted by 12 offspring, split into two food availability treatments, varying in the number of flies provided. Thus, 3 out of 12 offspring from each maternal family were assigned to the rich environment, being given 3× the amount of food provided in the poor (or standard) environment. Initially, a single fly was offered to the spiders in the poor treatment and 3 flies in the richer treatment. This quantity was adjusted to 3 and 9 when individuals were approximately 6 months old due to higher food demand at that stage.

After hatching, spiderlings of wolf spiders climb to the female back and, in L. fasciiventris, remain with it for a period of a few weeks (Parellada 1998). Due to logistic reasons, all spiderlings were removed from the female back within one week, that is approximately 42 ± 8 (mean ± SD) days after they hatched (age at isolation). To estimate and control for post-hatching common environmental effects occurring on the female back, the age at isolation was included in all models. This variable was never significant (data not shown). Spiderlings were carefully collected from the female back with the help of a paintbrush. We took 12 spiderlings from each female and placed them separately in cylindrical containers (5 cm height and 6 cm diameter). Each container had the bottom covered with filter paper, providing a substrate for both locomotion and absorption of excreta, inside the growth chamber. Filter papers were checked weekly and replaced if necessary. A plastic tip was inserted at the bottom of the container, filled with cotton connected to a reservoir, providing water ad libitum to spiders by capillarity (Moskalik and Uetz 2011). The 1248 spiderling containers were then randomly arranged within the growth chamber to ensure that individuals belonging to the same family were spatially interspersed. This allowed mitigating possible common environmental effects after spiderling isolation from their mothers.

Morphometry

Body components were divided between structural body size (carapace width; Hagstrum 1971) and body condition (residuals of abdomen width on carapace width; (Jakob et al. 1996). Body condition reflects energy and nutrient storage independently on the size of the spider and thus reflects hunger level (Moya-Laraño et al. 2008). Structural body size may reflect the strength to subdue prey (e.g., Moya-Laraño et al. 2002). Both carapace and abdomen width were measured at their widest point.

Body size and body condition were measured in two instances: after individuals were taken from their mothers and isolated, and immediately before the trials for acceptance. Morphometric measurements were taken to the nearest 0.1 mm with a dissection microscope (Leica MZ125). While structural body size measured at the time of trial was needed to calculate prey-to-predator size ratio, body condition at the time of the trial was used to control for the hunger state of each spiderling (i.e. its motivational state). These traits were also measured early in life and used to calculate genetic and maternal correlations, to test how maternal investment in both offspring body size and condition could affect behavioural patterns of the spiders later in life.

Prey acceptance

This experiment aimed to measure the maximum relative size of a prey cricket (Gryllus assimilis) that a spider accepted, considering a range of cricket lengths (in mm) decreasing from 5× to 1×  (in units of 1) the carapace width of the spider. For that, we placed them in experimental arenas where each spider was offered crickets in a decreasing order of relative size until it subdued and killed a cricket. The response variable, prey-to-predator size ratio (PPSR) is the ratio at which the spider attacks and kills the cricket. This measure corresponds to the maximum PPSR (PPSR_max) at which predators kill their prey and the larger the relative size of the prey killed, the higher the PPSR. Spiders were measured in blocks of 17 ± 5 (mean ± SD) individuals. Each block was defined as the experimental batch of individuals assessed in each day.

Although this cricket species does not occur in the study site, L. fasciiventris is able to effectively prey on it, and a similar species with similar body size, Gryllus bimaculatus, is highly abundant in the collection area (Moya-laraño personal observation). As it was not feasible to collect G. bimaculatus in numbers enough to carry out this study, we used G. assimilis individuals from an established laboratory population. Note that this approach allowed testing the response of spiders that were naive to this prey, as all spiders had been fed with Drosophila to that point. Thus, this approach minimized environmental variation due to potential effects of previous experiences with cricket prey. 

In the trial, we used crickets with a length that differed from the target PPSR (5×, 4×, 3×, 2× or 1× of the width of the spider carapace) by less than 0.2 units. Crickets were weighted, and their length determined from a calibration curve, previously generated with the weight and length of 40 crickets: L = 3.22 + 0.32log(M); R² = 0.99; p < 0.0001; where L is cricket body length (in mm) and M is cricket body mass (in mg).  Mass was measured to the nearest 0.1 mg using a high precision scale (Mettler Toledo XP26). None of the crickets were used in more than one trial.

To standardize hunger levels across individuals, spiders were left to starve for seven days before being tested, similarly to other studies (Persons and Rypstra 2000). As it was not possible to standardize age across trials, individuals were randomly assigned to each trial. Spider age at the time of each measurement (331 ± 30 days old, mean ± SD) was recorded and later controlled for in the statistical analysis as a covariate (see below). A single spider and one cricket were placed inside the arena (7.5 cm diameter), in opposite sides, within enclosed inverted plastic vials (3 cm diameter). Then, both vials were gently lifted simultaneously, and crickets and spiders were allowed to interact for 6 minutes. If the cricket was not captured and subdued, the spider was enclosed in the vial and the cricket was removed. Spiders were then left to recover in the vial for 30 minutes until a new cricket from the next immediately lower size was presented (lower PPSR). Trials ended as soon as the spider attacked and killed a cricket or if the spider did not catch the smallest (1×) cricket.

Data used to assess the sources of variation of the maximum prey-to-predator size ratio (PPSR_max) in juveniles of the wolf spider Lycosa fasciiventris using a paternal half-sib split brood design.